Reflective metal or glass encoder disc? Choosing by optical path, accuracy and environment
For high resolution and long-term accuracy, choose a glass encoder disc; for a thin, vibration- and shock-resistant, cost-controlled encoder, choose a reflective metal encoder disc. This article compares the two disc types in terms of optical path, accuracy and thermal stability, and mechanical environment.
2026-10-02 · Langfang Jingzheng Optoelectronic Technology Co., Ltd.
Bottom line: for high resolution and stable long-term accuracy, choose a glass encoder disc; for a thin, vibration- and shock-resistant, cost-controlled encoder, choose a reflective metal encoder disc. The three reasons are explained below.
1. Different optical paths
With a transmissive disc, the light source and receiver are on opposite sides of the disc; light passes through the transparent areas and is blocked by the opaque areas. Glass encoder discs are the most common transmissive discs: the glass is chrome-coated and then patterned by photolithography, and the chrome layer forms the opaque areas.
With a reflective disc, the light source and receiver are on the same side, and position is read from the light/dark difference between the high- and low-reflectance areas on the disc surface. Because space is needed on one side only, the encoder can be made thinner, which is why reflective discs are more common in collaborative robot joints and slim servo motors. Glass can also be made reflective, but metal substrates are more common.
2. Accuracy and thermal stability
Glass substrates are flat and the chrome film is thin, so photolithographic line edges are clean, and high line counts and absolute multi-track patterns are easier to produce.
Thermal expansion also differs. Typical coefficients of linear expansion are roughly: quartz about 0.5×10⁻⁶/K, borosilicate glass about 3.3×10⁻⁶/K, soda-lime glass about 9×10⁻⁶/K, and 304 stainless steel about 17×10⁻⁶/K. For a temperature change of 40 K, the grating pitch of a 304 stainless steel disc changes by about 0.07%, and that of a soda-lime glass disc by about 0.04%. This has little effect in ordinary servo applications; in high-resolution applications with a small readhead gap, you also need to consider the stress and eccentricity caused by mismatched expansion between the disc and the hub.
3. Mechanical and environmental factors
Glass is sensitive to drops and impact, and clamping stress must also be controlled during assembly. Metal does not shatter, making it the safer choice under heavy vibration or where collisions may occur. Both disc types are vulnerable to oil and dust covering the graduations, so the sealing requirements are the same.
| Criterion | Glass Encoder Disc | Reflective Metal Encoder Disc |
|---|---|---|
| Typical optical path | Transmissive | Reflective |
| High line counts, absolute multi-track | Easier | Possible, limited by etching and coating processes |
| Thermal expansion | Small | Larger |
| Shock resistance | Sensitive to drops | Good |
| Encoder thickness | Needs space on both sides | Can be made thin |
| Cost | Higher | Lower |
How to choose
- CNC machine tools, rotary tables, metrology and surveying instruments: glass encoder disc
- Collaborative robot joints, slim servo motors, hollow-shaft encoders: reflective metal encoder disc
- High volumes, moderate resolution, weight-sensitive: PC encoder disc
- If in doubt, order a sample batch of each and measure the signals with your own readhead
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.