Lamps, LEDs and Circuits
8.2 Tungsten halogen lamps
Key attributes
For mains or low-voltage operation
Longer rated life and higher luminous efficacy than
incandescent lamps
Easy to dim
Brilliant light
Low-voltage types are very small and are ideal for precise
direction of light (but do require a transformer)
Excellent colour rendition
Fig 8.2 Tungsten halogen lamps
Key application areas
Retail and domestic
Restaurants and catering
How they work
Current flows through a filament and heats it up, just as
in incandescent lamps. These lamps therefore generate a
relatively large amount of heat. The halogen cycle increases
the efficiency and extends the rated life compared with
traditional incandescent lamps. (The halogen cycle is a
chemical mechanism that causes tungsten that evaporates
from the filament during operation to be deposited back onto
the filament, thereby reducing blackening of the bulb wall.
Chemicals used in the halogen cycle also slow down the rate
of diffusion of filament material, thereby increasing the filament
life, which is the principal failure mechanism)
8.3 Fluorescent lamps
Key attributes
High to very high luminous efficacy
Good to excellent colour rendition
Long rated life
Extensive range of types
Dimmable
Key application areas
Extensively used in most application areas
Fig 8.3 Fluorescent lamps
How they work
An alternating electric field generates UV radiation (which is
in itself invisible to the human eye) between the two electrodes
in the discharge tube. This UV radiation is converted into
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visible light in the phosphor coating on the tube wall. The
colour rendering and colour temperature attributes of the light
produced depend upon the chemical composition of the
phosphors. The lamp needs a starting aid and a current limiting
device, which may be combined in an electronic ballast. The
luminous flux is highly dependent on the ambient temperature
around the lamp.
Application notes
T16 fluorescent lamps differ from T26 versions in several
characteristics that the user should be particularly aware of.
1. Luminous flux vs. temperature curve
As with all fluorescent lamps, the luminous flux produced
by the lamp is temperature dependant. An optimum
ambient temperature exists for which the light output is a
maximum, and the light output decreases as the ambient
temperature moves away from this optimum. Both the
T16 and T26 lamps have the same basic shaped curve,
however the optimum temperature for a T16 is 35°C,
whereas the optimum temperature for a T26 lamp is
25°C. The reason for this is that the lamp cool spot for a
T16 lamp is at the end of the tube with the manufacturers
label printed on it, whereas the cool spot for a T26 lamp
is in the centre of the tube.
One effect of this differing optimum temperature is that
the rated luminous flux quoted by manufacturers is at a
standard temperature of 25°C. For the T16 lamp the
maximum value of flux lies above this value, and therefore
the luminaire light output ratio (LOR) may have levels
greater than 100%.
Relative luminous flux %
110
100
90
80
70
60
50
40
30
20
10
0
T16
T26
5
10
15
20
25
30
35
40
45
50
55
Ambient temperature ˚C
Fig. 8.4 C
urves relating luminous flux to ambient temperature for T16 and T26
linear fluorescent lamps
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Lamps, LEDs and Circuits
2. Lamp Orientation
Owing to the two electrodes (tube ends) not being
identical in design it matters how one or more lamps
are fitted in the luminaires, In general, lamp ends should
always have the same orientation (so that the lamp labels
should be at the same lamp ends for all luminaires). In
cold environments it could be a benefit to lamp output to
have the lamp labels at opposite ends to aid heating of
the lamp cold spot.
3. Ageing/burning in
Brand new lamps stabilise during the initial aging phase.
This is the period immediately after the lamps are switched
on for the first time, when the initially encapsulated
mercury is vaporised and evenly distributed throughout
the lamp. Unstabilised lamps may differ in brightness and
light colour, and may exhibit flickering at low dimming
levels. To ensure perfect operation a period of two to four
days of operation without switching or dimming should be
allowed, particularly in installation which allow dimming.
One should also wait for proper ageing before assessing
an installation for illuminance levels and light quality.
T16
T26
Length
Power
549mm
14W
24W
Rated
luminous
flux (25°C)
1200lm
1750lm
849mm
21W
1900lm
39W
3100lm
28W
2600lm
54W
4450lm
1149mm
1449mm
35W
3300lm
49W
4300lm
80W
6150lm
Length
Power
590mm
18W
Rated
luminous
flux (25°C)
1350lm
895mm
30W
2350lm
1200mm
36W
3350lm
1500mm
58W
5200lm
Table 8.2 Summary of selected lamps
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