A Guide to: Driving Your Calibration Lamp

Calibration Lamp Stability

Nearly all photometers, colorimeters and spectroradiometers are calibrated using a tungsten halogen lamp that provides a known and traceable output - either spectral radiant flux (luminous flux) or spectral irradiance (illuminance). Tungsten bulbs remain the preferred choice of reference source due to their excellent long-term stability. However, what of the power supply that is used to power your calibration lamp? A calibration lamp delivers a known light output when a specified electrical current passes through the bulb. What is perhaps not so widely known is just how much lamp output is affected by relatively small changes in the drive current.

A tungsten lamp emits light as a result of incandescence - as current passes through the filament, it experiences resistance, which in turn causes the filament to heat up. At a colour temperature of about 2000 Kelvin, we regard the light emitted as "white" light, albeit of a distinct orange hue. Tungsten lamps which are employed as optical calibration standards are normally driven at a current which causes them to emit light of a colour temperature of 2856 Kelvin. A tungsten lamp of this colour temperature is referred to as a CIE Illuminant A standard. If the current is altered, the resistance of the filament will change, and so the colour and amount of light will also vary. Failing to drive your calibration lamp at its specified current will introduce a potentially large error into your subsequent measurements.

It is generally recommended that you should operate your tungsten calibration lamp at a current between 95 and 105% of its Rated Current. Within this range, there is an approximately exponential relationship between the various lamp parameters. If you deviate from the rated current, you can expect other lamp operating parameters to vary as follows:

- Colour temperature (Kelvin): exponent = 0.8
- Lifetime (hours): exponent = -25
- Luminous efficacy (lumens/Watt): exponent = 3.6
- Luminous flux (lumens): exponent = 6.5
- Power (Watts): exponent = 2.9
- Voltage (Volts): exponent = 1.9

To illustrate the significant change in lamp output if you were to drive your calibration standard at just 5% above its rated current, consider the following lamp which provides a colour tempearture of 2900K and a luminous flux of 1,040 lumens when operated at a rated current of 6.02 Amps. The lamp is also expected to have a life of 1,000 hours and a luminous efficacy of 16 lumens/Watt.

- Operating current = 6.02 x 1.05 = 6.32 Amps
- Colour temperature = 2900 x 1.05^0.8 = 3015 Kelvin
- Lifetime = 1,000 x 1.05^-25 = 295 hours
- Luminous efficacy = 16 x 1.05^3.6 = 19 lumens/Watt
- Luminous flux = 1,040 x 1.05^6.5 = 1,428 lumens

As can be seen in the example above, by applying a current of just 5% over the rate current, you reduce the lamp's life to 30% of the expected value and the luminous flux increases by 37%. The importance of operating your calibration lamp at its precise rated current is why our Labsphere calibration lamp power supplies used in our lamp measurement and uniform light sources systems provide a fixed current output which is regulated to within 0.1%. You can be confident that your calibration lamp will give its specified output over an extended period.

Pro-Lite wishes to credit The SPIE "Field Guide to Illumination" by Arecchi, Messadi & Koshel as the source material for much of the information presented above. Angelo Arecchi served as Labsphere's Vice President of Engineering for a number of years and continues to serve in an advisory capacity. To order your copy of this useful text book, click here.