The basic concept of lighting

    1. the nature of light
    Light refers to a part of the radiant energy emitted by the light source, that is, the radiant energy that can produce vision, so it is also called "visible light". From the point of view of physics, light is a kind of electromagnetic radiation energy, namely electromagnetic wave. The various electromagnetic waves are spread out according to the wavelength in turn, which becomes the electromagnetic spectrum.

    2. Visible light
    It is known from the electromagnetic spectrum that visible light accounts for a small part of electromagnetic waves, and its wavelength is in the range of 380~780nm (1nm=10-9m), and different wavelengths of light give people different color feelings. As the wavelength increases from 380nm to 780nm, the color of the light starts from purple and gradually changes in the order of blue, cyan, green, yellow, orange, and red. There is no clear division between the two colors, but a transition by the gradual decrease of one color and the gradual increase of the other. All visible light waves are mixed together to form daylight (white light) The electromagnetic wave emitted by the sun has a wavelength greater than 1400nm and is strongly absorbed by water vapor and carbon dioxide in the lower atmosphere; Wavelengths smaller than 290nm are absorbed by ozone in the upper atmosphere. The wavelength of an electromagnetic wave that reaches the Earth's surface matches that of visible light. This shows that the visual response of the human eye to light is the result of adapting to the light transmission effect of the Earth's atmosphere in the course of human evolution.

   3. Luminous flux
   The effect of lighting is ultimately assessed by the human eye, so it is not enough to describe the optical characteristics of various types of light sources only with energy parameters, but also must introduce a light quantity parameter based on human vision - luminous flux to measure.

   The energy that a light source radiates into the surrounding space and causes vision per unit time, called luminous flux, is expressed in symbols and is measured in lumens (Im).

    Because the sensitivity of the human eye to different wavelengths of light is not the same, such as in the daytime or where the light is strong, the wavelength of 555nm of yellow, green light is the most sensitive, the farther the wavelength from 555nm, the lower the sensitivity, so the luminous flux is not only related to the strength of the radiation, but also related to the wavelength of the radiation.

     It has been proved by experiments that when the wavelength of 555nm yellow and green light radiation power is 1W, the human eye sensation is 680lm, visible 1lm is equivalent to the wavelength of 555nm monochromatic radiation power is (1/680) W luminous flux.

    Luminous flux is an important parameter of electric light source and an essential data for lighting design. However, comments on the characteristics of electric light sources are often based on optical apparent efficiency. The optical efficiency of an electric light source is expressed in terms of the number of lumens produced by consuming 1W of electrical power, and the unit is current/watt (lm/W). The higher the optical efficiency, the better.

    4. Luminous intensity
    There is an uncovered incandescent lamp above the table, and after adding the lamp shade, the table looks much brighter. The same bulb without a lampshade and a lampshade, it emits the same luminous flux, but with the lampshade, the light reflected by the lampshade, so that the distribution of luminous flux in space has changed, and the luminous flux to the desktop has increased than that without a lampshade. Therefore, in electrical lighting technology, it is not enough to only know the total luminous flux emitted by the light source, but also to understand the distribution of luminous flux in all directions in space.

    The luminous flux space scale radiated by the light source in a unit solid Angle (per sphericity) in a specific direction of space is called the luminous intensity of the light source in that direction (referred to as light intensity), which is represented by the symbol 10 and the unit is Candela (cd). Its calculation formula is as follows:

    Iθ=φθ/ωθ the solid Angle (sr) corresponding to the sphere of ωθ---- in the formula

    Luminous flux (Im) radiated by φθ---- in ω solid Angle

    We know that the light emitted by the light source radiates in all directions of space, so the three-dimensional Angle must be used as the measurement unit of the space beam to calculate the density of the luminous flux. The Angle formed by a certain area of the sphere towards the center of the sphere is called a solid Angle. The unit of solid Angle is "sphericity" (sr), that is, to make a sphere with r as the radius, if the area S cut out by the cone on the sphere is equal to r2, the solid Angle is a unit solid Angle, called "sphericity" (sr). The solid Angle corresponding to the whole sphere of the circle is: ω=4πr2/r2=4π(sr)

    Because the luminous flux emitted by the light source to the space is not uniform, the light intensity should be indicated in which direction, and the subscript Angle is indicated, such as 1θ, l180° and so on. In the lighting design, the selection of illuminator and the calculation of illumination must be based on the light distribution curve. That is, the curve of the luminous intensity of the light source in all directions is represented by the polar coordinates, which is called the light distribution curve of the light source.    

