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The Complete Handbook of Sunglass Lenses
A practical field reference for opticians, eyewear retailers, product designers, and serious end users who need to select, specify, and maintain sunglass lenses with confidence.
by Alumigogo Books
Chapter 1: Understanding Sunglass Lenses
A sunglass lens is, at its most basic, a precision optical filter mounted in a frame. But that simple description hides a lot of engineering. A lens has to block harmful radiation, manage visible light to a level comfortable for the eye, preserve color perception to a degree that is safe for the activity, and do all of this without introducing optical distortion that would cause eye strain, headaches, or outright visual errors. When you buy a lens, you are not buying a piece of dark plastic; you are buying a series of engineering compromises, and your job is to choose the set of compromises that fits your situation.
The first thing to understand is what a lens actually does to light. Sunlight contains ultraviolet (UV) radiation, visible light, and infrared radiation. The UV portion is further divided into UVA (315-400 nanometers), UVB (280-315 nanometers), and UVC (100-280 nanometers), although the atmosphere blocks most UVC. The human cornea and lens absorb UV radiation, and chronic exposure contributes to cataracts, photokeratitis (a sunburn of the eye), and pterygium (a growth on the white of the eye). A sunglass lens's primary job is to reduce UV exposure to the eye to safe levels. This is done either by the lens material itself absorbing UV, or by a coating that absorbs or reflects it. Polycarbonate, for example, inherently absorbs nearly 100% of UVB and a large portion of UVA without any coating. CR-39, a common plastic, absorbs some