Waterloo Optics offers range of reflective mirror coatings to meet your system requirements. Rotating polygon and monogon optics are critical components in high-speed scanning systems, used widely in imaging and laser printing. The performance of these polygons hinges significantly on their surface coatings, which must not only enhance reflectance but also be robust. The most common coatings for polygon optics are protected aluminum, gold, and silver coatings, as well as advanced dielectric multilayers.
The Short Answer
- Aluminum – for most applications in the visible spectrum, and where cost is very important.
- Gold – for most applications above 600 nm wavelength
- Silver – if you must, must, must have higher reflectivity in the visible range, and can risk potential corrosion issues due to imperfections
- Dialectic Multi-Layers – if you are using monochromatic light, and can afford to maximize the reflectivity.
Protected Aluminum Coatings
Aluminum, inherently a robust reflector across the visible spectrum, forms the base of many polygon mirrors. However, its soft surface is prone to scratches and wear, particularly detrimental in high-speed rotational use. A common solution is to apply a thin layer of silicon monoxide—a durable dielectric protective coating. This layer significantly enhances the aluminum’s durability. Protected aluminum coatings maintain a high reflectivity, over 88%, across the 450-650 nm range, making them a cost-effective and reliable option for many scanning applications.
Gold Coatings
For applications extending into the infrared spectrum beyond 600 nm, gold coatings are preferred due to their excellent reflectivity characteristics. Gold reflects about 90% at 600 nm and maintains high reflectivity (>98%) well into the infrared spectrum. This makes it particularly valuable in applications involving deep infrared scanning. The inherent resistance of gold to oxidation and corrosion also contributes to the longevity and reliability of the coating, a crucial factor in maintaining the precision of polygon optics.
Silver Coatings
Silver coatings offer superior reflectance across a broad spectrum, which includes the visible and near-infrared regions. Their reflectivity exceeds that of aluminum, making them initially attractive for high-performance applications. However, silver’s susceptibility to atmospheric contaminants and the propensity for tarnishing limits its long-term reliability. Minute imperfections can lead to significant degradation, making it less favorable for durable applications despite its high initial reflectivity.
Dielectric Multilayers
To achieve tailored reflectivity at specific wavelengths, dielectric multilayers are employed. These coatings involve a series of high and low refractive index materials, such as silicon dioxide, hafnium oxide, and titanium dioxide, layered in a quarter wave stack. This design allows for the customization of the coating to enhance reflectivity precisely where needed. The versatility of dielectric stacks in enhancing the base metal’s reflectivity is invaluable, especially in applications that require stringent wavelength-specific performance.
Conclusion
The choice of coating for rotating polygon optics significantly influences their performance and longevity in demanding scanning applications. Protected aluminum remains a staple for its balance of performance and durability. Gold and silver offer higher reflectivity but come with considerations of cost and long-term stability. Advanced dielectric multilayers provide customizable solutions to meet specific operational demands. Each option presents its own set of advantages, necessitating careful consideration based on the specific requirements of the scanning system and operational environment.