Transmittance refers to the amount of light that can pass through the lens of a pair of glasses. This parameter is crucial for different types of glasses, such as sunglasses, sports glasses or dioptric glasses. A low transmittance means stronger darkening, which is ideal for bright sunny days, while a higher transmittance is suitable for low light conditions. Transmittance is usually expressed in percentages or categories S0-S4, where S0 means the highest transmittance and S4 the lowest.