Transcript: The way astronomers observe and calibrate the apparent brightness of something is through the technique of photometry. Photometry allows astronomers to measure the number of photons per second coming from an astronomical source in some specified wavelength range or pass band that’s defined by a filter. A filter is simply a colored piece of glass sitting above the CCD detector in a telescope that isolates a narrow range of wavelength. CCD detectors are sensitive to a wide range of wavelengths, and so the pass band must be specified by a separate optical element. Most photometry is done at optical and near infrared wavebands, so some of these wavebands are beyond the sensitivity of the human eye. The traditional wavebands are named after letters that do not follow the alphabet, they exist for historical reasons. In the optical bands we have U, B, V, R, and I at 350, 450, 550, 700, and 850 nanometers, and in the near-infrared the J, H, and K pass bands at 1.25 microns, 1.65 microns, and 2.2 microns. Relative brightness in absolute units is then determined by measuring bright stars where the absolute brightness has been measured by spacecraft.