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
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
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,
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,
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).
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