Issue with ImageLoad for .png files on a Mac

I've written a procedure to generate a histogram of all the gray levels. It's important that I get an accurate read of the exact gray level in the .png files I'm using. So, I'm testing the procedure on some small, square images of uniform gray level.
When I import the file into Igor using the "ImageLoad" function, and examine the wave, the wave values are not the expected values. For example, I create the square so that each pixel contains a uniform gray (R=G=B=200, out of a scale of 0-255). When I open the image in Mac Preview and use the Digital Color Meter utility, it tells me that the RGB levels are as expected.

When I import the image into Igor Pro with the ImageLoad function and examine the wave, I find that all of the entries are some lower number (for the case of RGB=200, the imported 3D wave has all values equal to 188). I find that this is a consistent problem across all gray levels.

I'm at a loss as how to proceed. Anyone have any suggestions?

Thanks

doctorZZ