Radiation and Photometry

This chapter deals with several fundamental properties of electromagnetic radiation. We first define the basic notions of radiometry, namely, the physical quantities associated with the energy transported by electromagnetic radiation. Those photometric quantities and units related to lighting techniques, photography, and so on, are all adapted to the sensitivities of the human eye. This means that they refer to visible wavelengths, and thus are not suitable for astronomical observations, which cover the whole electromagnetic spectrum and must therefore be independent of the properties of the human eye.