Using a colourwheel to set colours in photo

Report · Feb 27, 2018

Hello All,

I have some photos that I have taken for scientific observations of fungi, which I now want to correct to light/colour of so I can extract info on colour and size. The camera that I had access to only took JPG images (no raw) but I did include a colour wheel and grey standard in each photo (see attached example). Apologies for non-technical language, but I have not used photoshop a lot before. I have figured out how to adjust to lighting of the photo in GIMP program using the black, white and 18% grey in the GIMP program, but someone suggested that I could use photoshop to, for example, say that the red on the colourwheel equals a certain RGB and the blue another and the green a third, and then photoshop could adjust the image accordingly. However I have had no luck trying to muddle out how to do this myself. Could anyone help?

Thanks, Sam

Report · Feb 27, 2018

Hi Sam

I don't think trying to correct that by adjusting red green and blue using that chart is going to work. For a start you've no idea what the true RGB values on that target are, I imagine.

A click on a "true grey" object would likely help, but I doubt that strip you captured is providing a true spectrally neutral grey, unfortunately. You can test thst using Photoshops grey eyedropper (in levels or curves).

BUT - it's not too late to save your images using a more elegant and advanced method, I hope, what you need is an ICC profile to characterise camera and lighting.

Is there any chance you could photograph a camera profiling chart (I recommend the very capable Munsell colorchecker SG chart, but other, cheaper, alternatives may help too) - shoot that with the same lighting using the same camera?

That way you could use that captured colour chart to make an input (camera) profile using specialist software.

I prefer this one:

basICColor Input - software profiling software for camera and scanner characterisation | colourmanag...

Then you can assign that ICC input profile to your previously captured fungi jpeg images and that should, to a great extent, "fix" the appearance for you..

It goes without saying that the monitor display system you are using to view needs to be properly calibrated and profiled too

A bit more about those here:

http://www.colourmanagement.net/advice/about-icc-colour-profiles/

especially this section

about icc colour profiles, icc profiles explained | colourmanagement.net

I hope this helps

if so, please do mark my reply as "helpful" and if you're OK now, please mark it as "correct" below, so others who have similar issues can see the solution

thanks

neil barstow, colourmanagement

Report · Feb 28, 2018

Thanks for that comprehensive answer. That would be good if I just had one set of light conditions - but the photos were taken over many days with different light conditions on each. However, I am able to use intstruments at the university to obtain true RBG values for the colour wheel. If that helps?

Thanks, Sam

Report · Feb 28, 2018

Hi Sam,

Just to be clear, actually you can't read RGB values from a print. (Or a colour chart for that matter).

The spectrophotometer used to measure prints usually reads out in Lab colour (calculated from a spectral result & a light setting, D50 by default)

Lab values can then be translated to RGB via a colourspace like Adobe RGB. So, the colourspace is an important component in this process.

However, even with RGB values to use in photoshop I fear you'll struggle to attain accurate appearance.

God luck trying though, I hope you manage to get soem of the way.

But that is why photographers doing critical colour work find that accurate camera profiles are vital. Aligning a few colours is normally not enough, thats why a good colour chart has hundreds of patches and why we need profiling software to make the calculations.

clicking on a good spectrally neutral grey card should help also.

if you can shoot a good profiling chart in a representative light it should help.

It is vital, though, to be aware that changes to lighting of an object affect even visual appearance.

It does seem important in your case to attempt uniformity in the lighting.

Hope this is helpful

neil barstow, colourmanagement