A FUNDAMENTAL STUDY ON CURRENT INDUSTRIAL LIGHTING SYSTEMS
AND ANALYSES OF RETROFIT OPTIONS
by
MATTHEW BLAIR CLINTON
A THESIS
Submitted in partial fulfillment of the requirements
for the degree of Master of Science in the
Department of Mechanical Engineering
in the Graduate School of
The University of Alabama
TUSCALOOSA, ALABAMA
2008
Copyright Matthew Blair Clinton 2008
ALL RIGHTS RESERVED
CONTENTS
ACKNOWLEDGEMENTS ............................................................................................... iii
LIST OF TABLES ............................................................................................................ vii
LIST OF FIGURES ......................................................................................................... viii
ABSTRACT ........................................................................................................................ x
CHAPTER 1: INTRODUCTION ...................................................................................... 1
CHAPTER 2: HIGH-INTENSITY DISCHARGE LAMPS .............................................. 5
2.1 HID Lamp Construction ....................................................................................... 6
2.2 HID Ballasts ........................................................................................................ 11
2.3 HID Evaluating Criteria ...................................................................................... 12
2.4 HID Technological Advancements ..................................................................... 16
CHAPTER 3: LINEAR FLUORESCENT LAMPS ........................................................ 29
3.1 Linear Fluorescent Lamp Construction .............................................................. 29
3.2 Linear Fluorescent Lamps Types ........................................................................ 33
3.3 Fluorescent Ballasts ............................................................................................ 38
CHAPTER 4: FUTURE ALTERNATIVES .................................................................... 43
4.1 Induction Lighting .............................................................................................. 43
4.2 Light-Emitting Diodes ........................................................................................ 45
CHAPTER 5: LIGHTING CONTROLS ......................................................................... 49
5.1 Occupancy Sensors ............................................................................................. 49
5.2 Photocells ............................................................................................................ 53
v
CHAPTER 2
HIGH-INTENSITY DISCHARGE LAMPS
High intensity discharge lamps are commonly used in both high- and low-bay
applications to provide, powerful, efficient, high-quality light. Suffice it to say, HID
lamps have long been the standard for industrial facilities, and continue to be the lighting
workhorse for the illumination of manufacturing plants around the country. Their
compact design, attractive color characteristics, and high lumen efficacy make HID
lamps an appealing option for industry applications.
Three primary lamp types comprise the family of industrial HID lamps. These
lamp types include mercury vapor, high-pressure sodium, and metal halide. Each lamp
utilizes a unique set of materials, namely metals, to produce light, but all three adhere to a
common form of operation. Table 1 compares each lamp according to several features
[2]. Mercury vapor lamps will not be discussed because they have long become obsolete
and, for the most part, are not used in manufacturing facilities.
5
6
Table 1. HID Lamp Comparison
HID Lamp Comparison
Features
Mercury
Probe Metal Halide
High Pressure Sodium
Color
White
White
Yellow
Efficacy (L/W)
50
60 – 85
100 – 125
Lumen Maintenance
50%
65%
90%
Lamp Wattages
50 – 1000
175 – 1000
35 – 1000
Lamp Life (kHrs)
24+
6 – 20
24+
Source: Taken from Ref. 2.
2.1 HID Lamp Construction
There are two main parts to a HID lamp: an outer bulb, and an arc tube. Light is
formed in all HID lamps through the discharge of an electric arc inside the arc tube.
Contained inside the arc tube are two tungsten electrodes, a starting gas, and arc metal.
The starting gas, either argon xenon, or a mixture of neon, argon and xenon, will
facilitate starting the lamp at low pressure and ambient temperatures [1]. The arc metal
consists of metals and halide components of metals. An electric field is passed between
the starting electrode and the main electrode which allows the starting gas to volatize the
arc metal. The subsequent decrease in resistance between the main electrodes caused by
the volatized arc metal creates an arc in the tube [1]. As the arc metal evaporates into the
arc discharge, different metals will release distinct lines of radiant energy [1]. Because of
this, each type of HID lamp produces light characteristic of the metals contained in the
arc.
7
The outer bulb, made of either a soft or hard glass, encompasses the arc tube,
providing it protection from the environment. The outer tube also functions to filter out
unnatural light and short wavelength ultraviolet energy, absorbing a majority of the
unwanted radiation while allowing the visible light to pass through [1]. The inside of this
outer bulb can also be coated with a diffusing material to reduce brightness or a layer of
phosphors which absorbs the UV radiation to improve color-rendering properties.
Between the arc tube and the outer bulb, a low-pressure gas or vacuum is present. Fig. 1
depicts the basic structure of an HID lamp [21].
Fig. 1. A basic HID lamp [Taken from Ref. 21].
8
2.1.1 Metal Halide
Metal halide lamps were initially created in the 1960’s for use in industrial
settings to provide a compact, efficient, and intense source of light [1]. They have now
replaced the much outdated mercury vapor lamp, and are available in a wide array of
sizes and configurations to meet the varying demands present in commercial, residential,
and industrial applications. They are commonly referred to as a “point” source of light,
operating under high temperature and pressure. Because of the blue light they emit,
metal halide lamps are often the most ideal option for providing a primary source of light
in situations where no natural light is available. Most manufacturers offer metal halide
lamps in low, mid-range, and high wattages. Metal halides are most commonly seen in
wattages of: 175-W, 200-W, 225-W, 300-W, 320-W, 350-W, 360-W, and 400-W [1].
