Soft Coating

Properties of Soft Coatings:

    • Materials: Typically use softer materials like zinc sulfide, cryolite, or occasionally silver.
    • Structure: Layers are laminated together or separated by air gaps, making them more susceptible to damage and environmental factors like humidity.
    • Applications: Often used for narrow and ultra-narrow bandpass filters where precise control of specific wavelengths is crucial. Less common for broader bandpass or neutral density filters.
    • Advantages: Offer good performance at lower cost compared to hard coatings, can achieve sharper transitions in the passband.
    • Disadvantages: More delicate and less durable than hard coatings, require careful handling and protection, have shorter lifespans, and are not suitable for harsh environments.

Coating Techniques for Soft Coatings:

    • Electron Beam Evaporation (EBE): Precisely vaporizes materials using an electron beam, ideal for accurate layer control.
    • Thermal Evaporation: Heats materials to evaporate them, less expensive than EBE but offers less control.

Important Notes:

    • Soft coatings generally require additional protection, like encapsulation or cementing, for longer lifespan and environmental resistance.
    • Hard coatings, made with durable materials like tantalum oxide or hafnium oxide, are becoming more common due to their improved durability and wider range of applications.
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