    5. Illuminance

    Illuminance is used to indicate the intensity of the illuminated surface (point), the ratio of the luminous flux projected to the illuminated surface and the area of the illuminated surface is called the illuminance of the surface, expressed by the symbol E. The definition is as follows:

    E = phi/S

    Where φ---- receives luminous flux (Im) on the exposed surface

    S---- illuminated area (m2) where the above formula is illuminance on a vertical plane. If the illuminated surface is not perpendicular to the incident light, the calculation formula is: E=Iθcosθ/r2, the light intensity of the incident direction of the Iθ---- light (cd); θ---- the incidence Angle of light; r-- The distance from the light source to the illuminated surface (m) The unit of illuminance is lux (lx), indicating that the luminous flux of 1 lumen is evenly distributed on the illuminated surface of 1 square meter, that is: 1(lx)=1(Im)/m2 In order to have a perceptual understanding of the illuminance value, cite several actual illuminance values for reference. The illumination at 1 meter from the 40W incandescent lamp is about 30lx, and increases to 731x after adding an enamel lamp shade; The outdoor illumination at noon on a sunny day can reach 8x104~12x104 lx; At noon on a cloudy day, the outdoor illumination is about 8x103~20x103 lx; The full moon illuminance on the ground is 0.2lx.

    6. Brightness
    In the same position in the room, and discharged a black and a white object, although their illuminance is the same, but the human eye looks much brighter white objects, which shows that the illuminance of the surface of the object can not directly express the visual sensation of the human eye. This is because the visual perception of the human eye is produced by the illuminance formed on the retina of the eye by the light or reflection of the object being illuminated. The higher the amount of illumination formed on the retina, the brighter the human eye perceives it. White objects reflect light much more strongly than black objects, so white objects feel much brighter than black objects. If the object is viewed as a luminous body, the illumination on the retina is caused by the luminous intensity of the object in the direction of the line of sight.

    The luminous intensity of the luminous body in the line of sight on the unit projector plane is called the brightness of the surface of the object, expressed by the symbol L, the unit is Candela per square meter (cd/m2). The expression is: l-lθ /Scosθ formula lθ---- the luminous body's light intensity in the line of sight direction (cd); Scoseθ---- the projected area of the luminous body in the line of sight direction m2; θ---- The Angle between the line of sight and the normal (vertical) of the luminous plane.

    Brightness is a parameter that indicates the brightness of the light source, the larger the L, the brighter. But being able to see an object isn't all about brightness. If the brightness of the luminous surface is too large, it feels dazzling and can not see the object clearly.

    While receiving light, the illuminated surface reflects part of the luminous flux into space, so the illuminated surface is actually a bright light source. In the case that the orientation of the light source is certain and the light received by the illuminated surface is also certain, the luminous flux reflected in different directions will also be different, and then the light intensity reflected in different directions will be different. Because of this, under the same light source, the brightness of the light is different in all directions. When the orientation of the light source and the reflection coefficient of the illuminated surface are fixed, the brightness of the illuminated surface facing a certain direction is proportional to its illumination.

    The brightness of the light reflected by the illuminated surface is related to the direction, and it is difficult to determine, usually do not specify what standard the brightness of the illuminated surface should reach, but specify what standard the illuminance size should reach.

    In the above introduction, the last four are commonly used luminosity units, which express the optical properties of objects from different angles. light

    Flux indicates the amount of light emitted by the luminous body; Luminous intensity is the luminous flux density emitted by the luminous body in a certain direction, which indicates the distribution of luminous flux in space. Illuminance represents the luminous flux density accepted by the illuminated surface, which is used to identify the illumination of the illuminated surface. Brightness indicates the luminous intensity per unit surface area of the luminous body, which indicates how bright an object is.