A metal halide lamp is constructed in a similar manner to its predecessor, the
mercury vapor lamp, but boasts some significant improvements. Where light is produced
through the vaporization of mercury in a mercury vapor lamp, a process that continues
until the amount of mercury contained in the arc tube is completely vaporized and
depleted, a metal halide lamp utilizes a regenerative cycle to maintain light output [1]. In
a metal halide lamp, a compact arc tube contains a mixture of gases, commonly a highpressure mixture of argon, mercury and a variety of metal halides. As voltage is applied
to the electrodes and an arc is formed, the argon gas is easily ionized, sustaining the arc
and producing intense heat to vaporize the mercury and metal halides contained within
the tube. As the temperature and pressure increase inside the tube, and the halide vapors
converge on the intense central core of the discharge, the halides separate from the metals
allowing the metals to produce a bluish-white light. After the metal atoms have radiated
9
their spectrum, they return to the cooler arc tube wall to recombine with halogen atoms,
repeating the cycle [1]. Fig. 2, depicting normal metal halide lamp construction, is
displayed below [1].
Fig. 2. Metal halide lamp construction [Taken from Ref. 1].
2.1.2 High-pressure Sodium
High-pressure sodium lamps are one of the most efficacious light sources
available, with the ability to attain efficacies of nearly 150 lumens per watt. They are
commonly used outdoors because of their unmatched efficacy, long life, and poor color
rendition. In industrial applications they are most often seen in warehouses or low task
areas where minimal color contrast is sufficient.
10
The production of light in a sodium lamp is achieved when an electric current is
passed through sodium vapor [1]. In a typical high-pressure sodium lamp there exists an
arc tube, inside which an amalgam of metallic sodium and mercury resides to produce
light through vaporization, and a small amount of xenon functions as a starting gas. The
arc tube is made of a refractory material, namely aluminum oxide (alumina), to resist the
corrosive effects of the high-temperature sodium [1]. The outer envelope is typically
evacuated to isolate internal electrical components from environmental effects and
maintain a steady arc temperature.
While low-pressure sodium lamps typically radiate energy in the principal double
D lines of sodium at 589nm, the high-pressure sodium bulb will radiate energy across the
visible spectrum because of the pressure broadening associated with higher sodium
pressures in the lamp [1]. The resultant light output is no longer monochromatic, making
colors distinguishable and thus more suitable for indoor applications. Another important
feature inherent to the sodium lamp is the absence of a starting electrode and the presence
of an ignitor, which provides a high voltage, high frequency pulse to start the lamp. The
current application of this feature in pulse-start metal halide bulbs has seen
groundbreaking results. Fig. 3 displays the typical construction of a high-pressure
sodium lamp [1].
11
Fig. 3. High-pressure sodium lamp construction [Taken from Ref. 1].
2.2 HID Ballasts
A HID lamp must also have a ballast in order to operate effectively. Because the
light source does not regulate its own current consumption well enough, it must utilize
the services of a ballast to perform this operation. All HID and fluorescent lamps present
a negative resistance to the power supply when connected in an electrical circuit. If the
ballast were not used to provide a positive resistance in the circuit, the lamp would draw
an increasing amount of current until it failed or destroyed the power supply. Ballasts
have the reputation of being fairly intricate devices even though their operation is quite
simple.
Ballast types are numerous, but there are two categories for establishing their
function: magnetic or electronic. Magnetic ballasts are similar to transformers in that
they use an inductive core and coil to slow down changes in current. An inherent
annoyance with magnetic ballasts is their tendency to produce a flicker because of their
12
low frequency modulation. They also vibrate which causes a noticeable hum. Electronic
ballasts use solid state electronic circuitry to produce starting and operating currents for
the lamp. They are favored over magnetic ballasts because of their ability to take the
frequency of power flowing from the mains (50 or 60 Hz) and transform it to 20,000 Hz
or more [1]. This eliminates the stroboscopic effect (flicker) and constant humming
prevalent in the operation of magnetic ballasts. In addition, electronic ballasts offer
higher system efficacy because of efficiency gains in the ballast itself, such as the use of
a capacitor for lower line losses over an inductor [1].
2.3 HID Evaluating Criteria
There are several basic criteria upon which the two lamp types can be
distinguished for their performance in those areas. Some of those criteria will be used to
discuss the performance of both lamps.
2.3.1 Lamp Warm-up and Re-strike Times
An important aspect of any light source is its ability to yield light quickly. An
attractive light source will reach its full output of light and maintain its normal operating
color in the shortest amount of time, whether that be from first start or hot start (restrike).
2.3.1.1 Metal Halide
Although metal halide lamps below 150-W use ignitors, which provide enough
voltage across the main electrodes to start the lamp, most metal halide lamps above 150-
13
W in operation today use a standard starting probe which is part of the lamp circuit. It
takes time for the metal halides in the arc tube to warm up and evaporate into the arc; the
time it takes a lamp to reach 90% of full light output (warm-up) can vary between 2 and
15 minutes. Re-strike times are generally a lot longer because it takes a while for the
traditional pinched-body arc tube to cool down. In order to reignite, the temperature and
vapor pressure inside the arc tube must reach a reasonable level; otherwise, the high
required starting voltage cannot be applied without destroying the lamp. Re-strike times
for metal halide lamps typically vary between 5 and 20 minutes. Pulse start systems,
which utilize an ignitor have superior warm- up and re-strike times over standard probestart systems [22]. Fig. 4, taken from the National Lighting Product Information Program
(NLPIP), illustrates this point [22].
Fig. 4. Warm-up and re-strike comparison for probe-start and pulse-start lamps [Taken
from Ref. 22].
14
2.3.1.2 High-pressure Sodium
To start a high pressure sodium lamp, an ignitor has always been used for creating
a high voltage, high frequency pulse. Generally, the vapor pressure is lower and the
starting gas is ionized more easily with this configuration than it is with convention probe
start metal halide lamps. The arc is, therefore, created faster, allowing for shorter starting
times, and consequently, warm-up times. Re-strike times are also shorter because of a
lower vapor pressure and the presence of an ignitor.
