Principles of Light Production
Light Sources
Dr. Lale Erdem Atılgan
Prof. Dr. Serhat ŞEKER
Illumination Techniques
At Djibouti University
Fall 2018-2019
Luminous Flux, Φ [lm]
  K0.F.V
• Luminous flux is the rate at which light is
emitted by a light source.
• It describes the visible light radiating from a
light source in all directions and is measured
in lumens [lm].
• Luminous flux is denoted by the Greek letter
φ.
• F is the energy flux in W, the electromagnetic
energy in unit time produced by unit power.
• K0 is the photometric equivalent of energy flux
and is equal to 683 lm/W .
• Vλ is spectral sensitivity.
• The lumen is, in fact, a unit relating radiant
flux (in watts) to visually effective radiation
(i.e., light) for a standard human observer [2].
Spectral Power Distribution
• Spectral power distribution expresses the radiant power emitted by a
source of optical radiation over a range of particular wavelengths.
• This is also referred to as «spectral power concentration» in the
international lighting dictionary.
• Relative spectral power distribution is the quantity most commonly
used in lighting to express the nature of radiant power emitted by a
source.
• To make the spectral power distribution relative, all the data are divided
either by the average value, by the maximum value within the wavelength
range of interest or some arbitrarily chosen value.
Spectral Power Distribution
• Some sources of optical radiation, such as incadescent sources, exhibit a
continuous spectrum of radiant power over a wide range of wavelengths.
Incandescent
The Sun
Spectral Power Distribution
• Some sources of optical radiation emit radiant power only at a few
discrete wavelengths or within very narrow ranges of wavelengths, each
range centered on a particular wavelength.
• These are called line spectra.
Spectral Power Distribution
• Many sources emit not only a continuous spectrum of optical radiation but
also emit strongly at certain wavelengths or in very narrow wavelength
bands.
• These spectra are represented as a continuous function with a
superimposed histogram.
• Metal halide and fluorescent lamps have this type of spectral power
distribution.
FL
HID
Luminous Efficiency
• Luminous efficiency is the ratio of the energy flux of a light source that is
perceived by the eye as visible light, to the total energy flux of the light
source.
Luminous Efficacy
• Luminous efficacy is a measure of how well a light source produces visible
light.
• It is the ratio of luminous flux to total consumed power, denoted by e,
given in lm/W.
• Luminous efficacy is one of the most important qualities of a light source.
The higher the efficacy value, the lower the consumption of electricity for
an equal amount of luminous flux.
Luminous Efficacy
Color
• Color is a result of spectra of optical radiation generated by light sources,
perhaps modified by objects and processed by the human visual system.
• Goals of color science are to quantify and predict human color experience.
• For this, the phsiological properties of the human retina are used.
• The CIE color specification system is employed for all colorimetric
measures that are related to light sources.
• The 1931 CIE color space maps a color perceived by the eye on a graph of
x, y.
• This graphic enables the perceived color to be expressed as ratios of the
three main colors, red, green and blue.
• These three main colors, or the tristimulus values, are the stimulus of the
humen retina as light is processed by the cones responsible for daytime
vision.
The cones in the retina each have a special sensitivity to light in the wavelengths
corresponding to red, green and blue. Therefore, human vision is called
«trichromatic». Here, the calculated X, Y and Z values show the trichromatic stimulus
or the tristimulus values the cones give to any lighting signal.
Blackbody Radiation
• A blackbody radiator is a perfect incandescent radiator.
• No known radiator has the same emissive power as a blackbody.
• The ratio of the output of a radiator at any wavelength to that of a
blackbody at the same temperature and the same wavelength is known as
the spectral emissivity ε(λ) of the radiator.
• Radiant power from a practical source, particularly from an incandescent
lamp, is often described by comparison with that from a blackbody
radiator.
Color Temperature
• The spectrum of optical radiation, and therefore the apparent color of a
blackbody is solely dependent upon its temperature.
