Image: NGC6744, a galaxy much like the Milky Way, 30 million light-years away, in an RGB image color balanced to look like it does visually.Many years ago I used to wonder what it would be like to see with eyes that perceived color differently. Of course, sight is more than a mechanical phenomenon. The human (primate) retina is a carpet of well-mixed "red" and "green" receptors that overlap greatly in their wavelength sensitivity, and a scattering of less than ten percent "blue" receptors that have very little overlap with either of the others (though "red" has a secondary peak at blue wavelengths). The visual center in the brain boosts the blue signal to be on a par with the other two, while constantly adjusting so in most lighting situations the incident light seems "white".
We see with other eyes when we make a color photograph, because the filters in the film or digital sensor strictly separate red from green from blue, and print or display narrow-band signals that stimulate the color receptors in our eyes separately. Once I understood this principle, I daydreamed about using different films and filters to make separate "R" and "G" and "B" negatives, from which I could build a color print the way very early color photography was done. When I had access to a darkroom, I did a few experiments, but with poor results; I have poor laboratory skills. Special Infrared sensitive film would produce a false color image that was shifted a little bit to longer wavelengths. That was at least usable by klutzy old me, but that was about it.
It is easier now. False color rendition of all sorts of images is common, given digital tools for producing and compositing images. This is particularly true of astronomical imaging. This view of NGC6744 is a far-infrared image with false color, wherein the wavelengths captured range from 3 to 22 microns, and are printed as if they were in the visible range of 0.4 to 0.7 microns. It was made with the WISE spacecraft, which operated from December 2009 until February this year.At first glimpse, the galaxy looks very similar, just more "technicolor". The shape is the same, and the bright areas are similar. The most noticeable feature is the bluish-appearing central bulge; the bar seen in the RGB image does not show. Then there are five major reddish areas, and a number of smaller ones, that mark sites of active star formation. The flanking areas that are green in the IR image are blue in the RGB image, and would be bright in an ultraviolet view. They mark young stars that have blown away the dust and gas that shrouded their infancy.
My first daydreams about false color imaging were just centered on the coolness of it, an interest without a focus. Others with more practical needs made it a reality. The fields in which it is used are many, from LANDSAT earth images to the thermal imaging an "energy consultant" will use to show you how your house is wasting energy. A pair of images like those above is still thrilling to me for the sheer coolness of it.
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