${\displaystyle R_A=\frac{R}{A}=\frac{I/E_{\perp }}{A}}$

where I is the luminous intensity of the retroreflector in the direction of observation, and ${\displaystyle E_{\perp }}$ is the illuminance at the retroreflector on a plane perpendicular to the direction of the incident light

Note 1 to entry: The coefficient of retroreflection is particularly suitable for describing materials in sheet form.

Note 2 to entry: Formerly, the symbol R′ was used for this quantity.

Note 3 to entry: The coefficient of retroreflection is expressed in candela per lux per square metre (cd·lx⁻¹·m⁻²).

Note 4 to entry: This entry was numbered 845-04-97 in IEC 60050-845:1987.