• The apparent color and temperature of a blackbody are linked and so the
temperature of a blackbody can be used to describe the color appereance
of a light source, as its Color Temperature.
• Blackbody temperatures are absolute temperatures, expressed in units of
Kelvin.
• A theoretical blackbody becomes yellowish white at 3000 K, white at 5000
K, bluish white at 8000 K and deep blue at 60.000 K.
• Thus color temperature is the absolute temperature in units of Kelvin, of a
blackbody radiator having a chromaticity equal to that of the lightsource.
Correlated Color Temperature
• In many cases, an exact match of source and blackbody chromaticities is
not possible.
• Then, Correlated Color Temperature (CCT) is used to describe the nearest
visual match.
• CCT is the absolute temperature a blackbody has when it has
approximately the same color appereance as the source.
Correlated Color Temperature
• As the temperature increases the light appears to shift from red to
reddish-yellow to yellowish-white to white to bluish-white at high
temperatures.
• Confusingly, light with a CCT between 2700 K and 3200 K is a yellowishwhite light and is described as “warm” while light with a CCT between
4000 K and 7500 K is a bluish-white light and is described as “cool”.
Color Rendering Index
• Color Rendering Index (CRI) is a measure of the degree of color shift that a
set of test-color samples undergoes when illuminated by the light source
in question, as compared with those same test color samples when
illuminated by a reference illuminant of comparable color temperature.
• It is denoted by Ra.
• Color rendering index is a measure of how colors of surfaces will appear
when illuminated by a light source.
• Light that has an even SPD (Spectral Power Distribution) across the visible
spectrum, such as daylight or incandescent light, has a high CRI (the
maximum is 100).
• Light that has gaps in its SPD has a lower CRI.
Color Rendering In Real Life
CRI = 62
CRI = 93
CRI = 80
CRI = 92
Principles of Light Production
• Incandescence
• Gas Discharge
• Luminescence
– Electroluminescence
– Photoluminescence
• Phosphoresence
• Fluorescence
Incandescence
• Incandescence is the process by which optical radiation is emitted by a
material due to its temperature alone.
• In this type of light production, radiation results from the irregular
excitation of the free electrons of innumerable atoms due to atomic
motion.
• Heat is atomic motion and temperature is a measure of heat.
• The higher the temperature of a body, the greater is the atomic movement
and the greater and more frequent is the atomic excitation and generation
of photons.
• This thermal excitation involves many differently-sized electron
transitions and energy levels and so gives rise to many wavelengths of
radiation, forming a more or less continuous spectrum.
Incandescence
• At temperatures below approximately 873 K (600 C), only optical
radiation in the IR range is emitted by a body. (i.e. A coal stove or an
electric iron)
• Electronic transitions in atoms and molecules at temperatures above appr.
600 C result in the release of optical radiation in the visible as well as IR
regions.
• The incandescence of a lamp filament is caused by the heating action of an
electric current.
• This heating action raises the filament temperature substantially above
600 C, producing visible optical radiation.
Gas Discharge Production of Optical
Radiation
• Gas discharge is the mechanism by which many modern lamps convert
electrical power to radiant power.
• The spectral composition and therefore the practical utility for lighting of
this conversion depends on the constituents of the gas and its pressure.
Gas Discharge
• A gas discharge produces optical radiation by having free or conducting
electrons, moving under the influence of a relatively high electric field,
strike an atom in the gas and raise it into an excited state by moving one or
more of its orbiting electrons to a greater orbit.
• When the atomic electrons return to a lower state, they emit
electromagnetic radiation.
• The wavelengths of the electromagnetic radiation emitted by this process
depend on the energy levels of the atomic orbits characteristic of the gas
in the discharge and the interaction between atoms determined by the
pressure of the gas.
• At higher pressures, the spectral distribution broadens and contains more
wavelengths.
