Test Instruments FAQ
Q: Why are there different light sources and why would more than one type be needed?
A: Light sources may produce multiple light levels (luminance) and are rated at different color temperatures. (To understand what is best for your application see color temperature and luminance questions or, ask our sales department.)
At the present time there is no singular light source that can produce all luminance levels and all color temperatures.
For this reason several calibrated light sources are available that produce the most commonly used outputs.
We also have light sources with broader capabilities that are used in research and development and for other purposes.
Q: What are Color Temperature and Kelvin Degrees?
A: Color temperature varies according to the source from which light is produced.
The variation of color temperature is measured in degrees known as Kelvin and abbreviated "K".
While there are many different color temperatures that could be measured in various
lighting conditions the two that are primarily used in photographic applications are "Daylight and Tungsten".
- Daylight (outdoors in sunlight) is usually measured at 5500º K or above.
- Tungsten (indoors under artificial light) is usually measured around 3200º K.
Color photographic film does not have the ability to record daylight and tungsten correctly. For this reason there are daylight and tungsten films or, the photographer must use filters for the desired result.
Since most films are daylight balanced a flash is used indoors to provide extra illumination and boost the color temperature.
Q: What is Luminance?
A: Luminance refers to the intensity or level of brightness.
Calibrated Light Sources have the ability to produce or output specified amounts of luminance.
There are various ways to measure luminance output. Some are listed below:
Foot Candle: a unit of luminance representing the light intensity over a surface placed one foot away from a standard candle.
Lux: a unit of luminance equal to .0929 foot candle.
Candelas Square: a unit of luminance intensity of 60 candles per square centimeter of a blackbody radiator operating at the melting temperature of platinum.
Exposure Value: a figure expressing camera exposure based upon the equation:
- 2 EV = n2/ +
- When EV = exposure value, n = the relative aperture, and + = the exposure time in seconds, also called Light Value.
In photo graphics all the above may be used however, the most common are Exposure Values and Candelas Square.
Exposure Values are derived by rating the luminance levels in
"photographic stops" as they would apply to film that has a sensitivity rating of ISO 100.
Candelas Square is more accurate and is used more frequently with Digital Cameras.