Basic structure and optical properties of spherical concave metallic mirrors
Concave coating (commonly used): reflection occurs on the concave side, light convergence or divergence is determined by the radius of curvature.
Plano coating: reflection occurs on a plano surface, the concave surface is only used for refraction correction of the transmitted light path.

Parameter	Description
Radius of curvature (R)	Determines the focal length (f = R/2). Negative values indicate concave surfaces (converging light). Positive values are convex (uncommon for plano concave mirrors).
Aperture (D)	The effective clear aperture must match the beam diameter to avoid vignetting.
Focal Length (F)	Directly related to the radius of curvature, affecting beam control capability.

How are the materials for spherical plano concave mirrors selected?
Substrate Material	Thermal Expansion	Applicable Scene
BK7	~7.1×10⁻⁶/K	Lowcost visible applications with moderate accuracy requirements.
Fused Silica	~0.55×10⁻⁶/K	High power laser (low thermal distortion), UV band.
Silicon	~2.6×10⁻⁶/K	Infrared lasers (e.g. CO₂), but with anti-reflective coating.
D263T	~7.1×10⁻⁶/K	Ultra-thin, lightweight systems (e.g., portable devices) requiring chemical reinforcement.