Gas Discharge
Spectrum of mercury discharge at different pressures
http://www.lamptech.co.uk/Documents/M3%20Spectra.htm
Luminescent Production of Optical
Radiation
• Luminescence is the process by which optical radiation is emitted by a
material when it absorbs energy that is re-emitted as photons.
• Radiation from luminescent sources results from the excitation of single
valence electrons of an atom, either in a gaseous state, where each atom is
free from interference from its neighbours exerts a marked effect.
• In the first case, line spectra results, such as those of mercury or sodium
discharge.
• In the second case, such as with light emitting diodes, narrow emission
bands result, which cover a portion of the spectrum, usually in the visible
region.
Luminescence
• Two kinds of luminescence are used in modern electric sources.
• Photoluminescence describes the process by which a substance absorbs
a photon (electromagnetic radiation) of a particular wavelength and reradiates electromagnetic radiation at a longer wavelength.
• Electroluminescence describes the process by which a substance
absrobs an electron and radiates electromagnetic radiation. The electron
absorption process of electroluminescence is usually part of electrical
conduction in the substance.
Luminescence
• In some electric sources, both gas discharge and photoluminescence are
used, as with the fluorescent lamp.
• In this case, a conductive low pressure mercury discharge produces UV
optical radiation.
• Photoluminescence of a phosphor layer on the lamp’s bulb wall absorbs
the UV optical radiation and re-radiates visible optical radiation.
• Some LEDs use both electroluminescence and photoluminescence.
• Electroluminescence at the semi-conductor junction produces short
wavelength optical radiation.
• Photoluminescence of phosphor on top of the junction absorbs this optical
radiation and re-radiates visible optical radiation.
Photoluminescence: Fluorescence
• Fluorescence describes a type of photoluminescence in which a molecule
of a substance absorbs a photon and immediately emits a photon of longer
wavelength.
• Fluoresence is the basis of light production the the fluorescent lamp; UV
optical radiation produced by an electric discharge in mercury vapor is
converted to visible optical radiation by the lamp’s phosphors.
Photoluminescence: Fluorescence
For the phosphor to emit light, it must first absorb radiation.
In the fluorescent lamp this is chiefly at a wavelength of 253.7 nm.
The absorbed energy transfers an electron to an excited state.
After loss of excess energy to the lattice of the phosphor as vibrational
energy (heat), the electron oscillates around a stable position for a very
short time, after which it returns to its original orbital position and energy
level, with simultaneous emission of a photon of radiation.
• Stokes’ Law states that the radiation emitted by this process must be of a
longer wavelength than that absorbed.
• Because of the electron’s oscillation around both a stable and excited
orbital position, the excitation and emission processes cover ranges of
wavelength, commonly referred to as bands.
•
•
•
•
Photoluminescence: Phosphorescence
• Phosphorescence describes a type of photoluminescence in which the
time between absorption and emission of photons is significantly longer
than that observed in fluorescence.
• The transition form an excited to a stable state in phsporescent materials
can take minutes to hours, exhibiting long luminous persistance.
• Phosporescence is not common in electrical light sources but is used in
some emergency markers for wayfinding.
Electroluminescence: Electroluminescent
Lamp
•
•
•
•
•
•
Certain phosphors convert energy directly into optical
radiaton, without using an intermediate step as in a
gas discharge, by utilizing the phenomenon of
electroluminescence.
An electroluminescent lamp is composed of a two
dimensional area conductor on which a dielectricphoshpor layer is deposited.
A second two-dimensional area conductor of
transparent material is deposited over the dielectricphosphor mixture.
An alternating electric field is established between the
two conductors with the application of a voltage
across the two-dimensional conductors.
Under the influence of this field, some electrons in the
elecrroluminescent phosphor are excited.
During the return of these electrons to their ground
state, the excess energy is emitted as optical radiation.
Electroluminescence: Light Emitting
Diodes and Organic Light Emitting Diodes
• Light Emitting Diodes (LEDs) produce optical radiation by
electroluminescence when free electrons moving in a semiconductor
material become attached to an atom that has an outermost layer or shell
that can accept an electron.
