[APML] Synthetic Luminance Channel for Film Images

[Jerry Lodriguss <jml@astropix.com>]

I am re-posting this at the suggestion of Mike Cook because some film 
people may have ignored the original post because it had the words CCD in 
the title.

In my original post with the subject line "ESO Tarantula CCD Image" I put 
forth the opinion that creating a synthetic luminance image from the 
individual RGB images would not gain any increase in signal to noise ratio 
when composited into an LRGB image.

I did some experiments that seem to prove my opinion wrong.

Creating a synthetic luminance image actually does improve the signal to 
noise ratio in an LRGB composite, and is applicable to film and is 
illustrated with film examples.

The experiments and examples are at:

http://www.astropix.com/HTML/TEST/TEST1.HTM

Some results that may even be of interest (even to the CCD folks) show that 
simply converting the color image to gray scale in Photoshop with Image > 
Mode > Grayscale produces the best synthetic luminance image.

Another very interesting result shows that the two most common methods of 
creating an LRGB image, substituting the luminance channel for the 
Lightness channel in Lab color, and blending the same luminance channel as 
a layer via Luminosity mode, produce different results.

I have the original TIFF file available for download on the page for those 
who would like to repeat the experiments to verify or disprove them, or 
develop other methods.

Can anyone find a better way to average the three individual color channels 
to create a synthetic luminance image with even lower signal to noise?

Also, I still do not understand why this works as it does. If anyone has an 
explanation I would like to hear it.

Gee, this is almost like science. <G>

Jerry



Astronomical photography: http://www.astropix.com

Sports Photography: http://www.astropix.com/SPORTSPIX/INDEX.HTM


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