Incident energy is usually measured in which unit?

• A. calories per square centimeter
• B. joules per square centimeter
• C. calories per square meter
• D. joules per square meter

Answer: (A)

Incident energy is the radiant energy that lands on a surface per unit area, so it’s an energy density expressed over a small area. In laser safety and OSHA training, the traditional unit used is calories per square centimeter because it clearly communicates how much energy is delivered to a tiny area of material or tissue. You may also encounter joules per square centimeter as the SI alternative, since 1 cal/cm^2 equals about 4.184 J/cm^2. The other options—calories per square meter or joules per square meter—describe energy per a much larger area and aren’t the standard way this exposure is reported.