Photochromic lenses are lenses that can get darker as one goes into brighter light. Inside or on the lens a component (either a silver halide crystal in the case of glass or an organic molecule in the case of plastic) gets dark depending on the amount of UV light in the area.
Hot temperatures decrease the darkening effect. As an example, we took a glass photochromic lens called Photogrey Thin and Dark ® (link). This lens is the darkest glass photochromic we are aware and gets down to 14% transmittance – a good sunglass grade. This means that 86% of the visible light is filtered. UV light is screened out for both temperatures.
We measured the lens at 40 degrees F after a 3 minute exposure on a clear sunny day. The lens performed as expected (see illustration – lower white graph). We next measured the same lens at 100 degrees after a 3 minute exposure. The transmittance only reached 54% (upper grey graph). We used our Humphrey Instruments automated lensometer which has a spectrometer to measure the absorbance of light in the visual and UV range.
Hot lenses transmit more light (top) than cool lenses (bottom)
All photochromic lenses exhibit this behavior called temperature dependency. The process is most efficient for skiers on snowfields than sunbathers on a beach on a hot sunny day.