The measurement of quantities associated with radiant energy. The quantities may also describe the variation of the energy with respect to other variables such as wavelength, time, position, direction (solid angle), area normal to the light or projected area of emitting or receiving surfaces. If the light is monochromatic, it is sometimes convenient to replace the radiant energy by the corresponding number of photons (or quanta) which is obtained by dividing the energy by h.c/λ where h is the Planck constant, c the velocity of light and λ the wavelength of the light.