http://imagej.273.s1.nabble.com/Grayscale-values-for-RGB-images-tp3700260p3700264.html
in the book "Digital Imageprocessing" ("Digitale Bildverarbeitung") by Wilhelm Burger and Mark James Burge I read:
"Because the subjective birghtness of red and green are much higher than that of blue the result would be too dark in the regions with a high green and red ratio if you would just use the formula (R+G+B)/3.
This is why usually a weighted sum, the so called luminosity (freely translated from the German word "Luminanz") is used.
In most cases (also in ImageJ) the weights used in coding the analog TV-color-signals are used."
I also don't know how these weights were calculated, but I hope I could lead you into the right direction.
> Dear Karsten, Tony and Wayne,
>
> Thanks very much for your replies. Thanks also to Wayne for adding more
> information to the documentation.
>
> I'm a bit embarrassed to ask but where does the ratio for the RGB
> conversion come from (V=0.30R+0.59G+0.11B)?
>
> I know that with a Bayer mask, you get twice as many "green" pixels as
> blue and red but I'm not sure how this weighting of V=0.30R+0.59G+0.11B
> has been arrived at. I did have a bit of a look around but couldn't find
> the answer.
>
> My images are FITC fluorescence images (mainly green) which are captured
> using an RGB camera. There is some data also in the red channel because
> of the spectrum of FITC and the use of a longpass barrier filter. It is
> a series of treated and untreated samples so they will be relative to
> one another.
>
> Therefore, is it appropriate to convert them to grayscale using the
> V=(R+G+B)/ 3 giving each channel equal weight? This is what I would
> normally do except that now I don't actually have to include this step
> since the Measure command does it anyway.
>
> Or should I be using RGB weighting? If so, how should this be determined
> experimentally for the camera?
>
> If someone can clarify when weighting should be applied, that would be
> helpful.
>
> Cheers,
>
> Jacqui
>
> Jacqueline Ross
> Biomedical Imaging Research Unit
> School of Medical Sciences
> Faculty of Medical & Health Sciences
> The University of Auckland
> Private Bag 92019
> Auckland, NEW ZEALAND
>
> Tel: 64 9 373 7599 Ext 87438
> Fax: 64 9 373 7484
>
>
http://www.health.auckland.ac.nz/biru/
>
> -----Original Message-----
> From: Rasband Wayne [mailto:
[hidden email]]
> Sent: 15 February 2007 06:09
> To: Jacqui Ross
> Subject: Re: "Grayscale" values for RGB images
>
> > I think I must have missed an update somewhere. Usually, if I want to
> > measure intensity of RGB images, I have to change them to grayscale
> > first (8bit) but it seems that I don't need to include this extra step
>
> > now. I seem to get the same results whether I change my green image to
>
> > grey or not. Is this correct?
>
> Jacque,
>
> I added this paragraph to the Measure command documentation to make it
> clearer how it handles RGB images.
>
> "With RGB images, results are calculated using brightness values. RGB
> pixels are converted to brightness values using the formula V=(R+G+B)/
> 3, or V=0.30R+0.59G+0.11B if "Weighted RGB Conversions" is checked in
> Edit>Option>Conversions. The three weighting factors can be changed
> using the setRGBWeights() macro function."
>
> -wayne