Isophote -- from the Greek "isos" (equal) and "phos" or "phaos" (light) -- is a scientific term meaning literally "points of equal light" or "equal brightness". Isophotes are contours of constant brightness on an image. Much like the contours on a topographic map try to show the elevation of a land form, isophotes allow us to quantify where the light in an object comes from, and how much we observe.

It is, as far as I can tell, a relatively recent addition to the English language, perhaps originating in German scientific circles and then migrating into English. It arose from the relatively new science of using images as data for scientific research, primarily in astronomy from the quantitative analysis of photographic plates, and in medicine, from the analysis of X-ray photographs. Given that photographic and X-ray technology date from the mid- to late-nineteenth century, the term is probably not much younger than that.

In astronomy, the isophote is commonly used to define two things: the shape of an object, and the amount of light it gives off. The first, determining the shape of an object is not as trivial as it sounds. Very few objects in the universe are solids with well-defined boundaries, like planets. Everything from stars to galaxies to giant clusters of galaxies are mostly very tenuous objects. Stars often have a giant corona -- a faint halo of hot gas -- surrounding them, like our own Sun. Galaxies are made up of billions of stars and clouds of gas and dust; all of this matter is spread out over a huge volume of space so that there is no well-defined "boundary" of the galaxy. And even clusters of galaxies are made up of a mixture of galaxies and gas; the gas is invisible in optical light but incredibly bright in x-rays, and it doesn't have a well-defined boundary, either. Isophotes let astronomers say "where can we reasonably place a boundary around an object, based upon where we can detect light and attribute that light to the object we're interested in?"

The second point -- quantifying the amount of light received -- was not so trivial either. In the early days of photographic astronomy, astronomers had to make quantitative measurements of images using plate densitometers, which use a microscope to count the number of exposed grains on a photographic negative (an incredibly tedious task as I'm sure you can imagine). Later, the photomultiplier tube allowed astronomers to actually count photons as they arrived at the telescope. These tubes were mainly used to "photometer" stars (which are point sources, and don't need to be "imaged" per se), but they could also be used to make rough measurements of more extended objects. Later still, Reticon and CCD cameras allowed astronomers to take pictures and count the photons at the same time. Thus generating isophotes from images became just a matter of a few keystrokes on a computer.

The most common use of isophotes in astronomy is in the imaging and classification of galaxies, particularly of elliptical galaxies. The isophotes of elliptical galaxies provide information on a galaxy's shape, and hence upon its structure and dynamical behavior. Isophotes can be used on spiral galaxies, too, particularly to measure their radii, or to map the structures within their spiral arms. Isophotes are also used to measure the size, structure, and brightness of many gaseous or tenuous objects, such as X-ray galaxy clusters, radio jets from quasars, and the distribution of dust in our Galaxy. They have even been used to map the light reflected from the Moon and other planets to understand the properties of their surfaces (before we actually went there and studied them in person).

Greek help from http://www.perseus.tufts.edu/cgi-bin/resolveform
Astronomy help from http://adsabs.harvard.edu/abstract_service.html

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