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Tungsten Lamps
Excerpt from Field Guide to Illumination
Tungsten filament incandescent lamps, particularly tungsten halogen lamps, are often used in illumination systems. Paired with stable, current-controlled power supplies, they provide extremely stable sources. If the current is altered, the filament temperature changes, with a resulting change in filament resistance. Therefore, the voltage change is not linearly related to the current change. Similar nonlinear relationships hold for other characteristics of the lamp, such as power consumed, luminous flux output, efficacy (lumens/watt), color temperature, and lifetime.
Tungsten halogen lamps should be operated between 95% and 105% of their rated current. Within this operating envelope, the relationships between the operating parameters are approximately exponential. Modifying the current to a value different from the rated current changes the other parameters according to the exponents shown in the table below:
| Parameter | Exponent |
| Color temperature (K) | 0.80 |
| Lifetime (hours) | -25 |
| Luminous efficacy (lumens/watt) | 3.6 |
| Luminous flux (lumens) | 6.5 |
| Power (watts) | 2.9 |
| Voltage (volts) | 1.9 |
An example of a lamp operated at 5% over its rated current is shown below:
| Parameter | Rated | Operated |
| Current | 6.02 A | 6.02 × 1.05 = 6.32 |
| Color temperature | 2900 K | 2900 × 1.050.8 = 3015 |
| Lifetime | 1000 hrs | 1000 × 1.05-25 = 295 |
| Luminous efficacy | 16 lm/W | 16 × 1.053.6 = 19 |
| Luminous flux | 1040 lm | 1040 × 1.056.5 = 1428 |
| Power | 65 W | 65 × 1.052.9 = 75 |
| Voltage | 10.8 V | 10.8 x 1.051.9 = 11.8 |
A. V. Arecchi, T. Messadi, and R. J. Koshel, Field Guide to Illumination, SPIE Press, Bellingham, WA (2007).
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