planoconvex

A planoconvex lens has a flat surface on one side and a curved (convex) surface on the other. This asymmetry makes it useful in optical systems where light needs to be focused or collimated in a specific direction. The curved side does most of the work; the flat side serves as a mounting surface or as the exit face in certain configurations.

Planoconvex lenses come in two orientations depending on the application. When light enters the curved side first, the lens behaves like a weak focusing element; when light enters the flat side first, it becomes a collimating lens or produces a different focal length. The choice matters because spherical aberration, which causes off-axis light rays to focus at slightly different points, varies significantly with orientation. Incoming light through the curved surface distributes aberration more evenly across the field.

Common materials and sizes

Crown glass and borosilicate are standard choices for visible light applications, with diameters typically ranging from 6 mm to 50 mm for bench optics. Infrared systems use zinc selenide or germanium. Manufacturing involves grinding and polishing the curved surface to a precise radius while keeping the flat side flat to within lambda/4 or better (around 0.13 micrometers for green light). Larger planoconvex lenses suffer from sag at the flat surface; very large diameter versions require mechanical support or must be mounted in cells that hold them horizontally.

In practice, planoconvex lenses appear in laser beam shaping, collimation of LEDs, and as condenser optics in microscopy. They are cheaper than symmetric biconvex designs because one surface requires no curvature grinding. However, they introduce more coma and astigmatism than biconvex lenses of equivalent focal length, which limits their use in high-precision imaging. Pairing a planoconvex with another lens element often corrects these aberrations better than using two planoconvex lenses together.

The focal length of a planoconvex lens depends on radius of curvature, glass refractive index, and thickness. A typical crown glass planoconvex with radius 50 mm and thickness 5 mm has focal length around 100 mm. Thickness variations matter; thick lenses deviate from the thin lens equation and require correction factors when high accuracy is needed.