Fresnel lenses
Short definition
Fresnel lenses are thin, lightweight lenses made of concentric rings, which for years were the most common optics in virtual reality headsets.
In plain language
They replace a thick lens with a series of concentric steps that have the same optical effect, saving weight and cost. The drawback is that they can produce ray- or halo-shaped glare around bright areas and have a smaller sharp zone. Many recent devices have replaced them with pancake lenses.