Optical power is a function of both the number of photons and the wavelength. All light measurement units are spectral, spatial, or temporal distributions of optical energy. As you can see in figure 2.1, short wavelength ultraviolet light has much more energy per photon than either visible or long wavelength infrared. Since silicon photodiodes are more sensitive to light at the red end of the spectrum than to light at the blue end, radiometric detectors filter the incoming light to even out the responsivity, producing a flat response . This is important for accurate radiometric measurements, because the spectrum of a light source may be unknown, or may be dependent on operating conditions such as input voltage.
Most sources are continuums, emitting over a broad band of the spectrum.
Incandescent lamps are a good example. The color temperature and output of these lamps vary significantly with input voltage. Flat response detectors measure only output power in watts, taking into consideration light at every wavelength. Another approach is to use a narrow band filter to measure only within a small wavelength band. This is acceptable if the lamp has been fully characterized and the color temperature is carefully monitored.