# finding equiluminance for a pair of colors **URL:** \ **Category:** Uncategorized\ **Created:** [April 11, 2007, 10:46pm UTC](https://psychtoolbox.discourse.group/t/finding-equiluminance-for-a-pair-of-colors/807 "2007-04-11T22:46:35Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![xeos](https://avatars.discourse-cdn.com/v4/letter/x/ecae2f/32.png) [@xeos](https://psychtoolbox.discourse.group/u/xeos)\ **Post date:** [April 11, 2007, 10:46pm UTC](https://psychtoolbox.discourse.group/t/finding-equiluminance-for-a-pair-of-colors/807/1 "2007-04-11T22:46:35Z")
Does anyone have matlab software using the psych toolobox or standalone that allows measuring equiluminance points for arbitrary colors? IE something like this java demo: [http://www.cs.ubc.ca/nest/imager/contributions/flinn/Illusions/CB/cb.html](http://www.cs.ubc.ca/nest/imager/contributions/flinn/Illusions/CB/cb.html) Thanks! ---
**Author:** ![roberta.fagioli](https://avatars.discourse-cdn.com/v4/letter/r/c37758/32.png) [@roberta.fagioli](https://psychtoolbox.discourse.group/u/roberta.fagioli)\ **Post date:** [May 24, 2007, 8:59am UTC](https://psychtoolbox.discourse.group/t/finding-equiluminance-for-a-pair-of-colors/807/2 "2007-05-24T08:59:50Z")
Hi Sharon, I have to prepare an experiment in which the isoluminant point is found in a similar way. I have to create chromatic-contrast patterns (red-green or blue- yellow) superimposing (out of phase by 180 degrees) red-black and green-black (or blue-black and yellow-black) gratings. I would like to determine the equiluminance point for every subject with a variant of the method of heterochromatic flicker photometry. While the red-green gratings were sinusoidally reversed at 15 Hz, subjects adjust ratio of red to total luminance to minimized the perception of flicker. My question is: how can I control the luminance of gratings in the monitor? What's the way to change luminance as in the Flinn demo? Have I only to vary the Y coordinate of xyY CIE 1931, taking x and y constant at a fixed chromaticity for red and for green? Thanks a lot in advance!!! Roberta Fagioli University of Pavia --- In [psychtoolbox@yahoogroups.com](mailto:psychtoolbox@yahoogroups.com), "sharon.sally" \ wrote: > \> > \> Hi Alan, > \> > \> Sure, I've done that. > \> > \> The Flinn demo is to find equiluminance for moving images only. If you have static > \> images you'll use heterochromatic flicker photometry. > \> > \> Either way making the stimulus is very easy –first create your patterns. In the Flinn demo > \> (from Anstis' work) there are just 4 block images shown in sequence. Eg. > \> > \> function g= fillpattern > \> a=zeros(140,20); > \> mp=ones(10); > \> mp2=mp.\*-1; > \> nn=0; > \> for n=1:14 > \> mpp=mp > \> if mod(n, 2) > \> mpp=mp2; > \> end > \> a(1:10,1+nn:10\*n)=mpp; > \> nn=nn+10; > \> end > \> g=a; > \> > \> If you're doing flicker photometry you'll create two images of squarewave gratings 180 > \> degrees out of phase. > \> > \> For colored images mpp could be something like mp\*.25 and mp\*.57 > \> > \> > \> See driftdemo > \> SCREEN(w(1),'PutImage',gray+inc\*m); > \> > \> 127.5+127.5\*.57=200.1 > \> > \> for OS9 below > \> > \> clutss=SCREEN(window,'GetClut'); > \> clutss(200,1:3)=[255 0 0] % one (eg. red) stays constant > \> clutss(160,1:3)=[0 trialtheta 0]; > \> SCREEN(window, 'SetClut', clutss); > \> > \> For OSX use ReadNormalizedGammaTable and LoadNormalizedGammaTable > \> > \> That's it! > \> > \> Sharon Sally > \> > \> --- In [psychtoolbox@yahoogroups.com](mailto:psychtoolbox@yahoogroups.com), "Alan Robinson" \ wrote: > \> \> > \> \> Does anyone have matlab software using the psych toolobox or > \> \> standalone that allows measuring equiluminance points for arbitrary > \> \> colors? IE something like this java demo: > \> \> > \> \> [http://www.cs.ubc.ca/nest/imager/contributions/flinn/Illusions/CB/cb.](http://www.cs.ubc.ca/nest/imager/contributions/flinn/Illusions/CB/cb.) html > \> \> > \> \> Thanks! > \> \> > \> ---
**Author:** ![roux\_sebastien](https://avatars.discourse-cdn.com/v4/letter/r/d2c977/32.png) [@roux\_sebastien](https://psychtoolbox.discourse.group/u/roux_sebastien)\ **Post date:** [May 25, 2007, 6:54am UTC](https://psychtoolbox.discourse.group/t/finding-equiluminance-for-a-pair-of-colors/807/3 "2007-05-25T06:54:41Z")
Hi Roberta, Mario send me this email conerning a problem that I can not solve and I asked me to send you this response too: Ok, i have a hunch but i'm getting sick, so debugging must wait. Try to add this to the top of your script as work-around, and tell roberta fagioli to try the same with her problem. Screen('Preference', 'VBLTimestampingMode', -1); i'm offline, best, -mario Hope it can help you!!! Best, Seb ---