2.3.2 Lamp Life and Lumen Maintenance
The benefits from long lamp life are glaringly obvious. A lamp that lasts longer
will save money in replacement and maintenance costs. The average rated life, which is
provided by most manufacturers, is determined when 50% of a group of lamps initially
installed during a test are still operating. “The procedure prescribes operating cycles for
HID lamps of 11 h on, 1 h off” [1]. However, lamp life listed by manufacturers is
generally based on run times of both 3 and 12 hours. Factors affecting lamp life include
but are not limited to: burn position (vertical or horizontal), number of starts, high or low
operating voltage, ineffective control devices (ballasts, capacitors, etc.), and extremely
high operating temperatures.
Lumen output for lamps is normally tested by the manufacturer and measured
after 100 hours of operation (initial lumens) and at 40% of rated lamp life (mean lumens)
[1]. A lamp that maintains its initial lumen output throughout the course of its life would
certainly be ideal. However, this is only ideal, for lamps will lose lumen output gradually
over the course of their life. Fortunately, most causes of lumen depreciation can be
15
corrected and have been as technology has improved. Lumen maintenance curves for
three 400-W HID sources are displayed below in Fig. 5 [1].
Fig. 5. Lumen maintenance curves for 400-W HID light sources [Taken from Ref. 1].
2.3.2.1 Metal Halide
Of all HID lamps, metal halide lamps tend to experience a greater level of lumen
depreciation. Also because the electrodes used in metal halide lamps evaporate more
readily, they experience shorter life cycles. Toward the end of their life (the last 40%),
metal halide lamps can experience significant color shift. In color critical situations
where the color shift might be objectionable, the lamp will be replaced. Standard metal
halide lamps generally last from 10,000 to 20,000 hours [1].
16
2.3.2.2 High-pressure Sodium
High-pressure sodium lamps utilize smaller diameter arc tubes and an electrode
construction similar to that of mercury lamps. The electrodes contain metal oxides
embedded within the tungsten coil which allow for a slower evaporation process. This
slow evaporation process means less “sputtering” over the course of the lamp’s life
yielding higher lumen maintenance values than those present in typical metal halide
lamps [1].
“The life of a high pressure sodium lamp is limited by a slow rise in operating
voltage that occurs over the life of the lamp” [1]. This slow rise in voltage is caused by a
chain of events that cause termination of the lamp when the ballast can no longer supply a
high enough voltage to reignite the arc. The typical lifetime of a high pressure sodium
lamp is 16,000 to 24,000 hours [1].
2.4 HID Technological Advancements
Ever since both metal halide and high pressure sodium lamps were developed in
the 1960s to improve the operational shortcomings of mercury vapor lamps, there has
always existed a tradeoff between both lamps which inherently limited their versatility.
For decades high-pressure sodium lamps have been used as the energy saving alternative
to metal halide lamps. More specifically, they were considered the most viable choice
“when color quality is secondary to low operating and maintenance costs” [2]. In
manufacturing facilities, the characteristic yellow light produced by high-pressure lamps
is normally reserved for outdoor applications and color-indifferent areas where low
contrast light (poor color rendition) is tolerable, as is common in most warehouses and
17
storage areas. However, when higher quality white light is necessary, as is the case in
providing light for the surfaces in most industrial facilities, metal halide lamps were
chosen because they distribute crisp, white light with higher color rendition. For
specifiers and plant managers alike, it was always a choice between a source with good
color quality and a source with good operational efficiency; no single lamp presented
both merits.
2.4.1 Metal Halide
This dilemma in HID lighting existed for decades. While standard metal halide
lamps are still keeping most manufacturing facility floors lit, and high-pressure sodium
lamps maintain a relegated role in providing light to secluded areas which see little
occupation, revolutionary research and development is devoted to changing this scenario.
Within the last 10 years considerable research has led to significant innovations in the
development of metal halide lamps allowing them greater versatility. The metal halide
lamp was chosen because “of all the lamps in the high-intensity discharge (HID) family,
the metal halide family of lamps has the brightest future” [3]. Despite their limitations,
metal halides already offer much better light quality than mercury or sodium lamps and
have a potential for modification which far outweighs other lamps in the HID family.
Because they could already produce the coveted white light demanded by the industrial
sector, they were an obvious choice over mercury and high-pressure sodium lamps for
future development.
A company on the forefront of integrated luminaire technology, focused solely on
providing new metal halide lamp and ballast systems, Venture Lighting, strives “to be the
18
best metal halide lamp company in the world and to dominate in the development of
metal halide lamps for general lighting” [23]. Heralded as “the fastest growing metal
halide company in the world,” Venture Lighting is the only corporation concentrated on
this technology [23]. Presently the owner of many design patents on pulse-start lamps
and ballasts, they are providing the lighting industry with many innovative options,
rectifying the problems plaguing the old standard probe-start lamps. Another company
dedicated to solving this paralyzing dilemma in HID lighting, Advance, often considered
the industry leader in the manufacturing of new ballasts, is regarded as the top source for
innovations and designs in fluorescent, HID, and LED ballasts in North America.
Leading an aggressive pursuit of this desirable integrated systems approach to solving
key operational limitations of HID lighting, Advance coordinated a revolutionary
cooperative effort between itself and the top four U.S. lamp manufacturers to coalesce
issues facing the formerly distinct lamp and ballast industries. Lamps and ballasts had
always been seen prior as separate entities in the design process, which Advance
recognized as an impediment to the process needed to solve this dilemma. Advance has
performed substantial research in conjunction with these lamp manufacturers to
overcome many design flaws in probe-start metal halide lamps. Both of these sources
will be documented as references for most of the changes in metal halide lamp design and
operation.
2.4.1.1 Pulse-start Technology
The quest of Advance, and Venture Lighting alike, was a complete systems
approach to solving the major dilemma in HID lighting. Metal halide lamps must be
19
considered a complete system, more than any other lamp [3]. All key components
(ballast, ignitor, controls, lamp and fixture) must work together to get the best
performance [3]. Because almost every lamp is designed individually and is different
chemically, containing different doses of chemicals, careful attention must be paid in
designing the whole system. Thus, it was essential that the problem be tackled in this
manner.
