If you press a button in the grid, that pixel becomes the current point, which is then moved to the middle of the grid. The buttons colored red and blue contain the highest and lowest values out of those displayed.
The controls at the top are as follows:
File value: Turn this box on to have the continuous readout of pixel value at the mouse position come from the image file instead of from program memory. Taking data rapidly from random positions in the file could be problematic with a large file or slow access. If it is not possible to read single-pixel values from the current image file, this box will be grayed out and data will be taken from memory.
Grid: Turn this box off to have a small window with just the continuous readout.
Grid value from file: Turn this box off to have the grid values taken from memory instead of from the image file. If it is not possible to read single-pixel values from the current image file, this box will be grayed out and data will be taken from memory.
Convert RGB to gray scale: When color images are loaded, the grid will show the red, green, and blue components. Turn this box on to see a single gray scale value, converted with the standard NTSC RGB to luminance conversion used elsewhere in IMOD.