Dear Dr. Color:

I wish to measure the brightness of my LCD monitor so that I can set it to the recommended brightness for photo editing work. How can I convert the measurements from a hand held exposure meter to candles per square meter or NITS?

First, you must be using a spot meter; incident lightmeters won’t work for this task. (I suppose you could use an incident meter in a room with no other illumination…) Then compute the exposure value using your light meter, which shouldn’t be too hard: just collect f-stop, shutter time, and ISO setting. That’s easy. The tricky part is to determine the meter’s calibration constant. According to Wikipedia, it’s probably 12.5 (for Canon, Nikon, and Sekonic) or 14 (for Minolta and Pentax). Take all of those numbers and put them into this equation:

((calibration constant) * (f-number)^2) / ((time) * (ISO speed))

The resulting value is luminance in candelas per square meter. There’s your radiometry lesson of the day.

But I wonder, doesn’t your monitor calibration tool perform this as one of the first steps? Mine, the consumer-level Monaco EZ-Color suite, does. And if you’re that worried about brightness, you’ll probably need to worry about other aspects of the ISO standard: surround illumination, specular highlights, etc. Those other factors determine apparent brightness and color values, which is probably what you’re worried about. And isn’t Photoshop’s soft-proofing good enough?