The problem with probe-start technology surfaced when lamp manufacturers
began to focus on improving performance features in metal halide lamps, specifically
lumen efficacy and lamp life. In order to reach this end, manufacturers desired greater
fill pressures and different chemistries inside the arc tube; however, the benefits of these
improvements inside the arc tube could not be realized unless a modification was made to
allow for higher starting voltages. Probe start systems, which only allowed for 600-volt
peak starting voltages, could not accommodate the arc tube changes. In other words, to
see an increase in lumen efficacy and lamp life, a higher starting voltage would need to
be applied to the lamp, which the standard probe-start system was incapable of doing.
That is when researchers seized the opportunity to apply present technology to a different
lamp type. By using an ignitor, the integral device already used in high-pressure sodium
lamp circuits to apply starting voltages, manufacturers were able to revolutionize the way
metal halide lamps operate.
Pulse start technology has existed in metal halide lamps in the 150-W range and
smaller for quite some time, yet within the last five years manufacturers have started
applying the technology to a broader range of lamp sizes. At first glance, the difference
in a standard probe start lamp and the modified pulse start lamp seems minor, yet the
20
consequent gains in overall lamp operation are vast. A conventional metal halide system
consists of a lamp and probe start ballast. Inside the arc tube are three electrodes, “a
starting probe electrode and two operating electrodes” [22]. The ballast applies a high
starting voltage to the probe, normally greater than the voltage applied to the main
electrodes during operation, creating a discharge in the small gap between the probe and
one of the main electrodes. “Electrons then jump across the arc tube to the other
operating electrode to help start the lamp” [22]. Once the main arc is established and the
pressure and temperature begin to rise in the arc tube, the probe electrode is removed
from the circuit by a bi-metal switch. A pulse-start system does not use a starter
electrode. Instead, an ignitor sends a series of high voltage pulses (usually 3 to 5
kilovolts) across the two main electrodes [22]. It is important to recognize that the ignitor
is an integrated part of the ballast. Therefore, pulse-start lamps will not operate on
traditional probe-start ballasts.
By eliminating the probe to start the lamp, a level of performance from metal
halide lamps that never existed before is made possible. Because the probe is no longer
present inside the arc tube, “the amount of pinch (or seal) area at the end of the arc tube is
reduced, which results in a reduced heat loss” [22]. This distinction opened up a wealth
of opportunities, allowing manufacturers the flexibility to optimally reshape arc tubes and
increase fill pressures and temperatures inside, producing greater light output (arc tube
modifications are discussed below). Additionally, two of the key elements prone to cause
premature lamp failure, the bi-metal switch and probe electrode, are removed from the
circuit in a pulse start lamp, boosting lamp life to levels only attained previously by highpressure sodium lamps. Furthermore, the series of pulses heat up the lamp faster during
21
starting, creating better warm-up times. Finally, probably the most distinguishable
feature in the pulse-start lamp is a reduction of tungsten sputtering, an undesirable
characteristic in probe-start lamps which caused considerable lumen depreciation as
tungsten blackened the arc tube wall as a lamp aged [22].
Fig. 6 displays the subtle difference in probe-start and pulse-start lamps which
produces so many gains in performance [22]. The probe electrode and bi-metal switch
are easily visible in the figure.
Fig. 6. Construction of probe-start vs. pulse-start lamps [Taken from Ref. 22].
Manufacturers claim many achievements for pulse-start systems over their
predecessor. The following list of improvements obtained from Advance and NLPIP
yields many of these performance achievements [2, 22]:
•
Depending on lamp wattage and burning cycle, longer lamp life of up to 50%
over traditional probe-start metal halide lamps (20,000 – 30,000 hrs)
•
Better lumen maintenance – increases by up to 33% in most lamps
22
•
Greater efficacy – lumen output per watt increases by 25% to 50%
•
Superior color rendition – a 30% improvement of up to 85 CRI
•
Reduced color shift and improved lamp to lamp color consistency
•
Faster warm up and re-strike times – warm up time is often cut in half to 2
minutes and re-strike hovers around 3 to 4 minutes
•
Better cold starting capability – start at temperatures as low as -40°C (-40°F)
Table 2 yields a simple comparison of probe-start and pulse-start metal halide lamps
to high-pressure sodium lamps [2]. The gap in performance between probe-start metal
halide lamps and high-pressure sodium lamps is evident from these criteria.
Table 2. Standard HID Comparison with Pulse-start Metal Halide
Comparison with Pulse Start Metal Halide
Features
Probe Metal Halide
Pulse Metal Halide
High Pressure Sodium
Color
White
Whiter
Yellow
Efficacy (L/W)
60 – 85
90 – 110
100 – 125
Lumen Maintenance
65%
70% - 75%
90%
Lamp Wattages
175 – 1000
35 – 1000
35 – 1000
Lamp Life (kHrs)
6 – 20
10 – 30
24+
Source: Taken from Ref. 2.
2.4.1.2 Arc Tube Improvements
A direct consequence of the elimination of both the probe electrode and bi-metal
switch in pulse-start metal halide systems allowed for some major innovations in arc tube
23
design. Venture Lighting has patented and crafted an arc tube design which reshapes the
arc chamber to provide a contoured fit of the arc itself. This unique quartz sculpting
process, resulting in Venture Lighting’s patented Uni-Form body, eliminates the pinchedseal design which prevailed for nearly 30 years in the industry [23]. Fig. 7 displays this
contoured Uni-Form body arc tube in comparison to the pinched-seal design [23]. Not
only is the arc tube smaller and lighter, allowing it to heat and cool at speeds faster than
the previous pinched seal design would allow (better warm up and re-strike times), but it
creates a uniform geometry which produces uniform heating of the metal halides in the
arc tube. This allows the materials in the arc tube to reach a higher average temperature.
