The unit of measure for the brightness or intensity of light is lumens. For example, indoors, most of the artificial light intensity of 400 to 600 lumens. If the sun in the sun outdoors, the brightness will float in a wide range, the brightness of the shade at 1000 lumens, and like a large highway and other places where the concrete, brightness of 6000 lumens. In the 3500 lumens below the brightness, we are feeling very comfortable eyes. When the direct light or reflected light to reach the brightness of 4000 lumens, the human eye to receive light began to become difficult. When we try to see these very bright areas, only to see the white flash, which is glare. In order to reduce the discomfort caused by a lot of light into the eyes, we began to squint. Once the brightness reaches 10,000 lumens, your eyes begin to completely filter out the light when the received light is too strong. Exposure to such a long time in such a strong light can cause damage, resulting in temporary or even permanent blindness. On a clear day, a large snow reflects up to 12,000 lumens of brightness, and if we look at the light without any protective measures, it will lead to "snow blindness".
Visible light is the light that the human eye can perceive. We see the visible light emitted by the sun is colorless, that is, we often say that white light. In fact, it is made of a mixture of colored colors of colored light. The light that is mixed with all colored light in the spectrum of visible light is colorless, that is white. (See the principle of light for more information).