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Do we need Luminance for imaging? or only RGB?

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#51 mmalik


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Posted 16 February 2013 - 07:04 AM

Do we need Luminance for imaging? or only RGB?

According to an S&T article (Dec. 2012, page 71)...

"You create the color image by aligning and combining these (RGB) color-filtered stacks into a master RGB image called chrominance.

Although the chrominance image provides the beautiful color, it's the luminance image (shot without color filters) that can contribute some of the sharpest detail."

#52 Inverted


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Posted 16 February 2013 - 08:40 AM

In photoshop you can easily control the ratios of R, G, B, yellow, cyan and magenta.

For sure, but then you have "false color". As mentioned we can think of an image in 2 parts, luminance and chrominance. An nebula or galaxy etc.. has a certain ratio of color "chrominance". And we then perceive that color based on the relative intensities and sensitivity of our eyes to that color this is "luminance.

So, by definition, an image should have a certain amount of luminance which by definition should match "chrominance" in a specific ratio. In our hobby we modify luminance (and "chromonance") to enhance visualization of features and detail in the image. So, I think we are really generally always working with "false color" images. That is why I don't think there is a strong argument to using RGB to begin with and extracting the luminance (unless you just prefer processing that way, or like the results better).. Unless we only do a linear stretch of the data, equally on all channels (once white balanced), and maintain the luminance in a 3:59:11 ratio, then we really do have false color.

So, there are two benefits to using L i think. If we collect a separate luminance image, this allows us to increase the SNR of the image, as no light is being blocked by the filters for the L. And two, we can work on a luminance image, which is essentially adjusted to remove the limitations of our vision. with respect to sensitivity to and perception of certain colors.

So, yeah, we do sort of end up with a false color image, but I seems that the way we proccess images in the hobby, we do regardless. Also, as mentioned, we are again only really increasing the SNR of the L, not the color, but do we care if it looks better? The L is where the detail is, and the RGB is distorted either ways.... So, to me, I'm perfectly happy distorting it a bit more to sample the detail a bit better.

#53 neptun2


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Posted 16 February 2013 - 10:30 AM

Yes i completely agree that luminance should be used. I wonder if we will get any improvement if we extract another luminance from the combined R + G + B channels with 1:1:1 ratio and stack it with the normal luminance creating one let's say "super luminance". After this we can normally process the "super luminance" and RGB frames as ususal. I think that the added "color" luminance should create higher SNR combined "super" luminance. Of course this will be valid only if you take L , R , G and B frames with equal rsolution and duration and not binned the color.

#54 bill w

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Posted 16 February 2013 - 12:04 PM

.... We know that, with the exception of some planetary nebulae, there is no green object in the deep-sky. ....

Take a look at Hanny's Voorwerp.

dan k.

They mentioned O-III in the paragraphs I quoted "Oxygen III emission corresponds to a mix of blue and green. "

if memory serves, the voor werp is "red shifted" towards pure green

clearly an exception to the rule

#55 blueman


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Posted 16 February 2013 - 01:49 PM

Here is an RGB image, using the RGB to make luminace in CCDStack using my own process.

I can not compare it to LRGB, but I have done that with other images and I can not tell the difference.

The synthetic luminance is from 63 10 minute RGB images.



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#56 blueman


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Posted 16 February 2013 - 01:52 PM

Here is the Synthetic Luminance, small size due to image size constaints.

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#57 kfir Simon

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Posted 17 February 2013 - 05:24 AM

Thank you all for such great inputs.
As I understand, it isn't as "Black and White" in favor of LRGB over RGB as I suspected in the beginning.

Keep on the good ideas!


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