These mirrors have high reflectance for specific laser wavelength. That is to say that these mirrors are designed to work at a peculiar wavelength individually. And multilayer coatings are stacks intended to achieve the highest possible reflectance at specific laser line wavelengths and at particular angles of incidence. Specifications: - Material: BK7 glass or Fused silica - Diameter: +0/-0.1mm - Thickness: ±0.1mm - Flatness: λ/10 at 633 nm - Parallelism: <3 arc min - Surface Quality: 10/5