A reader asks:
Is there any kind of device I could use to see how much light comes off my light box at 24 inches? I am using an old one that a friend dug out of her attic, and the manufacturer does not respond to my inquiries.
Not easy. There is an industrial unit, called a photometer, which reads illumination in lux. Probably quite expensive, although some photo stores have them and might lend or rent one to you for a few days. If you do this, make sure that the sensor is a rounded, “cosine-corrected” probe, rather than a directional probe. The latter will inflate lux readings.