Standard road-marking glass beads have a refractive index around 1.5 – cost-effective and adequate for most markings. But on high-speed roads, tunnels and airport surfaces, wet-night visibility changes the requirement: water film on the line breaks retroreflection unless the bead returns light at a steeper angle. That is where higher-index beads enter the picture.
Refractive index, simply
R.I. describes how strongly the bead bends light. A bead of R.I. 1.5 returns light to the driver under normal conditions; higher-index glass (R.I. 1.7 or above) returns more light toward the source even when the bead is partly covered by water – the classic “wet-reflective” performance used for airport and expressway markings.
When standard 1.5 beads are enough
- Urban roads and parking areas with moderate speeds
- Projects where EN 1423 / AASHTO M247 Type I-II grading and budget lead
- Markings that are re-coated frequently
The numbers behind “good enough”
The decision between RI 1.5 and RI 1.7 beads is a decision about what conditions the marking has to survive, not about which product is better. Both are correct products for different specifications, and the cost difference is significant enough that specifying high-index where it is not needed is a straightforward waste of money.
| Condition the marking must survive | Standard RI 1.5 | High index RI 1.7+ | Recommendation |
|---|---|---|---|
| Dry night, good visibility | Adequate | Over-specified | Standard beads |
| Occasional rain, low-speed urban roads | Acceptable with correct application rate | Better, rarely required | Standard beads, review rate first |
| Frequent rain, highways above 80 km/h | Retroreflectivity drops significantly when wet | Maintains performance under water film | High index on critical sections |
| Continuous heavy rain, tropical climate | Fails the wet-night requirement | Specified performance achievable | High index |
| Tunnel and low-light approaches | Depends on geometry | Beneficial | Model before committing |
| Decorative or non-traffic applications | Sufficient | Unjustified | Standard beads |
When to evaluate higher brightness or high-index options
- Highways, expressways and tunnels
- Airport runways and taxiways
- Rain-prone regions with night traffic
A practical middle step
Before jumping to specialty glass, many contractors first upgrade within the R.I. 1.5 family: high-brightness grades with higher retroreflectivity (RL) and tighter grading. They are a cost-effective way to lift night visibility without changing the whole specification.
Specifying both grades on the same project
The most common practical answer is that a project does not need one grade throughout. High-index beads belong on the sections where wet-night performance is genuinely required, and standard beads are correct everywhere else. Splitting the specification can reduce bead cost materially without touching the performance requirement.
| Parameter | Standard grade | High-index grade |
|---|---|---|
| Refractive index | 1.50–1.52 | 1.70 and above |
| Typical mesh range for drop-on | 40–80 | 40–80, project-specific |
| Drop-on application rate | 350–500 g/m² | 350–500 g/m², may be tuned downward |
| Coating | Usually moisture-resistant coating | Coating choice affects adhesion under wet conditions |
| Cost position | Baseline | Multiple times baseline; verify the requirement before specifying |
| Best used on | General network, urban roads, dry climates | High-speed wet-weather sections, high-risk junctions |
When you split the specification, mark the sections clearly on the drawing. Mixed-up installation is the most common way a well-designed split specification fails in practice.
Ask the right questions
- What RL value does the spec require (mcd/m2/lx)?
- Is the wet-night test part of acceptance, or only dry retroreflection?
- What size band is permitted?
Linghang supplies standard R.I. 1.5 road-marking beads plus high-brightness and high-retroreflectivity grades (RL 400-500 mcd/m2/lx class) for demanding markings; availability of higher-index grades can be confirmed against your project requirement. See our retroreflective range or ask for RL test data.
For a fuller technical treatment of High-index and retroreflectivity, see our guide: What Is Retroreflectivity (RL) and Why It Fades on Real Roads.
How to verify the claim before you buy
RI is not a parameter you can check by eye, and a supplier statement is not evidence. If the specification depends on RI, the purchase order should require a test report that names the batch and the method.
| Verification step | What to ask for | What it proves |
|---|---|---|
| RI test report | Value plus test method, per batch | Confirms the grade actually supplied |
| Sieve analysis | Batch report, not a catalogue figure | Confirms gradation alongside RI |
| Roundness data | Percentage and method | Confirms the beads will peen/reflect as intended |
| Retained sample | Sealed sample from the delivered batch | Gives you physical evidence if field performance is challenged |
| Field trial section | Measured RL on a test strip | The only end-to-end proof of the specification |
Related reading
- Roundness — optical performance
- Night Visibility — road marking selection