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In the discussion of display luminance, the most fundamental of all units is the elemental quantity known as the lumen, and abbreviated as lm. Knowledge of the basic meaning and sense of this term is helpful in understanding the many metrics discussed in this text, for they are based on it.
The first thing to understand about the lumen is that it is nothing more than a measure of luminous power, but it is received power, tempered according to the light sensitivity of the human eye/brain system. McCluney speaks of the eye/brain perception of brightness as a psychophysical activity, something more than a simple matter of counting photons. One definition of the lumen is that it is the amount of light radiating from a 1/60-cm volume of platinum heated at 2500°C. A more useful definition of the lumen, however, is that it is 1.47×10−3 W of visible light at a wavelength of 555 nm. With watts being power (joules/second), this tells us that the lumen is a measure of light power, but light power normalized to a wavelength of peak sensitivity to the (average) human eye. Think of a light bulb that is rated both in watts and lumens.Watts informs us of the total energy available to the bulb (feed current and applied voltage, i.e., volts times amperes), some of which produces light in the nonvisible region (for example, the infrared, which produces heat) and some of which produces light in the visible region. Even in the visible region, however, the light is not one but multiple wavelengths, starting at approximately 430 nm and extending to approximately 690 nm. Now, if the human eye/brain system responded to this light according to the inherent power of what was transmitted, there would be nothing else to qualify; but that is not the case. Not only is the human eye insensitive to wavelengths outside this region, its sensitivity to wavelengths within the visible region varies according to the curve. The lumen, therefore, is not a quantity indicating only power, but power as attenuated by the human eye/brain system.
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