In astronomy, the term photometry is used for the study of the brightness (magnitude) of stars and other bodies, optionally using bandpass filters to measure magnitudes over specific spectral ranges, e.g., to determine color indices. The results of such observation may be called the star's photometry.
This is as opposed to a more complete characterization of the body's light via spectroscopy, which requires a more specialized instrument. Photometry has the advantage of requiring less light (apparent magnitude), thus can be done at a greater distance and can be used with many more stars. It also is naturally "multi-object", also making it much more efficient to survey many stars.
The passbands used have been standardized (photometric systems) to allow comparison and general use of photometric data.
Filters with different passband widths are used, according to need and light-availability, yielding some levels of spectral detail, but short of using full spectroscopy. For example, direct imaging of extra-solar planets offers limited light, making photometry more practical than spectroscopy. Some subtypes of photometry:
As photometry developed in the 20th century, electronic sensors were developed that are capable of capturing and counting a significant percentage of photons (quantum efficiency), first the photomultiplier tube (PMT) (a type of vacuum tube), and more recently, photosensitive solid state devices, often incorporated in CCDs. These practices are termed photoelectric photometry (PEP) and CCD photometry, but the former term is sometimes used to mean both.
Carrying out consistent, reproducible photometric measurements is a challenge, particularly from the ground, given the effects of Earth atmosphere (airmass). (Variable star measurements are in some sense not reproducible given constant variation, except that two different observers observing at the same time would aim to produce the same measurements. The methods to accomplish such consistency and accuracy are non-trivial. The aims of the techniques fall under two classes: attempts to measure on some absolute scale, and attempts to measure the relative magnitudes of a target star compared one or more others. The terminology used for these is not universally consistent.
Absolute photometry is clearly an attempt to get a photometric measurement on an absolute scale. Some writers reserve the term for direct comparison with lab sources of electromagnetic radiation, but others also include indirect comparison, comparing the target star with some star whose magnitude is very well established (a standard star aka standard source aka photometric standard star, whose magnitude may have been determined through direct comparison as above). The term relative photometry is sometimes use to distinguish the two above methods, to refer to the latter.
Differential photometry is comparing the photometry of a target star with one or more other stars. Comparisons of a variable star with a star with very little variation that is in the same field, i.e., in the same portion of the celestial sphere, can be of more use than attempts to translate the variable star's magnitude to an absolute scale: the accuracy of one comparison can be better than the resulting accuracy of a chain of comparisons. The term relative photometry is sometimes used to mean such differential photometry.