Higher temperatures equate to a higher light output from the arc tube. Venture Lighting
claims many performance improvements in their pulse-start systems because of the
formed-body arc tube, namely: higher efficacy, better color uniformity, faster warm up
and hot re-strike, and longer life [23]. Fig. 8 displays formed-body arc tubes inside two
different shaped metal halide lamps [27].
Fig. 7. Common metal halide arc tubes [Taken from Ref. 23].
24
Fig. 8. Formed-body arc tube inside two differently shaped metal halide lamps [Taken
from Ref. 27].
An additional finding, which was the result of a tinkering similar to that of the
installation of an ignitor in the metal halide ballast, came “when Phillips engineers
borrowed a ceramic arc tube from a sodium lamp and stuck it into a metal halide lamp”
[3]. Manufacturers soon discovered a superior improvement in color performance. This
improvement in color offers CRI values in the 80 to 90 range, approaching that of
halogen lamps. Possibly the most important consequence of this discovery is the almost
guarantee of complete color consistency (meaning no color shift during a lamp’s
lifetime). Lumen maintenance also improves in ceramic metal halide lamps. Though
ceramic metal halide lamps are essentially a perfect match for retail and residential accent
25
and downlighting applications because of their ability to replace halogen and
incandescent lamps, they are great for color-critical spaces often prevalent in industrial
facilities.
2.4.1.3 Electronic Ballasts
Traditionally, metal halide lamps have operated on a magnetic ballast, consisting
of a core and coil which are relatively heavy, hot (when operating), and bulky. Many
ballast manufacturers now offer electronic ballasts that are smaller, lighter, and provide
better performance [3]. Electronic ballasts have been successfully used with fluorescent
lamps for years mainly because of reduced energy losses and dimming capabilities, but
only recently have they taken flight in high-wattage HID applications. Electronic ballasts
use solid state circuitry to control voltages and currents for starting and operating the
lamp in a manner more efficient than magnetic ballasts. In other words, they generally
accomplish the task of current regulation with a lower input wattage to the system than
do their predecessors. They also have the ability to operate at higher frequencies, often
reaching 100 kHz and above. Higher frequency operation of ballasts reduces tungsten
electrode sputtering, and therefore consequent deposition on the arc tube wall. This
improves many facets of a lamp’s performance including: lumen maintenance, lamp life,
warm up and restrike, and color stability. All electronic ballasts use pulse starting and
therefore gain additional operational efficiencies from the use of an ignitor (as mentioned
previously pulse starting also reduces tungsten sputtering). Another appealing advantage
of electronic ballasts is their ability to allow lower dimming levels at higher efficiencies
than magnetic ballasts could attain. Conventional magnetic ballasts can only attain
26
dimming levels of 40% to 50% of full output, with significant reductions in system
efficiency [3]. Electronic ballasts can drop lighting levels lower while still maintaining
relatively good system efficiencies. The combination of these features of electronic
ballasts and the potential to apply ceramic arc tubes to metal halide lamps has produced a
viable alternative source for many retail applications. The future for electronic ballasts in
the industrial sector is somewhat uncertain because owners are hesitant to use them at
such a high initial cost. Presently, it is difficult to consider a major capital project for
replacing old magnetic ballasts on metal halide lamps because of high initial costs, but
recent trends show that the price of electronic ballasts is decreasing.
Fig. 9 is a photograph of Advance’s Dynavision electronic HID ballast. These
ballasts operate with 320-W and 400-W HID lamps. An innovative microprocessorbased design offers significant increases in lumen maintenance with continuous dimming
of the lamp.
Fig. 9. Advance’s Dynavision electronic HID ballast [Taken from Ref. 28].
27
The following is a list of advantages provided by electronic HID ballasts [13]:
•
Improved light color and output
•
Thirty percent longer lamp life and lower lumen depreciation
•
Continuous dimming capabilities
•
Reduced energy costs
•
Smaller size and lighter weight
•
Silent operation
•
Flicker-free
•
High-power factor (0.99) and low harmonic distortion
2.4.2 High-pressure Sodium
Because metal halide lamps were essentially the “chosen” HID lamp for future
development, not much has been done to improve the performance of high-pressure
sodium lamps. Several manufacturers like Phillips, GE, and Venture Lighting offer highpressure sodium lamps with better color rendition, but these retrofit options still provide
warmer color temperatures (2,200K) and a shorter life (15,000hrs). These options are
attractive because they yield better quality light with a simple lamp retrofit, but they will
never offer the benefits afforded metal halide lamps due to lack of new technology (see
aforementioned section on metal halide advancements). Fig. 10 displays a high-pressure
sodium retrofit option with improved color rendition manufactured by Phillips.
28
Fig. 10. Phillips Ceramalux® high-pressure sodium lamp [Taken from Ref. 29].
The characteristic yellow light provided by high-pressure sodium lamps is more
than just a limiting factor for development; it seems it has become responsible for their
demise. As the production of metal halide lamps with increasingly better efficacies and
lumen maintenance values continues, high-pressure sodium lamps will most likely be
utilized for fewer applications and eventually bumped from the market. Applications
where the unwanted yellow light source is chosen for reasons of efficacy and lumen
maintenance are diminishing. Metal halide lamp manufacturers are creating new lamps
with all the color benefits of previous sources and performance qualities approaching
those of high-pressure sodium lamps. Additionally, people are beginning to prefer higher
quality white light for outdoor and other niche applications controlled by high pressure
sodium lighting because of these advances in metal halide lamps. The T-12 fluorescent
lamp has been dealt similar circumstances. It presently dominates the fluorescent market
much like high-pressure sodium lamps still dominate their niche applications, but both
are fast approaching the end of their useful lives because of more efficient and better
quality alternatives.
