Finding the Luminance values of an image ?

[StrongBeaver]

What is the best method to find the luminance values of an image, similar to a vector scope as to find where parts of an image are too bright as to fix ?

[c.pfaffenbichler]

Please explain what you actually mean – what is »vector scope« supposed to mean for example? 

Maybe posting screenshots or sketches would help. 

[Stephen Marsh]

The L channel of Lab mode and the isolated RGB luminosity component are similar, but slightly different monotone representations of a full-colour image.

[D Fosse]

Yes, "luminosity" is an odd creature and nobody really knows how it's defined. Given how similar it is to Lab L, it's really strange that they're not simply using that.

I've seen a weighting formula mentioned here and there, something like 25 R, 60 G and 15 B (or thereabouts).

It's probably done this way for internal visual consistency between color spaces, but I don't really know.

[Stephen Marsh]

The "true" isolated/extracted luminosity component when blended in luminosity mode, is lossless and has no effect on the underlying image. Filling with white/black/gray in color, hue or saturation blend mode removes the colour component leaving the "true" RGB luminosity (not the same as desaturating). This is the luminosity value reported in the grey blend-if sliders (layer options) and on the composite/master RGB curve.

[Stephen Marsh · Accepted solution]

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@StrongBeaver wrote:

What is the best method to find the luminance values of an image, similar to a vector scope as to find where parts of an image are too bright as to fix ?

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When in levels or curves, one can hold down the alt/opt key while moving the white or black hightlight/shadow endpoint sliders for a "threshold" preview of the lightest or darkest tones in the image.

[StrongBeaver]

Right, forgot this option. 

[Earth Oliver]

set your info panel to HSL.

[NB, colourmanagement]

As Stephen writes, activating the Threshold option in Levels or curves is the way to go, it identifies the darkest or lightest pixels with an overlay.

I hope this helps
neil barstow, colourmanagement net :: adobe forum volunteer:: co-author: 'getting colour right'
google me "neil barstow colourmanagement" for lots of free articles on colour management