equiluminance and tiled images

[erowen]

I am a graduate student and I use simple high-contrast black and white patterns as visual stimuli to study visual attention. I am using photoshop to create new images from one specific image, by tiling the pixels of the original image, and using the texture tools in photoshop such as 'ripple' and 'raindrops'...BUT I want to make sure that the images I'm creating this way are equiluminant (avg luminance of whole image, not regions of image) to the original image pattern. I have created the original image using a luminance layer, but I'm concerned that by scrambling and tiling pixels, I'm changing the overall average luminance....any suggestions? If it helps, the reason I need my images to be equiluminant is that I am presenting them in distinct spatial locations but at the same time and I do not want differences in luminance to bias visual attention to one stimulus location over another.
Any comments/suggestions would be helpful!
Thank you.

[vladimir]

Do Filter -> Blur -> Average and measure resulting gray pixels.