• In the process of falling into such an orbit, the electron releases energy
and the material emits optical radiation.
• LEDs can also be made from organic semiconductor material.
• In this case, the structure is thin-film and layered rather than a small block
of material as in a silicon LED.
• Organic Light Emitting Diodes (OLEDs) are area sources of optical
radiation, rather than the tiny luminous jucntions of silicon as in LEDs.
• The active elements of an OLED can be deposited onto a substrate in
patterns, much like printing, and so provide for OLED driven displays,
signage and active fenestration systems.
Lamps
• Lamps used for providing illumination can be divided into three general
classes:
1.
2.
3.
Incandescent,
Discharge,
Solid-state lamps.
• Incandescent lamps produce light by heating a filament until it glows.
• Discharge lamps produce light by ionizing a gas through electric discharge
inside the lamp.
• Solid-state lamps use a phenomenon called electroluminescence to
convert electrical energy directly to light.
Choosing Lamps
Lamp Characteristics
• Power
– The electrical power consumption of the lamp as opposed to the power
consumption of a system comprising lamp and ballast.
• Luminous flux/luminous efficacy
– The luminous flux specifies the total amount of light generated by a lamp. The
rated luminous flux is measured at a standardized measurement temperature
of 25 °C in units of lumen [lm]. The ratio of luminous flux to electrical power
consumption gives the luminous efficiency [lm/W]. The system luminous
efficacy also includes the power consumption of the ballast.
Lamp Characteristics
• Service life
– The average service life is normally specified, being the time by which
statistically half the lamps are still working (mortality), or half the lamps have
failed. This test is subject to standardised operating conditions. Lamp
manufacturers display this failure rate by curves.
• Drop in luminous flux
– The initial luminous flux of a new lamp decreases over its time of operation
(lumen maintenance), due to the ageing of its chemical and physical
components. Lamp manufacturers display this drop in luminous flux by
curves.
Lamp Characteristics
• Light color
– The light colour describes the color impression made by a white light source
as relatively warm or relatively cool. It is affected by the red and blue color
components in the spectrum.
• Color rendition
– The spectral components of the light determine how well various object colors
can be reproduced. The higher the color rendition index (Ra or CRI), or the
lower the color rendition group number, the better the color rendition in
comparison with the optimum reference light.
Lamp Characteristics
• Warm-up time
– Discharge lamps in particular need between 30 seconds and several minutes
to warm up and output the full luminous flux.
• Re-start
– High-pressure discharge lamps need to cool down for several minutes before
they can be started again.
Lamp Characteristics
• Dimming capability
– Besides incandescent and halogen incandescent lamps, nowadays all
fluorescent and compact fluorescent lamps can also be dimmed over almost
any range. Metal halide lamps, however, are still not approved by the
manufacturers for dimming, because this may have uncontrollable effects on
light quality and lamp service life. The new series of special models for indoor
and outdoor applications constitute an exception. The power of high pressure
sodium- and mercury-vapour lamps can be varied, but only in discrete levels.
Lamp Characteristics
• Burning position
– Manufacturers specify the permitted burning positions for their lamps. For
some metal halide lamps, only certain burning positions are allowed so as to
avoid unstable operating states. Compact fluorescent lamps may usually be
used in any burning position; however, important properties such as the
luminous flux vs. temperature curve may vary with the position.
Filament Lamps
• Filament lamps consist of a wire filament mounted within a glass bulb that
contains a gas or a vacuum.
• Optical radiation is emitted when the filament is heated to incandescence
by the passage of electrical current.
• End of life is most commonly due to tungsten evaporation, which leads to
failure of the filament.
Filament Lamps
• Electric current passes through a thin filament of tungsten wire, heating it
until it emits optical radiation. The efficacy of light production depends on
the temperature of the filament; the higher the temperature, the greater
the portion of optical radiation emitted in the visible region.