**Author:** ![roberta.fagioli](https://avatars.discourse-cdn.com/v4/letter/r/c37758/32.png) [@roberta.fagioli](https://psychtoolbox.discourse.group/u/roberta.fagioli)\ **Post date:** [May 25, 2007, 4:44pm UTC](https://psychtoolbox.discourse.group/t/finding-equiluminance-for-a-pair-of-colors/807/4 "2007-05-25T16:44:33Z")
Dear Sharon, thank you very much for the suggestion!!! Now, I will try to create my stimulus with these coordinates. Greetings from Italy! Roberta --- In [psychtoolbox@yahoogroups.com](mailto:psychtoolbox@yahoogroups.com), "sharon.sally" \ wrote: > \> > \> Hi Roberta, > \> > \> You can work in hue, value, saturation coordinates and translate back and forth to RGB. > \> See Matlab's HSV2RGB / RGB2HSV mfiles. This way you can keep hue and saturation > \> constant and change value. Take a look at Matlab's colormap(hsv) to get started. > \> > \> colormap('hsv'); > \> ww=colormap > \> www=rgb2hsv(ww) > \> > \> All values are from 0 to 1. > \> > \> Best, > \> > \> Sharon > \> > \> --- In [psychtoolbox@yahoogroups.com](mailto:psychtoolbox@yahoogroups.com), "roberta.fagioli" \ wrote: > \> \> > \> \> Hi Sharon, > \> \> I have to prepare an experiment in which the isoluminant point is > \> \> found in a similar way. > \> \> I have to create chromatic-contrast patterns (red-green or blue- > \> \> yellow) superimposing (out of phase by 180 degrees) red-black and > \> \> green-black (or blue-black and yellow-black) gratings. > \> \> I would like to determine the equiluminance point for every subject > \> \> with a variant of the method of heterochromatic flicker photometry. > \> \> While the red-green gratings were sinusoidally reversed at 15 Hz, > \> \> subjects adjust ratio of red to total luminance to minimized the > \> \> perception of flicker. > \> \> My question is: how can I control the luminance of gratings in the > \> \> monitor? What's the way to change luminance as in the Flinn demo? > \> \> Have I only to vary the Y coordinate of xyY CIE 1931, taking x and y > \> \> constant at a fixed chromaticity for red and for green? > \> \> Thanks a lot in advance!!! > \> \> > \> \> Roberta Fagioli > \> \> University of Pavia > \> \> > \> \> > \> \> --- In [psychtoolbox@yahoogroups.com](mailto:psychtoolbox@yahoogroups.com), "sharon.sally" \ > \> \> wrote: > \> \> \> > \> \> \> Hi Alan, > \> \> \> > \> \> \> Sure, I've done that. > \> \> \> > \> \> \> The Flinn demo is to find equiluminance for moving images only. > \> \> If you have static > \> \> \> images you'll use heterochromatic flicker photometry. > \> \> \> > \> \> \> Either way making the stimulus is very easy –first create your > \> \> patterns. In the Flinn demo > \> \> \> (from Anstis' work) there are just 4 block images shown in > \> \> sequence. Eg. > \> \> \> > \> \> \> function g= fillpattern > \> \> \> a=zeros(140,20); > \> \> \> mp=ones(10); > \> \> \> mp2=mp.\*-1; > \> \> \> nn=0; > \> \> \> for n=1:14 > \> \> \> mpp=mp > \> \> \> if mod(n, 2) > \> \> \> mpp=mp2; > \> \> \> end > \> \> \> a(1:10,1+nn:10\*n)=mpp; > \> \> \> nn=nn+10; > \> \> \> end > \> \> \> g=a; > \> \> \> > \> \> \> If you're doing flicker photometry you'll create two images of > \> \> squarewave gratings 180 > \> \> \> degrees out of phase. > \> \> \> > \> \> \> For colored images mpp could be something like mp\*.25 and mp\*.57 > \> \> \> > \> \> \> > \> \> \> See driftdemo > \> \> \> SCREEN(w(1),'PutImage',gray+inc\*m); > \> \> \> > \> \> \> 127.5+127.5\*.57=200.1 > \> \> \> > \> \> \> for OS9 below > \> \> \> > \> \> \> clutss=SCREEN(window,'GetClut'); > \> \> \> clutss(200,1:3)=[255 0 0] % one (eg. red) stays constant > \> \> \> clutss(160,1:3)=[0 trialtheta 0]; > \> \> \> SCREEN(window, 'SetClut', clutss); > \> \> \> > \> \> \> For OSX use ReadNormalizedGammaTable and LoadNormalizedGammaTable > \> \> \> > \> \> \> That's it! > \> \> \> > \> \> \> Sharon Sally > \> \> \> > \> \> \> --- In [psychtoolbox@yahoogroups.com](mailto:psychtoolbox@yahoogroups.com), "Alan Robinson" > \> \> \ wrote: > \> \> \> \> > \> \> \> \> Does anyone have matlab software using the psych toolobox or > \> \> \> \> standalone that allows measuring equiluminance points for > \> \> arbitrary > \> \> \> \> colors? IE something like this java demo: > \> \> \> \> > \> \> \> \> > \> \> [http://www.cs.ubc.ca/nest/imager/contributions/flinn/Illusions/CB/cb.](http://www.cs.ubc.ca/nest/imager/contributions/flinn/Illusions/CB/cb.) > \> \> html > \> \> \> \> > \> \> \> \> Thanks! > \> \> \> \> > \> \> \> > \> \> > \>