Originally posted by ckhaos
While answering another thead on the colour temperature of halogen lamps, I thought about this question that I have no answer to:
How do you define a colour temperature for a light source that is not a blackbody, e.g., a street lamp or a tungsten bulb?
Or, don't tell me, street lamps and tungsten bulbs are blackbodies, because I remember from O level physics that they are not.
This is a really complex issue to attempt to explain without the use of a fair number of graphics, however here goes.
Photographs are generally taken where the light travels from the subject to the camera, that is the light is reflected light from the subjects surface. The sources of the light can be either natural such as the sun, sky and clouds or man made in the form of flash lighting, and other light sources.
Artifical lightsources are classified according to the method used to produce the light, that is by burning, heating, electric spark/arc or electric discharge.
Examples:
Burning: - candles, fires, oil lamps, flash powder and flash bulbs.
Heating: - Carbon and tungsten filament lights, eg household lights, studio lamps and tungsten-halogen lamps.
Electric Spark/Arc: - Carbon Arcs, spark gaps
Electric Discharge: Electronic flash, metal halide lamps, sodium and mercury vapour lamps.
Spectral Quality
The radiation from most sources comprises a mixture of light of various wavelengths. The
Spectral Quality or
Hue of the light from a source may vary considerably depending on the distribution of energy at each wavelength in the spectrum. Most of the sources of light used in photography give what is known as
white light. This is light that to the human eye appears to be white or close to white, it is not visually deficient in any particular band of wavelengths while not implying any definate colour quality. Most white-light sources vary considerably amongst themselves and from daylight. Howvever because of the perceptual phenominon of
colour constancy these differences matter little in everyday life, but they are very important in photography, especially when using colour emulsions and materials. Thus is it is desirable to describe the light source in precise terms and one of the most important for the photographer is the exact hue of the light. Since visible light is a particular region of the electromagnetic spectrum and a form of radient energy the colour quality may be described in terms of
Spectral Power Distribution (SPD) throughout the spectrum of visible light.
Spectral Power Distribution Curves
This is the sticking point area, without a several curve plots it's difficult to explain how little the actual variance is between many lightsources yet the difference in hue can be large. However after analysis of curves the scientists etc can classifiy the curve in to one of three basic categories, these being:
Continiuous Spectra Energy is present at all wavelengths in the region measured.
examples: All incandescent-filament lamps.
Discontinuous Spectra (Line Spectrum) Energy is confined to one or a few narrow bands, at these wavelengths the energy levels are high and at all other wavelengths the energy level is near almost nil.
examples: typical low pressure mercury and sodium vapour lamps.
The final type (name forgotten ARGH) comprises broad bands of energy across the spectrum accompanied by a continuous background spectrum of varying energy levels.
examples: Typical high pressure mercury and sodium vapour pressure lamps, some types of flourescent lamps, metal halide, xenon and argon vapour lamps.
COLOUR TEMPERATURE
For the purpose of photography a more common method of describing the light quality of an incandescent light source is by means of its
colour temperature. This is defined in terms of a Planckian Radiator, otherwise known as a Full Radiator or a black body. This is a light source that emits radiation in which the SPD depends only on its temperature and not on the material or nature of its source.
The colour temperature of the light source is the temperature of a full radiator that would emit radiation of substantially the same spectral distrubution in the visible region as the radiation from the light source. Colour temperatures are measured on the thermodynamic scale (ie: Deg Kelvin) with a zero reference of -273.15 deg C.
A simple way of appreciating the idea of colour temperature is to imagine the progressive change in colour of the light emitted from a piece of metal as its temperature is raised, going from dull black through red, orange and on to white heat. The quality of the light emitted is thus a function of the temperature of the metal. While its true that in theory the concept of colour temperature can only be applied to sources that are full radiators, in practice it is extended to include sources that have an SPD that is similar to a full radiator.
I could go on for a lot longer .. but by now I'm sure most will have got the general drift.
