draw graph of colour values across an image

[CVHManchester]

I am trying to assess and improve light distribution across the flat objects I am photographing.

Although one can get colour values at points as the cursor moves, it would be useful to create a contour map of the light distribution. Alternatively, a graph of colour values along a chosen line across an image would provide similar information.

Is there a tool that can enable that?

The next stage will be to capture that distribution using a white sheet and correct the lighting of the consequent images.

Again, is there a tool to correct light unevenness?

Thank you.

[c.pfaffenbichler]

Is this about luminance only or about color casts, too? 

Ultimately I don’t understand what good the information you want to acquire would do – could you elaborate or post sketches? 

[CVHManchester]

Thank you

As part of an archive project, I am photographing large C19th prints, which need to be evenly illuminated.

I am using an OM-D E-M1ii camera.

The first stage is to check and ensure that the lighting is as even as the resources available allow, so the initial photograph is good.

The next stage is to deal with any remaining lighting uneveness in post-processing.

The primary concern is illuminance.

However, I am having difficulty (?lack of understanding) in adjusting the levels using Photoshop.  In the photos, I include a calibrated colour swatch, including white/grey/black patches.  When I adjust the image using the white eyedropper on the white patch to achieve RGB 225,225,225, the reported levels of grey patch don't match the calibrated patch, and the green is way out.  I think there might be a metamerism problem, compounded by the properties of the camera white balance setting. The lights I have used are: photographic CFL bulbs (reasonable), tungsten filament GLS bulbs (slightly better), daylight colour matching fluorescent tubes (appalling). In order to increase evenness, I was trying the linear tubes, rather than point sources.

I was hoping that understanding the colour problem might help address the illuminance/ level evenness.

Thank you

[c.pfaffenbichler]

I am not a photographer so this is not my field of expertise but have you considered using Colorchecker Passport? (Adobe itself doesn’t offer a comparable product, as far as I know, so I hope me posting a link is OK.) 

https://www.xrite.com/categories/calibration-profiling/colorchecker-classic-family/colorchecker-pass...

As for the luminance variance:

Wouldn’t it make sense to photograph a neutral sheet (neutral in hue and definitively neither white nor black) and use that to either create Layer Masks for Adjustment Layers or use (blurred) instances of itself with appropriate Blend Modes etc. to level out the variations? 

That would probably take some experimentation and time, though. 

[Semaphoric]

There used to be a plug in that would trace the contour along a path, However, the interface looked like development had frozen ca.1990, and the demo only worked with a .PSD that they provided for evaluation. Did it work with mine? Who knows; I passed.

I did this to get a "contour map" of the Luminance. I double clicked on the RGB Composite channel to load luminance as a selection, made a new alpha channel, and filled the selection with black to get a luminance channel. I then blurred this slightly, and Image > Adjustments > Posterize, with 6 steps, followed by Filter > Stylize > Find Edges.

I loaded this channel as a selection, and then pasted black into a new layer (background is ghosted out for clarity):