CHAPTER 3
LINEAR FLUORESCENT LAMPS
Fluorescent lamps are manufactured in a variety of shapes and sizes. Linear
fluorescent lamps represent a distinct family of straight-tube light sources that generate
light through the excitation of mercury atoms and subsequent fluorescence of a phosphor
coating. Three particular types of linear fluorescent lamps will be described in
anticipation of the analyses that follow.
3.1 Linear Fluorescent Lamp Construction
In 1857, a French physicist named Alexandre E. Becquerel began experimenting
with the luminescence of substances when placed in a Geissler tube. These discoveries
would lead to theorizations about the construction of fluorescent tubes similar to those
seen today. Heinrich Geissler, a German glassblower, had earlier created a mercury
vacuum pump capable of inducing a vacuum in a glass tube to a level not previously
attainable. When current was passed through this tube, a green glow could be seen at the
cathode end of the tube, creating a device known commonly as the Geissler tube.
Becquerel built on this discovery by experimenting with different coatings on the inside
of the tube. In 1901, the American, Peter Cooper Hewitt, patented the first mercury
vapor lamp. This is considered “the very first prototype of today’s modern fluorescent
lights” [11].
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30
A fluorescent lamp, commonly categorized as an arc or vapor lamp, operates in a
completely different way than does a standard incandescent lamp. In an incandescent
lamp, light is simply a byproduct of electricity running through a highly resistive
filament. As electricity flows in the circuit of an incandescent lamp, high temperatures in
the filament cause it to glow and emit light. In a fluorescent lamp, light is produced
when electricity is applied to different gas mixtures contained in a vacuum tube. So the
direct result of operating a fluorescent lamp is visible light, while an incandescent
dissipates 90% of the input electrical power as heat.
Fluorescent lamps are low pressure, gas-discharge devices that utilize electricity
to excite mercury vapor suspended in a gas (normally argon or xenon). An arc is created
when electricity runs through a cathode (coiled tungsten electrode typically coated with
barium, strontium and calcium oxides – electron-emitting substances) at each end of a
tubular lamp, sending current through mercury vapors in the tube. As electricity passes
through the lamp circuit, the cathode heats up and emits electrons that collide with
mercury atoms, displacing electrons in these atoms from their natural orbit and forcing
them into a higher, unstable energy state [12]. As the displaced mercury atom’s electron
returns to a stable energy level, it gives off some visible, but mostly invisible (ultraviolet)
radiation (generally along principle lines of 254, 313, 365, 405, 436, 546, and 578 nm).
This ultraviolet radiation is transformed into visible radiation by a special phosphor lining
on the inner side of the lamp tube which lengthens the “UV wavelengths to a visible
portion of the spectrum” [12]. In other words, ultraviolet photons are absorbed by
electrons in the atoms of the phosphor coating which causes a similar energy level jump,
and drop, emitting another photon of wavelength perceptible by the human eye. “The
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phosphors are generally selected and blended to respond most efficiently to 254 nm, the
primary wavelength generated in a mercury low-pressure discharge” [1]. Fig. 11
illustrates this phenomenon in fluorescent lamps [12].
Fig. 11. Illustration of light production in general fluorescent lamp [Taken from Ref. 12].
Fluorescent lamps are commonly designated by their shape and diameter which
follow a particular nomenclature. A letter designates the shape of the lamp while a
number (indicated in eights of an inch) designates the diameter. For instance, T-8 refers
to a tubular bulb 8/8 in. (1 in. in diameter). Lamps can either be tubular or U-shaped.
Most lamps seen in the industrial sector are tubular. Lamps vary in diameter from 2 mm.
(0.25 in. T-2) to 54 mm. (2.125 in. T-17). Their lengths vary from 100 to 2440 mm. (4 to
96 in.) [1].
Similar to HID lamps, fluorescent lamps use two electrodes which are
hermetically sealed inside the tube, one located at each end. These cathodes are designed
for either cold or hot operation, but because hot operation (arc mode operation) is
inherently more efficient, most fluorescent lamps are designed in this way. In arc mode
operation, the hot cathode is typically constructed from either a single strand of tungsten
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wire or a coiled piece of tungsten wire around which another finer piece of wire is
uniformly wrapped. Thus, the electrode is called either a coiled-coil electrode or a triplecoil electrode, respectively. Both electrode types are coated with an electron emissionenhancing substance, which is commonly a mixture of alkaline earth oxides. As current
passes through the tungsten coil, temperatures of 1100 °C (2012 °F) are reached, causing
the coating to emit a large quantity of electrons [1].
The lamp tube is commonly filled with mercury vapor, typically maintained at
1.07 Pa (the vapor pressure of liquid mercury at 40 °C – the optimum bulb wall
temperature of operation for which most lamps are designed), and a rare gas or
combination of gases at low pressure to facilitate ignition of the discharge [1]. Common
rare gases used in lamps include: argon, krypton, neon, and xenon. An integral part of
operation is the ability of the lamp to develop and maintain a discharge between the two
electrodes. Thus, the attainment of optimum temperatures and pressures inside the tube
is critical for the ionization of mercury gas, which ultimately determines the fate of a
consistent discharge.
The phosphors which coat the walls of each fluorescent tube will establish the
color of light produced by each lamp. Lamps with markedly different spectral power
distributions are available for purchase, and thus, it is often a chore selecting a lamp with
the most appropriate color characteristics. “Popular fluorescent lamps use three highly
efficient narrow-band, rare-earth activated phosphors with emission peaks in the short-,
middle-, and long-wavelength regions of the visible spectrum. These triphosphor lamps
can be obtained with high color rendering, improved lumen maintenance, and good
efficacy with correlated color temperatures between 2500 and 6000 K relative to
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halophosphate lamps” [1]. However, rare earth phosphors are rather expensive and thus
longer lamps (T-5 – T-12) typically employ a two-coat system. The less expensive
halophosphate is located on the inside, while the more expensive rare earth blend resides
on the outside, closest to the mercury discharge. A lamp utilizing this configuration of
phosphors will typically have a spectral power distribution with a closer resemblance to
that of the rare earth phosphors. Fig. 12 was taken from the Illuminating Engineering
Society of North America [1]. The three distinct illustrations within the figure elucidate
some key areas of function in a fluorescent lamp.
