What is Retroreflectivity?

Retroreflectivity is the property of a surface that returns light toward the source it came from.

How it works

Retroreflection is light that returns on a path parallel to the one it came from. Glass beads do this with refraction and reflection. Light enters a bead, reflects off the paint behind it and leaves toward the source. Microprisms use reflection from three faces. The FAA specifies glass beads for airport marking paint in P-620.

Measurement

For pavement markings the quantity is RL, in mcd/m²/lux. ASTM E1710 is a field method for horizontal markings. It fixes a viewing distance of 30 m, a headlight height of 0.65 m and an eye height of 1.2 m. The entrance angle is 88.76 degrees and the observation angle is 1.05 degrees. These are road-vehicle dimensions. They do not represent an aircraft cockpit.

FAA AC 150/5370-10H, Item P-620, takes RL with a portable retroreflectometer to ASTM E1710. A vehicle-mounted retroreflectometer may also be used. The P-620 table gives minimum initial values for white and yellow paint by bead type, for preformed thermoplastic, and a level to remark below.

What the standards say

ICAO Annex 14 and EASA CS ADR-DSN.L.520 recommend reflective materials for pavement markings at aerodromes with night operations. For signs, Annex 14 and CS ADR-DSN.N.775 require or recommend retroreflective and/or illuminated signs in the cases they list. Annex 14 also requires taxiway edge markers to be retroreflective blue and taxiway centre line markers to be retroreflective green. The FAA covers runway and taxiway retroreflective markers (L-853) in AC 150/5345-39.

In the United States, 14 CFR 139.311(d) requires certificate holders to clean, replace or restore any faded, missing or nonfunctional marking or sign. The rule sets no RL value. The FAA gives the methods in its advisory circulars.