Photometry is the modification of the scheme described above so as to reflect the variable sensitivity of human vision to light of different wavelengths, in contrast to the radiometric measurements, which weight equally the energy transmitted at every wavelength of the electromagnetic spectrum.
This adjustment is based on empirical measurements of the relative sensitivity of human vision to light of the various wavelengths. This weighting curve is referred to as the spectral luminous efficiency function, usually denoted by V(λ) or Vλ. The photometric measurement of the radiant energy output or input in a system is the summation (or, better, integral) of the energy transfer at each wavelength multiplied by the value of Vλ for that wavelength.
Unfortunately, for historical reasons, unlike the case for radiometric measurements, the derived units for each geometric variant of this process for photometric measurements have been given different names, further complicating these discussions.
The photometric equivalent of the measurement of the total radiant output of a point source is the lumen (lm), the Vλ-weighted version of the watt. This quantity is referred to as the luminous flux. For example, the output of an ordinary incandescent or light-emitting diode (LED) light bulb (which can be regarded for most purposes as a point source) is now often indicated in lumens. A 60-W incandescent bulb might typically have a light output of 800 lumens. (In contrast, an 800-lumen LED bulb might consume only 12 W of power.)
The photometric equivalent of the measurement of the radiant output of a point source per unit of solid angle is the candela (cd), or lm/sr. This quantity is referred to as the luminous intensity of the point source. For historical reasons, the candela is the primary SI (metric system) unit for the measurement of light, intended to approximate the light output of the standard wax candles which were originally used for this purpose. One cd is defined as the luminous intensity of a source emitting monochromatic radiation at a frequency of 540 THz and a radiant intensity of 1/683 W/sr. Note that 540 THz corresponds to approximately 555 nm (= c × 109 / f = 3 × 108 × 109 / 540 × 1012), the wavelength to which the human eye is most sensitive under light-adapted or photopic lighting conditions, and that 1/683 was chosen to make 1 cd equal to the old unit, the “candle,” which was based on the actual luminous intensity of a wax candle flame.
The photometric equivalent of the measurement of the radiant output of an extended planar source is the lumen/m2. This quantity is referred to as the luminous exitance.
Conversely, the photometric equivalent of the measurement of the radiant energy incident on an extended planar surface is also measured in lumen/m2. This quantity is known as the illuminance. As a unit for illuminance, the lm/m2 is known as the lux.
The spectral luminous efficiency function depends in practice on the state of light-adaptation or dark-adaptation of the eye. Most measurements are made under light-adapted conditions, and use the light-adapted (or photopic) efficiency function. Under dark-adapted (scotopic) conditions, the efficiency function shifts toward shorter wavelengths (the Purkinje shift), reflecting the difference between the absorption spectrum of rhodopsin (the visual pigment in retinal rods) and the summed absorption spectra of the 3 visual pigments in the retinal cones. The scotopic spectral luminance efficiency function is often referred to as V′(λ) or V′λ. It has a maximum at 507 nm, whereas the photopic efficiency function has a maximum at 555 nm. In precise work, the choice of luminance efficiency function should be specified.
Further note that the terms radiance or luminance are not used simply as the counterparts for emitting surfaces of the terms irradiance or illuminance for absorbing surfaces. Rather, radiance refers to the radiant intensity per unit area, and thus is expressed in units of W/sr·m2. The corresponding photometric quantity, luminance, refers to the luminous intensity per unit area, and is thus expressed in units of cd/m2 = lm/sr·m2. This last unit is also referred to as the nit. Table 2-3 lists other alternative units commonly used to measure illuminance and luminance. To help you develop a “feel” for photometric units, Table 2-4 lists photometric values associated with visual functions.
Table 2-3 Various Photometric Units and Their Conversion Factors
Table 2-4 Luminance Levels Associated With Various Visual Functions
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.