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Light and the Human Eye > Measurement of Light - Pg. 32

20 Light and the Human Eye vision, and we will see later that the basic principles of color television are quite similar to the processes of human trichromatic vision. 2.6 Measurement of Light Light is a form of energy, and consequently we could measure it by using dif- ferent spectroscopic methods, but the results of such measurements would be of no importance in the domain of television shooting. The photometric mea- surement methods used in film and television are essentially based on different comparisons--on comparing the situation in front of the camera with some pre- determined reference values. In that respect, two values are generally routinely measured--the color temperature and the intensity of the incoming light. We have seen already how color temperature is defined and measured. The other operational parameter, light intensity, is also measured by comparing the light on the scene to an arbitrarily selected reference light. One of the first reference values used for a comparative measurement of light intensity was candlepower, the light of one candle whose composition was precisely defined. As a much more precise and reproducible reference, we use today the 1/60-part of the light intensity radiated by an ideal black body (see part on color temperature) heated to the melting temperature of platinum. That reference value is still called "one candle." Theoretically an ideal punctual source, that is a theoretical source shaped as a dot having no physical dimensions, radiates equally in all directions. The amount