3.2 Linear Fluorescent Lamps Types
The following discussion highlights some aspects of linear fluorescent lamps in
the most common available sizes. Descriptions are pithy and not meant to provide a
comprehensive review of each lamp type.
34
Fig. 12. Cutaway view of some common fluorescent lamps: (a) typical rapid start
fluorescent lamp and the production of light (b) lamp electrode construction (c) detail of
the electrode [Taken from Ref. 1].
3.2.1 T-12 Lamps
Despite the influx of newer, more efficient fluorescent lamps and the passing of
the National Energy Policy Act of 1992 (EPACT) which banned their production after
1995, T-12 fluorescent lamps are still the most commonly used source of fluorescent
light. T-12 lamps, possessing a tube diameter of 1.5 in., are commonly seen in lengths of
4-ft. and 8-ft [1]. More specifically, the 40-W, 4-ft, rapid-start T-12 lamp had
35
considerable control of the market for fluorescent lighting until more efficient and
efficacious lamps exposed its inefficiencies. Today many of the nation’s industries still
use the inefficient T-12 lamp, mainly in a reduced 34-W configuration, but most are
making the switch to more efficient T-8 models when replacements are needed. Despite
a growing awareness of more viable replacements to the T-12 lamp, many facilities are
hesitant to invest in expensive capital projects to replace these lights. T-12 lamps have
clearly seen their days of dominance, however; and today, T-8 and T-5 lamps are slowly
uprooting the industrial sector’s workhouse due to numerous energy cost saving benefits.
3.2.2 T-8 Lamps
T-8 lamps represent a viable, energy-efficient replacement to the obsolescent T12 lamp. They are a family of 1 in. diameter straight tube fluorescent lamps which are
manufactured in many of the same lengths as T-12 lamps. The 4-ft version is the most
common, consuming approximately 32 W. Other common lengths are 2-,3-,5-, and 8-ft.
Though T-8 lamps are interchangeable with T-12 fixtures of similar length, a different
ballast must be used in replacement. Most T-8 lamps utilize higher efficiency electronic
ballasts which facilitate even better performance. Additionally, these thinner lamps
“enable better photo-optic control of the light produced by the fixture, increasing
efficiency and providing uniform distribution of its light output” [5]. The smaller
diameter also allows for a more efficient use of the expensive rare earth phosphors. T-8
lamps are also offered in popular energy savings wattages of 25 W, 28 W, and 30 W
where a combination of ballasts with varying ballast factors can be employed to achieve a
desired quality and quantity of light for numerous applications [1].
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T-8 lamps installed in multi-lamp fixtures have recently gained momentum in the
market for fluorescent high-bay fixtures because of their proven ability to provide good
optical control and steady, uniform light. For high fixture mounting heights (those
typically seen in high-bay applications), high-lumen, 32-W T-8 (Super T-8) lamps are
often specified. A drawback of using T-8 lamps at this height is that more lamps may be
required to produce the same wattage as a 400-W metal halide in a retrofit. Currently
multi-lamp fixtures of both T-5 and T-8 lamps are being employed for both high and low
bay applications (although T-5 fixtures are considered a better option at heights above
20ft.). Common high-bay applications utilize 6- high-lumen, T-8 lamps (typically 3,100
initial lumens per lamp) with high ballast factor ballasts (1.14-1.20) and very efficient
reflectors.
3.2.3 T-5 Lamps
T-5 lamps also represent a suitable, energy-efficient replacement to the T-12
lamp. These lamps pose an even greater threat because they are smaller in diameter than
the T-8, offering the same benefits but on a different level. According to the National
Electrical Manufacturers Association (NEMA), T-5 and T-5HO lamps comprise 2% of
the fluorescent lamp market. This should, however, come as no surprise considering it
has taken almost a decade for T-8 lamps to gain their 33% share of the market from the
entrenched T-12 lamp.
The T-5 lamps compose a family of 5/8 in. diameter straight tube lamps, 40%
smaller than T-12 lamps, that utilize the appealing triphosphor technology. They are
designed to operate solely on electronic ballasts, and for most multi-lamp systems the
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newly refined programmed start ballast is considered optimal. This fact, in conjunction
with the fact that they are only available in metric lengths and bipin bases, renders them a
more difficult replacement for the T-12 lamp. Rather than just replacing the ballast for a
T-12 to T-8 change, the entire fixture must be replaced to change from either of these
lamps to a T-5 lamp. Despite making a retrofit rather difficult, these features are
generally beneficial to the T-5 lamp. Mainly, they allow for it to provide a higher source
of brightness and better optical control. Additionally, because they are so much smaller,
they are designed to provide optimum light output at 35 °C (95 °F) rather than the
common 25 °C (77 °F) [1]. For this reason it has been stated that T-5 lamps are better
than T-8 lamps for high-bay applications where greater ambient temperatures are more
common. Also, this higher optimum operating temperature allows for the design of more
compact luminaires, an extremely attractive feature responsible for the rise of T-5 highbay fixtures. Another important advantage of T-5 lamps is its use of barrier coating
technology. The smaller diameter lamp makes it more economical to use expensive
coatings that reduce the absorption of mercury by the tube’s glass interior and phosphor
coating. This barrier coating has dramatically reduced the amount of mercury needed to
operate the lamp (from 15mg to 3mg), and consequently, by reducing mercury absorption
which causes lamp light depreciation, increased maintenance of initial lumen output
values.
Typically fluorescents were not considered a viable option for high-bay
applications because they could not provide the source brightness that HID lamps could
at heights above approximately 20 feet. But the emergence of higher-output fluorescent
lamps with excellent performance qualities, and cooler-operating, multi-lamp (4 to 6
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lamps) fixtures has allowed T-5 (and in most cases T-8) luminairies to compete at these
heights.
