Weak signal and large interpolation error with a reflective disc? Check the high/low reflectance contrast first
The signal of a reflective encoder comes from the difference in light intensity between the high- and low-reflectance areas of the disc. This article gives the contrast formula with examples, explains why lowering the low reflectance is more effective than raising the high reflectance, and lists the optical parameters to state clearly in an RFQ.
2026-10-02 · Langfang Jingzheng Optoelectronic Technology Co., Ltd.
The signal of a reflective encoder comes from the difference in light intensity between the high- and low-reflectance areas of the disc. The larger the difference, the larger the signal amplitude, the higher the signal-to-noise ratio, and the smaller the error after interpolation. The metric for this difference is contrast:
Contrast C = (reflectance of high-reflectance area − reflectance of low-reflectance area) ÷ (reflectance of high-reflectance area + reflectance of low-reflectance area)
| 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 works better than raising the high reflectance. With the high-reflectance area held at 65%, reducing the low-reflectance area from 10% to 1% raises the contrast from 0.73 to 0.97; with the low-reflectance area held at 10%, raising the high-reflectance area from 65% to 90% only raises it to 0.80.
Three easily overlooked points
- Reflectance varies with wavelength. The same disc gives different values depending on whether the readhead uses 850 nm infrared or 460 nm blue light. State the readhead wavelength in your RFQ.
- Uniformity matters as much as the average. If reflectance fluctuates around the revolution, the signal amplitude fluctuates with it and interpolation error increases.
- Stray light raises the reading of the low-reflectance areas. Using an absorbing layer for the low-reflectance areas, rather than relying on scattering from a rough surface, absorbs the excess light.
Common approaches
- Mirror-finish metal substrate + absorbing film, with the absorbing film removed from the high-reflectance areas by photolithography and etching
- Absorbing substrate + reflective film, with the reflective film removed from the low-reflectance areas by photolithography and etching
- A reflective film and an absorbing film stacked on any substrate, followed by photolithography
The Jingzheng Optoelectronics utility model patent CN217210914U “An optical reflective encoder disc, encoder and linear scale” (granted in 2022) uses exactly this layered structure of reflective and absorbing materials. According to the patent text, the reflective film has a reflectance of >90% in the 460 nm and 850 nm bands, and the absorbing film <1%. For production parts, the values in the specification sheet apply.
What to specify in your RFQ
Readhead wavelength, separate requirements for the high- and low-reflectance areas, reflectance uniformity, and film adhesion and wipe resistance requirements.
This article was compiled by the engineering department of Langfang Jingzheng Optoelectronic Technology Co., Ltd. Jingzheng Optoelectronics manufactures glass encoder discs, reflective metal encoder discs, PC encoder discs and custom encoder discs. Website: www.encoderdisk.cn; phone: 0316-8933331.