Check/match the luminance of cogent-created images

I've got someone else's code that creates two circular visual stimuli, which are very similar to pie charts. They are drawn while the code is running (i.e. they aren't just image files being loaded from a drive) using the Cogent toolbox, specifically the cgarc command.

I want to be able to match the luminance of these two visual stimuli. Is there any Matlab code that can be used to report or match the luminance of two stimuli that are drawn 'on-line' rather than loaded from a file?

Thanks Rob

Answer 1

[Image Analyst]

If you (or the code) drew them, then you know the color of them and you can make sure they're the same.

[Robert]

Hi, do you mean using the formula: Luminance = 0.2126 R + 0.7152 G + 0.0722 B?

The proportions of the pie chart/circles are defined dynamically in the code, so I guess I could calculate the Luminance of the first pie chart and then adjust the exact RGB specifications of the second chart so they match - is that what you are suggesting?

Thanks Rob

[Image Analyst]

Yes, exactly. If the RGB are the same, you know they "should" be the same, however you know from checking your monitor screen with a lightmeter or radiometer that the light emitted everywhere on the screen isn't the same even when it's supposed to be, because of side lighting by the fluorescent or LED lights. So how are you going to take that into account? Take a picture of a uniform screen and divide the image by that? No, as you should know, cameras have shading, mostly due to the lens which means they'll be darker at the side than they should be. As you can see, it can be quite a complicated issue to do what you're trying to do and that's why calibrated viewing environments and monitors are expensive to buy and take some effort to calibrate. They're not "just plug it in and it works" like other arbitrary monitors that don't have good luminance control. For really good, high quality, calibratable monitors, try Barco: http://www.barco.com/en/Products-Solutions/Displays-monitors-workstations