I had an idea to ensure that user’s monitors are adjusted correctly. This file was constructed on a calibrated monitor (Macbeth Eye1). The lightest (1%) and darkest block (97%) should be almost invisible. If they are not, your monitor is likely wrong.
Feel free to use it - I’m going to include it with my image postings.
Out of interest, for publishing purposes, my images have to be between 4–93% to be printable.