PHOTOMETRY

Photometry is the measurement of the intensity of electromagnetic radiation emitted by an object. In astronomy, the concept of magnitude is used to describe the brightness of a celestial body. This is a quantity logarithmically related to the luminosity of the objects. If an object is 100 times more luminous than another object, they differ by only five magnitudes.

Apparent magnitude (m) is the magnitude of an object as viewed from Earth. If two identical objects are at different distances, their apparent magnitudes will be different. Absolute magnitude (M) is the magnitude of the object as viewed from 10 parsec (32.6 ly). The relation between the apparent and absolute magnitude is M=m - 5 log (d/10), where d is the distance of the object from Earth in parsec.

Brighter objects have magnitudes expressed by smaller numbers. Thus, the brightest objects have negative magnitude, and the fainter the object, the greater the magnitude.

Stars emit in all frequencies and their luminosity in the different frequencies is different. Photometric devices are calibrated to measure the brightness in specific frequencies (red, green, blue, x-ray, infrared, ultraviolet, radio, etc.).