Abstract: We develop a general optical model and describe the absolute radiometric calibration of the readings provided by two widely-used night sky brightness sensors based on irradiance-to-frequency conversion. The calibration involves the precise determination of the overall spectral sensitivity of the devices and also the constant G relating the output frequency of the light-to-frequency converter chip to the actual band-weighted and field-of-view averaged spectral radiance incident on the detector (brightness). From these parameters, we show how to define a rigorous astronomical absolute photometric system in which the sensor measurements can be reported in units of magnitudes per square arcsecond with precise physical meaning.

Abstract: Digital photography is based on the conversion of light by sensitive matrix (array of electronic photodetectors) to capture the image which is then digitized and stored as a computer file for further processing and printing. The spectral sensitivity of the cameras is in good agreement with the spectrum of action of the photosensitive hormone melatonin. Digital photography can be used to quantify light pollution acting on the physiology of living organisms. The chapter discusses the principles of spectrophotometry. A portable spectrophotometer for the measurement of light pollution is proposed by Cinzano. It consists of a cooled CCD camera and a small spectrographic head which is equipped with a De Amici prism composed of two external crown prisms and an inner Flint prism. Sky quality meter (SQM) is a portable photometer for measuring sky brightness and for light pollution monitoring. This device collects the light from a wide solid angle.