Optical filters used in plant lamps
1. Introduction
Plant photosynthesis shows strong selectivity for blue-violet (400–470nm) and red light (630–750nm), while yellow-green light (500–580nm) is barely absorbed and may hinder growth. This study introduces a broadband negative filter designed to optimize natural sunlight by blocking detrimental yellow-green wavelengths for enhanced plant cultivation.
2. Technical Specifications
Key performance requirements:
- >90% average transmittance in 400–470nm (blue-violet) and 630–750nm (red)
- <0.2% transmittance in 500–580nm (yellow-green)
Deposited on 100×100mm K9 glass, the filter balances spectral precision with large-substrate uniformity.
3. Coating Design & Material Engineering
3.1 Multi-Material Layer System
Starting from a high-reflection base model sub|H(HL)⁹|air, the design incorporated symmetric matching layers to eliminate passband ripples. The final architecture: sub|(3H₂3M)²(3H₁3M)⁶(3H₂3M)²2H₂L|air, featuring: - H₁: TiO₂ (n=2.34), H₂: ZrO₂ (n=1.97) - M: Al₂O₃ (n=1.56), L: MgF₂ (n=1.38)
3.2 Material Properties
| Material | Refractive Index @550nm | Transparency Range (μm) | Melting Point (°C) |
|---|---|---|---|
| TiO₂ | 2.2–2.4 | 0.35–12 | 1850 |
| ZrO₂ | 1.97 | 0.34–12 | 2700 |
| Al₂O₃ | 1.53–1.60 | 0.20–8 | 2045 |
| MgF₂ | 1.38 | 0.21–10 | 1395 |
4. Deposition Process
Executed on a ZZS660 electron-beam evaporator with ion assistance:
- Substrate heating: 150°C (180°C for MgF₂ layers)
- Corrected baffles for thickness uniformity on 100mm substrates
- Oxygen flux to prevent oxide de-oxygenation
Process Parameters
| Material | Deposition Rate (nm/s) | Beam Current (mA) | Ion Source |
|---|---|---|---|
| TiO₂ | 0.35 | 220 | Yes |
| ZrO₂ | 0.25 | 180 | Yes |
| Al₂O₃ | 0.40 | 200 | Yes |
| MgF₂ | 1.05 | 25 | No |
5. Performance Evaluation
Tested via Lambda900 spectrophotometer: - 400–470nm/630–750nm: >90% average transmittance - 500–580nm:<0.2% transmittance Minor passband ripples (e.g., 486nm dip) attributed to layer-thickness accumulation errors, but overall met design criteria.
6. Conclusion
Successfully developed a large-area broadband negative filter blocking yellow-green light while enhancing blue-violet/red transmission. Key innovations:
- Multi-material design (TiO₂/ZrO₂/Al₂O₃/MgF₂) for spectral precision
- Ion-assisted deposition for mechanical durability
- Baffle correction for uniform coating on 100mm substrates
