Selection guide
Reflective Metal or Glass Encoder Disc? How to Choose
Pick a glass encoder disc for the highest resolution and long-term accuracy; pick a reflective metal encoder disc for thin, shock- and vibration-resistant, cost-controlled encoders; pick PC for high-volume, moderate-resolution designs.
Choosing between a glass and a reflective metal code wheel comes down to three questions: which optical path your readhead uses, how much accuracy and thermal stability you need, and how harsh the mechanical environment is.
1. Optical path: transmissive or reflective
Transmissive discs sit between the light source and the detector. Light passes through the clear areas and is blocked by the opaque ones. Chrome-on-glass is the classic transmissive disc: the glass is chrome-coated and patterned by photolithography, and the remaining chrome forms the opaque areas.
Reflective discs have the source and detector on the same side. Position is read from the difference between high- and low-reflectance areas on the disc surface. Because the optics need space on one side only, the encoder can be much thinner — the reason reflective discs dominate in collaborative robot joints and slim servo motors.
2. Accuracy and thermal stability
Glass is flat and the chrome film is thin, so lithographic line edges are clean and high line counts or absolute multi-track patterns are easier to produce.
Thermal expansion also differs. Typical coefficients of linear expansion:
| Material | Approx. ×10⁻⁶/K |
|---|---|
| Fused silica (quartz) | 0.5 |
| Borosilicate glass | 3.3 |
| Soda-lime glass | 9 |
| 304 stainless steel | 17 |
Over a 40 K temperature change, 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 %. For ordinary servo use this hardly matters. In high-resolution designs with a small readhead gap, also consider the stress and eccentricity caused by expansion mismatch between disc and hub.
3. Mechanical environment
Glass is sensitive to drops and impact, and clamping stress must be controlled during assembly. Metal does not shatter, so it is the safer choice under heavy vibration or where collisions can happen. Oil and dust on the graduations hurt both types equally — sealing requirements are the same.
Side-by-side
| 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 |
| Thermal expansion | Low | Higher |
| Shock resistance | Sensitive to drops | Good |
| Encoder height | Needs space on both sides | Can be very thin |
| Cost | Higher | Lower |
Quick selection
- 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 volume, moderate resolution, weight-sensitive → PC encoder disc
- Still undecided → order a small sample lot of each and measure the signals with your own readhead.