Basic terms of light

1. The essence of light: the essence of light is electromagnetic wave, which is a very small part of the whole electromagnetic spectrum. Light is a form of energy; Visible light is the part of electromagnetic radiation spectrum that can cause human vision. Visible light forms part of the so-called electromagnetic spectrum that exists in radio and television signals, including infrared and ultraviolet radiation, X-rays, nuclear radiation and cosmic radiation. Among these electromagnetic radiations, only light waves are visible to the eyes of animals and human beings. The spectrum also includes standard 50 Hz AC (wavelength 6000 km) and visible light at 380-780 nm (= 10-9 m). Different wavelengths give the human eye different color sensation, from red, orange, yellow, green, blue, indigo (i.e. blue violet) to purple.

2. Luminous flux (beam): refers to the light (number) emitted by the light source, and the unit is lumen (LM). For example, if an energy-saving lamp emits 780 (beams), the total luminous flux (beam) is 780 lumen.

3. Brightness: the definition is almost the same as illumination. If we regard every object as a light source, then brightness is to describe the degree of brightness of the light source, and illumination is to regard every object as a illuminated object. Take a board as an example to illustrate how much illumination the board has when a certain light beam hits the board, and then how much light beam the board reflects to the human eye, It's called the brightness of the board, so there is the following formula: the brightness is equal to the illuminance times the reflectivity. In the same room, at the same location, the illuminance of a white cloth and a black cloth are the same, but the brightness is different.

4. Illuminance: the light passing through each unit area, unit: LX. (LM / m2)

5. Light intensity: the number of light passing through 1 solid angle, (the density of passing beam). The unit of light intensity is the basic unit of photometry and one of the basic units of international system of units. In order to reproduce the unit of light intensity, the reference device of light was originally the candle, so the unit of light intensity has long been called (candlelight). Later, with the development of science and technology, the light standard was changed to tungsten filament lamp, and then to black body. After 1948, the light intensity unit was officially named candela (CD).

6. Glare, how to control glare: there are extremely bright objects or strong brightness contrast in the field of vision, which can cause discomfort or reduce vision, called glare. The glare that causes vision loss is called disability glare; The glare that makes people feel uncomfortable is called uncomfortable glare. There are two ways to control glare: 1. Directly control the brightness of the light source or use transparent materials to reduce glare; 2. Glare is controlled by protection angle of lamps.

7. Color temperature: because people use the absolute temperature of the complete radiator which is equal to or similar to the chromaticity of the light source to describe the color table of the light source, the color table of the light source is also called the color temperature of the light source. Color temperature: when the color of light emitted by the light source is the same as that radiated by the blackbody at a certain temperature, the temperature of the blackbody is called the color temperature of the light source. 

8. Color temperature psychology color temperature below 3300K has stable atmosphere and warm feeling; The color temperature between 3000-5000k is the middle color temperature, which has a refreshing feeling; When the color temperature is above 5000K, it feels cold. Under the illumination of high color temperature light source, if the brightness is not high, it will give people a kind of cloudy atmosphere; Under the illumination of low color temperature light source, high brightness will give people a muggy feeling. When two kinds of light sources with large color difference are used in the same space, the contrast will have a hierarchical effect. When the light color contrast is large, the brightness level and the light color level can be obtained at the same time. 

9. The physiological basis of color vision: the light emitted by an object passes through the cornea, aqueous humor, lens and vitreous body, making the object image focus on the retina. The photoreceptor cells of the retina turn into nerve impulses, and then transmit to the advanced visual center of the brain to produce vision. There are two types of photoreceptor cells in human retina: rod cells and cone cells. The light sensitivity of rod-shaped cells is very high, while that of cone-shaped cells is very low. Therefore, only rod-shaped cells work under weak illumination. With the increase of brightness, the role of cone-shaped cells increases, and finally cone-shaped cells play a leading role. Rod cells can't distinguish colors, only cone cells have color sense.