Optics
Reflective Encoder Disc Contrast: Why Low Reflectance Matters More
Signal amplitude on a reflective disc is set by contrast C = (R_high − R_low) ÷ (R_high + R_low). Cutting the low-reflectance value from 10 % to 1 % lifts contrast from 0.73 to 0.97 — far more than raising the high reflectance from 65 % to 90 % (0.80).
A reflective encoder’s signal comes from the light-intensity difference between the high- and low-reflectance areas of the disc. The larger the difference, the larger the signal amplitude, the better the signal-to-noise ratio, and the smaller the error after interpolation.
The contrast formula
C = (Rhigh − Rlow) ÷ (Rhigh + Rlow)
| High-reflectance area | Low-reflectance area | Contrast C |
|---|---|---|
| 90 % | 1 % | 0.98 |
| 70 % | 1 % | 0.97 |
| 65 % | 10 % | 0.73 |
| 50 % | 20 % | 0.43 |
Lowering the low reflectance beats raising the high reflectance. Holding the high area at 65 %, reducing the low area from 10 % to 1 % raises contrast from 0.73 to 0.97. Holding the low area at 10 %, raising the high area from 65 % to 90 % only gets you to 0.80.
Three points engineers often miss
- Reflectance depends on wavelength. Specify whether your readhead uses 850 nm infrared or 460 nm blue.
- Uniformity matters as much as the average. If reflectance varies around the revolution, signal amplitude varies with it and interpolation error grows.
- Stray light lifts the “dark” reading. A true absorbing layer in the low-reflectance areas works better than relying on scattering from a rough surface.
How reflective discs are built
- Mirror-finish metal substrate + absorbing film, with the film removed from the high-reflectance areas by photolithography and etching
- Absorbing substrate + reflective film, with the film removed from the low-reflectance areas
- Reflective and absorbing films stacked on any substrate, then patterned
Our utility model patent CN217210914U (“An optical reflective encoder disc, encoder and linear scale”, granted 2022) uses the layered reflective/absorbing structure. According to the patent text, the reflective film exceeds 90 % reflectance at 460 nm and 850 nm and the absorbing film is below 1 %. For production parts, the values on the specification sheet apply.
What to put in your RFQ
Readhead wavelength; separate requirements for the high- and low-reflectance areas; reflectance uniformity; film adhesion and wipe resistance. See the reflective metal encoder disc page.