ABSTRACT
The measurement of the intensity of electromagnetic energy coming to us from an astronomical object has traditionally been called ‘photometry’. It is harder to separate imaging from photometry, especially with array detectors, because much of photometry is now just imaging through an appropriate filter or filters. The system used by astronomers to measure the visible brightness of stars is an ancient one and for that reason is an awkward one. Numerous filter systems and filter-detector combinations have been devised. The relationship between UBV magnitudes and absolute physical quantities has been determined using laboratory sources such as platinum furnaces. Measurements in one photometric system can sometimes be converted to another system. The usual purpose of making measurements of stars in the various photometric systems is to determine some aspect of the star’s spectral behaviour by a simpler and more rapid method than that of actually obtaining the spectrum.
