Conversion Between Radiometric and Photometric Outputs
So now that we defined the radiometric and photometric measures of light, how do we convert from one to the other? In other words, if a light source has a known radiometric output, can we determine its corresponding photometric output? Yes, if we know the spectral properties of the lamp, that is, the radiometric output at each wavelength. Then, the output at each wavelength is multiplied by the sensitivity of the eye to that wavelength as well as by the conversion factor that is given by the definition of the candela (according to which there are 683 lumens per watt for 555-nm light), and the results are summed to obtain the total response of the eye to light from that source.
For example, if a light source has a known output in watts, how much is its output in lumens? If we assume a monochromatic light source, such as a 650-nm (red) laser pointer with a photometric power of 5 mW, the photoptic weighting factor is 0.096, and therefore the corresponding luminous flux is 0.096 × 0.005 W × 683 lm/W = 0.33 lm. If we had a green (532 nm) laser pointer, this value would rise to 0.828 × 0.005 W × 683 lm/W = 2.83 lm.
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.