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Finding the Luminance values of an image ?

Forum|Forum|3 years ago
June 23, 2022
6 replies
2592 views

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 ?

Windows

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Correct answer Stephen Marsh

@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 ?

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.

NB, colourmanagement

Community Expert

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

Earth Oliver

Legend

set your info panel to HSL.

Stephen Marsh

Correct answer

Community Expert

@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 ?

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.

StrongBeaverAuthor

Legend

Right, forgot this option.

D Fosse

Community Expert

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

Community Expert

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

Community Expert

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

c.pfaffenbichler

Community Expert

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

Maybe posting screenshots or sketches would help.

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