Re: luminance
- From: Walter Roberson <roberson@xxxxxxxxxxxx>
- Date: Mon, 13 Oct 2008 15:02:21 -0500
Jessica wrote:
Does anyone know whether it is possible to measure the overall luminance
of an image using Matlab?
Yes, I know that it is *not* possible, not without additional information.
Take a single picture of a star, Use any magnification and exposure you want.
Now, using just that one picture, tell us what the absolute luminance of the
star is.
If you could do that, you could revolutionize astronomy.
When you take a picture, you get out a group of sensor measurements.
However, just knowing the measurement readings doesn't tell you what was being measured. Are the sensors linear or non-linear?
Are they full-spectrum or narrow-frequency spectrum or broad spectrum?
For example, a single pixel lit at intensity 42 (maximum 255) in
an image from a gamma-ray sensor photographing an extra-galactic object
represents *much* *much* more luminance than an entire image lit
at intensity 254 in an far-infrared sensor on long exposure of
a terrestrial object shot close-up.
In order to measure luminance, you have to have calibrated
the sensors, and you have to know the distance to the object
photographed, and you have to know about impediments (e.g.,
galactic dust clouds or terrestrial water clouds) that
are attenuating the reception; and you have to have a good
(and well-founded) model of the relationship between the
inherent luminance of the object and the luminance as measured
on the frequency bands your sensors are sensitive to. Is the
object emitting classical Boltzmann black-body radiation?
(Amswer: probably not really: it probably has emission spectra
and absorption spectra...)
.
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