**2. Optical radiation quantities**

To understand the measurement of several basic optical radiation quantities used to determine absolute amounts of optical radiation, it is useful to study the two aspects of quantitative optical radiation measurements: the geometrical optical radiation that we wish to measure and the spectral components of this special geometrical compound of radiation [1]. Our discussion concentrates on electromagnetic radiation with wavelengths from approximately 100 to 2500 nm, which is an extension from the visible wavelength range, which is approximately from 380 to 760 nm.
