Lamps, LEDs and Circuits 8.2 Tungsten halogen lamps Key attributes
For mains or low-voltage operationLonger rated life and higher luminous efficacy than
Easy to dimBrilliant lightLow-voltage types are very small and are ideal for precise direction of light (but do require a transformer)
Fig 8.2 Tungsten halogen lamps Key application areas
Retail and domesticRestaurants 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 wal.
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 efficacyGood to excellent colour renditionLong rated lifeExtensive range of typesDimmable
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 Lamps, LEDs and Circuits
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
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 al 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 100 5 10 15 20 25 30 35 40 45 50 55
Fig. 8.4 Curves relating luminous flux to ambient temperature for T16 and T26 Lamps, LEDs and Circuits
Owing to the two electrodes (tube ends) not being identical in design it mat ers 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 al 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.
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 initial y 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 al owed, particularly in instal ation which al ow dimming. One should also wait for proper ageing before assessing an installation for illuminance levels and light quality. Power luminous Power luminous flux (25°C) flux (25°C) Table 8.2 Summary of selected lamps
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