Thank you for the quick reply.
I had no particular reason to use the “opacity” instead of “contrast” parameter. According to the Psychopy website “contrast” and “opacity” can both be used to control contrast and “opacity” seemed easier to control for me:
Set the contrast of the stimulus, i.e. scales how far the stimulus deviates from the middle grey. You can also use the stimulus opacity to control contrast, but that cannot be negative.
I guess I should clarify why I’m asking my question. I’m not particularly interested in Michelson contrast as such. I’m currently writing a paper about my experiment and would like to indicate what we did exactly to manipulate our Gabor patches. I thought that Michelson contrast and opacity may be the same.
Do you think there is a more straightforward way of expressing (using an equation) what opacity does exactly? I’d be happy to substitute opacity into the Michelson equation. Which parameters of the GratingStim would be required for that?
Thanks - rbr