• The major factors that affect filament temperature are:
– The filament material, microstructure and geometry,
– The composition of the atmosphere and its pressure,
– The magnitude of electrical current.
• All else equal, lamp life is inversely related to filament temperature.
Filament Lamps
• The basic components are a filament, bulb,
gas fill and base.
• When the gas fill includes a halogen, usually
bromine, the lamp is referred to as a
tungsten halogen lamp.
• When a special coating is applied to a
tungsten halogen capsule to redirect
infrared radiation back to the filament, it is
then known as a halogen infrared lamp.
Spectrum
• Filament lamps produce proportionally more long-wavelength optical
radiation than short. Most of the radiation is in the infrared part of the
spectrum.
Relative Power Conversion
Efficacy
Lifetime
Fluorescent Lamps
• Fluorescent lamps are the most widespread and verstaile of discharge
lamps.
• They are employed universally in offices, educational facilities, healthcare,
and other commercial applications, while finding widespread use in
industrial, retail, institutional and residential lighting.
• This is due to the fact that fluorescent lamps are available in a wide variety
of lumen outputs, shapes and colors, while having desirable characteristis
that include good to excellent life, luminous efficacy, lumen maintenance
and color rendering.
Fluorescent Lamps
• The fluorescent lamp is a low-pressure gas discharge source, in which
light is produced predominantly by fluorescent powders, also known as
phosphors, that are activated by UV energy generated by a mercury arc.
• The electrodes of most fluorescent lamps are pre-heated prior to ignition,
causing them to emit electrons, which collide with mercury atoms
contained within the discharge tube.
• Collisions may happen with such force to free electrons from mercury
atoms, a process known as ionization, which is necessary to maintain the
arc.
• Collisions at lower force may elevate an electron of the mercury atom to a
higher energy level, which is known as excitation.
Fluorescent Lamps
• When the electron of an excited mercury atom returns to its rest state, a
photon is released.
• In a low pressure mercury discharge, most of these photons are in the UV
region of the spectrum.
• Phosphors on the inside of the tube convert the UV radiatio into visible
optical radiation.
Fluorescent Lamps
• Because the mercury discharge has a negative volt-ampere relationship,
fluorescent lamps must be operated in series with a current-limiting
device, commonly called a ballast.
• A ballast limits the current to the value for which the lamp is designed,
provides the required starting and operating lamp voltages and may
provide dimming control.
Fluorescent Lamps
• The tube of a normal linear fluorescent lamp is made of soda-lime glass
doped with iron oxide to limit the emission of UV radiation.
• Low sodium content glass is also used for very higly loaded lamps, such as
compact fluorescent lamps (CFLs).
• Tube length and diameter have been standardized.
• Diameter is determined first by the desired loading on the phosphors;
higher loadings increase lumen output per unit area and are associated
with small tube diameters.
• Length is dictated first by the luminous flux to be produced by the lamp.
• All else being equal, higher lümen output requires more surface area of
phosphors and therefore longer tubes.
Fluorescent Lamp
• Fluorescent lamps are generally classified according to their diameter.
• In this classification, the inch from the American measurement system
is used.
• According to this, the diameter of the T5 lamp is 5/8 inches, the T8 lamp
8/8 inch and T12 lamp 12/8 inches.
Lamp
Standard
Tube
Diameters
(mm)
T1
3.2
T2
6.4
T5
16
T6
19
T8
26
T10
32
T12
38
T17
54
Fluorescent Lamps
• Two electrodes are hermetically sealed at opposit ends of the fluorescent
bulb.
• The electrodes conduct electrical power into the lamp and provide the
electrons necessary to maintain the arc discharge.
• Constructions vary but all are made of tungsten coated with a mixture of
alkaline earth oxides, which readily emit electrons when heated to a
temperature of about 800 C.
