1 yr. ago • 98%

Is there an easy way to generate a list of CMYK color values that will appear identical to the human eye under 589nm light?

I picked up a low pressure sodium lamp and am working on a Halloween demonstration. I’m hoping to make a display that appears one way under normal light, but looks totally different under the monochromatic 589nm sodium vapor light.

So basically, I’m looking to generate a color wheel where I pick a shade of gray and get a list of colors that would look that gray under sodium vapor light.

…I feel like there must be a Python library for thing or something…

16 comments

Hmmmmmmmmm.... From a high level perspective you need to know the reflectivity of your combined pigments at that wavelength. If it's the same, they will look the same.

I don't know of anything easy you can use, but would suggest trying to find reflectance curves for each pigment you have available and making combinations that subtract to the same value at 589nm, or since 589 should be basically yellow, make up some colors where Y is constant and you change the ratio of C to R and try them out?

15
- That's true! Using RGB alone will not be enough to calculate this! Two materials that might appear equally yellow under white sunlight may appear different shades of yellow under sodium light. Technology Connections did a great video about the difference: https://piped.video/watch?v=uYbdx4I7STg
  
  edit: he starts talking about sodium light in particular at 11:14
  
  3
Low pressure sodium lamps have a pretty sharp spectrum: https://en.wikipedia.org/wiki/Sodium-vapor_lamp

Looking at the color spectrum, have you just tried and colors in the green to blue to purple range? I don’t think you need a Python library for this, I think you need to experiment. There’s a lot of dependence on the reflectivity of the material you’re looking at in addition to the color you see under sunlight or even indoor light with broad spectrum.

Try blue and green and see if both look the same under the lamp.

11
- I was hoping I could avoid experimenting. CMYK light responses should be well controlled/documented, no?
  
  3
  
  Thinking about this more , you probably want this to develop a curve in your color space that represents something with constant CMYK values for your chosen light source.
  
  https://python-colormath.readthedocs.io/en/latest/conversions.html
  
  E.g. your sodium light is 100% yellow, 10ish % magenta. Any color that varies cyan from 0%-100% and black from 0%-100% should presumably not reflect any additional color information (since the source light doesn’t have any cyan and black is just giving brightness)
  
  I also think this means that as long as you hold Y and M constant, you can vary cyan and black for your comparison colors that will look the same. If you try to vary cyan and yellow or magenta at the same time then your effect probably won’t work.
  
  This is tricky because you have multiple curves in the color space that are valid when just considering a single wavelength. The reality is, your lamp emits a spectrum of light (sharp, but still has a width). There’s also the variability in perception. But I’m not sure what the “bandwidth” of our eyes is and what color resolution humans are capable of detecting.
  
  5
So I'm really not sure how this works anymore but if you figure out which frequencies alias you can use https://academo.org/demos/wavelength-to-colour-relationship/ to do a conversion.

8
May be useful:

6
- Only realised when I went to reply. Using sync your video link doesn't show. Here it is for other sync users.
  
  https://youtu.be/fv-wlo8yVhk?si=cj3aaD8Ufm0Hlt6M
  
  11
  
  May be useful: ![Video about colors](https://youtu.be/fv-wlo8yVhk?si=cj3aaD8Ufm0Hlt6M)
  
  So that's why, Sync tries to load this as an image, but there's no image link..
  
  But apparently the link still works and is invisible in Sync.
  
  6
Different printers have CMYK primaries with different spectra, so there’s not going to be a generic solution. But in principle, CMY can only create a linear combination of three discrete frequency bands, not a continuous spectrum.

The same will be true of the appearance under monochromatic light: you can only make colors that blend the monochromatic appearance of the primaries. So if none of the three primaries has the desired effect, you can’t create the effect by mixing them.

6
Is there an equation for this? Like y = f(x) where y is your choice of gray and x is your color. Maybe you can empirically find “f” by fitting randomly created “x” with the resulting “y”.

If “f” can be approximated and maybe there’s something special about it so that you can find the inverse. Otherwise, you could always just generate a bunch of “x*” again, feed through “f”, and see whether the output “y*” matches your chosen gray.

2
Would it count as easy to make the colors in a photo editing tool (picking things of the same value and saturation but different hues) and then converting to the CMYK designation?

2
Here's the Wikipedia section one general colour space, with a pretty diagram of chromaticity, and the one on CMYK colour conversion. What you want is the preimage of a CMYK colour projected into the entire perceptual space.

CMYK actually sounds kind of complicated to do this with, so yeah look for a pre-made function to convert CMYK to CIE 1931 in whatever "normal" light you have. I can help you find the preimage from that once you do.

Edit: Oh wait, this was a Halloween thing. Maybe for next year?

1
Permanently Deleted

-9
- This is "ask science", not "ask hallucinating charlatans"
  
  11
  
  Permanently Deleted
  
  -2

You've viewed 16 comments.