When a spec sheet says a marking must reach RL 150 or RL 300, it is talking about retroreflectivity: how much light the line bounces back toward the driver’s eyes rather than scattering into the night. The unit looks intimidating, mcd/m2/lx, but it simply compares the light coming back to the light shining in. Higher RL means a brighter line at night.
Where glass beads fit in the physics
A bare paint line is mostly flat, so headlight beams hit it and scatter sideways. The driver sees a dim smudge. Add glass beads, and the light enters the bead, bends, reflects off the back surface, and returns roughly toward the source. That is the retroreflection in the name, and it is why beads are the standard answer to night visibility on road markings.
Two things control how much light comes back. One is the refractive index of the glass, standard road marking beads run at about 1.5, which balances brightness with cost. The other is the shape of the bead. Light has to enter and exit at useful angles, and a bead that is not round sends it off in the wrong direction. That is the direct link between roundness and RL, and it is why we test roundness on every batch rather than only checking sieve size.
Why the number drops over time
New markings often test well above the spec, and nobody complains. The interesting part is the curve after that. Traffic slowly polishes the exposed beads, dust and tire residue build up on the surface, and rain film over the beads scatters light before it can enter them. A line that started at RL 250 can drift below a 150 requirement within a year or two on a busy road.
This is exactly where the premix-versus-drop-on choice comes back into the story. As the top layer of drop-on beads wears off, premix beads inside the binder surface and take over the reflection. Markings built with only a drop-on layer lose their night visibility much faster, because nothing is waiting underneath to replace the worn beads.
Coating, the quieter factor
Moisture is a second enemy of RL. A plain glass bead collects a thin water film on its surface, and that film bends light in the wrong way, which is why wet roads are where markings fail hardest. Coated beads, usually treated with silane or silicone, shed water and keep reflecting in the rain. Projects in high-rainfall regions or on unlit highways often specify coated beads for exactly this reason, even though they cost a little more per ton.
What to ask for
If your tender names an RL requirement, ask the bead supplier for the sieve and roundness data, not just the RI number, because those two drive the real-world RL. If the tender only says “meets AASHTO M247” or “EN 1423”, those standards were written around producing a usable night line, so a genuinely compliant bead is a reasonable starting point. If you want the marking to stay bright into year three or four, that is when premix ratios, coated beads, and bead quality stop being technical details and start being the difference between a satisfied client and a callback.