I assume this would be rather easy: display "white" on entire screen, press Esc to quit.
It would allow you to use your monitor as a poor man's lightbox for viewing slides, x-rays, MRI's, and other tranparencies.
Don't laugh: I needed to look at some (important) MRI's for a family member the other day. I created a blank/white image file and viewed it in IrfanView's full-screen mode. Worked for me.