First, we need to know a word: transmittance.
The transmission ratio, known as the transmission coefficient, refers to the ratio of the luminous flux through the lens to the incident luminous flux. Simply speaking, it is the transmittance or penetration rate of visible light. For example, if the lens of a certain type of sunglasses has a 40% transmittance, that means only 40% of the visible light can pass through the lens and enter the eye.
Based on the transmittance of visible light, lenses can be classified into categories 1 to 4, ranging from shallow lens to deep lens. Sunglasses can be divided into categories 1, 2, 3, and 4 according to lens transmittance. According to the standard, the transmission score of sunglasses has 4 grades. If the grade is taller, the light blocking rate is higher. And these four categories, basically, correspond to three different types of sunglasses.
For the light-colored sunglasses
For the light-colored sunglasses, the projection ratio is 43%~80%, and shading and reducing light flux is the least, usually used as fashion accessories. These are usually light-colored sunglasses with the lightest lenses and are suitable for most everyday use. Light-colored sunglasses do not block the sun as well as sunshades, but they are colorful, suitable for use with all kinds of clothing, and have a strong decorative effect.
The second is light-colored sunshade sunglasses. With a transmission ratio of 18%~43%, they have a slightly lighter color, used on cloudy days.
Transmission ratio grade 3
Transmission ratio grade 3 usually refers to the sunshade lens, with a transmission ratio of 8%~18%. Most sunglasses fall into this category and are suitable for outdoor activities like hiking. Sunshade lenses are relatively dark in color and are only used when the sun is strong, especially in the hot summer sun. Many people use sunshades to block the sun from their eyes to reduce fatigue from eye conditioning or damage caused by bright light.
Transmission rating category 4
Transmission rating category 4 refers to special-purpose sunglasses with the darkest lenses and are only suitable for use in skiing, desert, beach, etc. According to the requirements, the transmission ratio of driving sunglasses should be greater than 8%, so category 4 sunglasses cannot be used while driving. Some sunglasses can cause the wearer to be unable to recognize traffic signals, resulting in accidents, because these sunglasses have a substandard traffic transmission ratio.
Sunglasses usually can be divided into sunshades, light-colored sunglasses, and sunglasses for special purposes. In outdoor activities, especially in the summer, many people use sunshades to block the sun to reduce fatigue caused by eye adjustment or injury caused by strong light stimulation. Light color sunglasses have rich color, diverse designs. Special purpose sunglasses have a strong ability to block out the sun, often used in the beach, skiing, mountain climbing, golf, and other fields with strong sunlight. Its anti-ultraviolet performance and other indicators have higher requirements.