Table 3. Comparison of Nominal Four-Foot Fluorescent Lamp Types
Comparison of 4-foot
T5HO
T8
82
85
3,000/3,500/4,100/5,000/
3,000/3,500/4,100/5,000/
6,000K
6,000K
5,000 lumens @ 35°C
3,000 lumens @ 25°C
ambient
ambient
54W
32W
92.6 lm/W @ 35°C
93.8 lm/W @ 25°C
ambient
ambient
Lumen Maintenance
95% @ 8,000 hours
95% @ 8,000 hours
Maintained System
86.1 lm/W @ 8,000
89.1 lm/W @ 8,000
Efficacy
hours and 35°C ambient
hours and 25°C ambient
Rated Life (3hrs/start)
20,000
24,000
Fluorescent Lamps
CRI
Color Temperature
Initial Rated Light Output
Nominal Lamp Wattage
Initial Lamp Efficacy
Source: Taken from Ref. 2.
3.3 Fluorescent Ballasts
The interaction between lamps and ballasts is one of the most significant factors
governing overall system performance. Various starting and operating methods for lamps
are provided by a multitude of different ballasts and it is of utmost importance to ensure
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that the ballast is chosen to match a particular application. These ballasts will often
provide a balance of certain characteristics like lamp life, energy efficiency, and
maintenance costs. Most fluorescent lamps used today operate on electronic ballasts
because of their inherent superiority over magnetic ballasts. They are significantly
cheaper for fluorescents than they are for HID lamps due to many decades of proven
performance and field acceptance. As previously noted, electronic ballasts will provide
energy savings over magnetic ballasts, but there are numerous reasons that their influence
on the fluorescent ballast market has been substantial. Among other advantages, they are
smaller, quieter, and lighter than their magnetic counterparts [25]. They operate at cooler
temperatures, are more efficient (better efficacy – high lumen per watt), provide longer
lamp life, and reduce the flicker and soft hum common in magnetic ballasts (because of
higher frequency operation) [25]. Three main types of fluorescent ballasts, categorized
by their lamp starting and operating methods, are seen consistently in application, but one
particular type, the programmed-start ballast, has garnered considerable acclaim for its
ability to operate with multiple controlling techniques in coveted high-bay applications,
and will thus be the focus of discussion.
3.3.1 Rapid Start Ballasts
A rapid start ballast applies continuous heat to the cathodes during the course of
operation of a fluorescent lamp. The heat and starting voltage are applied
simultaneously. This continuous heating of the cathodes reduces the necessary starting
voltage and allows the cathodes to reach a sufficient operating temperature. High starting
voltages are often the cause of tungsten sputtering and eventual lamp blackening; so a
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reduction in starting voltage likely increases the life of the lamp. The only drawback is
that additional power will be needed to provide continuous cathode heating.
Additionally, because the cathode heating and starting voltage are applied
simultaneously, a delicate balance must be reached for the optimal performance. The
cathodes need to be heated to the appropriate temperature before the arc is established.
But there is no guarantee that proper cathode temperatures will be reached prior to lamp
ignition. If the cathode temperature is below optimum and the voltage across the lamp
gets high enough to ignite the lamp, sputtering of the emissive material may ensue.
However, as will be discussed later, programmed start ballasts rectify this design
shortcoming (see section on programmed start ballast). Rapid start ballasts are very good
for lamps connected to occupancy sensors, photocells, or any other lighting control
devices where they might be switched on and off frequently. Note that additional
circuitry needed for cathode heating warrants a higher ballast cost [25].
3.3.2 Instant Start Ballasts
An instant start ballast will provide a high starting voltage to a fluorescent lamp
without prior cathode heating. Providing more of a “jump start” to the lamp, instant start
ballasts put undeniably more stress on the electrodes. An obvious reduction in energy
consumption will exist as extra power to provide heating is unneeded, but a shorter lamp
life because of repeated electrode material expulsion may outweigh the benefits of energy
efficiency if the lamp is employed for frequent switching applications [25].
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3.3.3 Programmed Start Ballasts
Programmed start ballasts are a variation or improvement to rapid start ballasts.
Their unique starting process markedly improves lamp life (50 percent in frequent
switching applications) and reduces the pitfalls associated with rapid start ballasts to
approach the energy saving potential of instant start ballasts. Programmed start ballasts
utilize this precise starting sequence, which is broken into well-established steps that
eliminate the deficiencies in other starting methods [26].
The ballast begins with the application of heat to the cathodes during a starting or
preheat interval, and voltage across the lamp is maintained at a low level. The duration
of this first step is preprogrammed in a ballast and is an integral part of successful
operation. The ballast provides heat (voltage) to the cathode until it reaches an optimum
temperature (at least 700°C) [26]. Voltage across the lamp remains at a low level until
this optimum cathode temperature is reached, preventing the lamp from igniting and
creating an unwanted glow current (lamp current that flows during the preheat interval –
causing lamp blackening and degradation) [25].
In the second step of operation, the ballast applies a voltage high enough to begin
ignition of the arc, but low enough to preserve emissive material on the cathodes. Both
step operating times and applied voltages are programmed into the ballast for better
performance, hence the name “programmed start.” Often, cathode heating is reduced
after lamp starting to provide additional energy savings with decreased power
consumption. Ultimately, programmed start ballasts are a better alternative for
applications that utilize lighting control devices as they are purported to provide 50,000
switching cycles (15 min. on/ 5 min. off) and a lamp life improvement of 10,000 hours in
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the these conditions [25]. They are simply gentler on the cathodes than either of the other
ballast types because they employ such a well-defined starting process. These ballasts
are currently being paired with multi-lamp T5HO fixtures for excellent optical control
and light quality when operated with a variety of light sensors and control devices.