• Elecrrodes may be,
– Preheated
– Continuously heated
– Cold
Fluorescent Lamps
Pre-heated
Electrodes
Continuosly
heated
Cold
Preheating causes electrodes to emit elecrrons that facilitate starting
with less loss of electron emissive material, compared to cold starting.
Once the lamp is operating, the ballast stops heating the electrodes. The
temperature necessary for continued electron emission is maintained by
electrons from the discharge that bombard the electrodes and this way,
energy can be conserved.
After the lamp is operating, the ballast continue to heat the electrodes.
In the cold mode, high voltage is used to start the fluorescent lamp
instantly, causing electrons to bombard the electrodes at high velocity.
Such collisions heat the electrodes and facilitate the emission of electrons
via thermionic emission. Ion bombardment also occurs, which causes
sputtering of the electron emissive amterial, leading to end blackening
and reduced electrode life.
Compact Fluorescent Lamps (CFLs)
• The compact fluorescent lamp family includes a
variety of multi-tube, single-based lamps.
• T4 and T5 tubes are typically used and there are
many techniques of adding, bending and
connecting the tubes to obtain the physical size
and lümen output desired.
• Because of the high power density in these
lamps, high performance phosphors are used
extensively in order to attain the desired lumen
output, lumen maintenance and color rendering.
• They were initially designed to replace
conventional 25 to 100 Watt incandescent
lamps, having greater efficacy values and much
longer lifetimes.
Compact Fluorescent Lamps
• CFLs are manufactured with pins, without
integral ballasts and with screw bases
including integral ballasts.
• Sockets may have 2 or 4 pin configurations.
• 4 pin versions are generally paired with
electronic ballasts that may be dimmable or
on/off.
• 2 pin versions may also be paired with
electronic ballasts but they cannot be
dimmed.
• The dimmability of the screw-based version
depends on the integral ballast.
Inductive Discharge Fluorescent Lamps
• Inductive discharge fluorescent lamps are low
pressure gas discharge fluorescent lamps that operate
without the need of electrodes.
• They use and electromagnetic field instead of an
electric current passing through electrodes, to excite a
gas in a bulb.
• As there are no electrodes to fail, they are sometimes
called electrodeless lamps and they have rated
lifetimes of up to 100.000 hours.
• These lamps are finding greater use in hard to reach
locations and where lamp or fixture maintenance
might be especially difficult.
Inductive Discharge Fluorescent Lamps
http://www.agtus.org/knowledgebase/
Cold Cathode Fluorescent Lamps
• Cold cathode fluorescent lamps
are often used in decorative,
sign lighting and other
architectural applications.
• Due to the high energy losses
associated with electrode
operation, they are not as
efficacious as the more
widespread hot cathode lamps.
• They are frequently
manufactured with small
diameter tubing so that they
can be bent into various shapes
and sizes.
"Cold Cathode Fluorescent Lamp" by Neotesla - Own work.
Licensed under CC BY-SA 3.0 via Commons https://commons.wikimedia.org/wiki/File:Cold_Cathode_Fluoresc
ent_Lamp.JPG#/media/File:Cold_Cathode_Fluorescent_Lamp.JPG
https://www.nelt.co.jp/english/products/ccfl/about.html
UV Lamps
•
•
A low pressure mercury discharge generates UV
radiation that in an ordinary fluorescent lamp is
converted to visible optical radiation by
phoshpors.
UV lamps that make use of the low-pressure
mercury discharge fall into two categories:
– Those that create UV-C for sterilization and
germicidal applications,
– Those that create UV-A for special illumination
effects, such as used in theaters and discotheques
(blacklights).
•
•
UV-C lamps do not use a phoshpor. These lamps
are used to purify water and surfaces, harden
paints, adhesives, plastics, expose printing plates
and assist with some inspection tasks.
UV-A lamps employ a phoshpor that converts
short wavelength UV-C and UV-B to longer
wavelength UV-A.
Spectrum
Relative Power Conversion
Efficacy
Lifetime