From the term "architectural lighting design standard GB 50034-2013" we know that the luminous flux d Φ incident on the bin containing this point is divided by the quotient of the bin area dA.

The unit is lux (lx), 1 lx = l lm / m2 (lumen per square meter). Illuminance is an objective quantity, which has nothing to do with the feeling of being photographed and people. For example, according to experience and continuous summarization, the standard stipulates that the office work surface illumination is 500lx. In such a visual environment, people can work for a long time. As for whether it is comfortable or not, There are other assessment criteria.

However, we must know that the quantity specified in the standard is to maintain the average illuminance and uniformity of illumination. At the same time, the standard also stipulates that, under normal circumstances, the illuminance value of the design may be compared with the standard value of the illuminance, and may not exceed ±10%. Deviation.

Illuminance is a vector with size and directionality. All illuminance can be decomposed into illuminance values ​​in different directions (ie, angles). Usually only horizontal illuminance and vertical illuminance are considered. The horizontal illuminance is the illuminance in the direction perpendicular to the illuminated surface (see the figure below).

Vertical illumination is the illuminance in the direction parallel to the illuminated surface (see figure below).

In addition, there is also the definition of semi-cylindrical illuminance, which refers to the average illuminance on the side surface of a sufficiently small semi-cylindrical body that is placed vertically on the calculation point, that is, an infinitesimal vertical half. The average illuminance on the cylinder (see figure below).

When we work, we emphasize the horizontal illumination of the working surface, but we can't see the observations. What color, shape and stereoscopic effect is the observation? This requires us to understand another very important concept, what is brightness?

What is brightness?

The amount defined by the formula L = d2Φ / (dA · cos θ · dΩ). The unit is Candela per square meter (cd/m2).

In the formula:

dΦ ----- the luminous flux (lm) transmitted by the beam element at a given point and containing the solid angle dΩ in a given direction;

dA----- includes the beam cross-sectional area (m2) at a given point;

θ ----- The angle between the normal of the beam section and the beam direction.

It can be seen that the brightness is consistent with the physical dimension of the illuminance. The general brightness is the amount of visible light reflected by the illuminated surface in the illuminance, determined by the reflection coefficient and the reflection angle of the reflective surface. The brightness is an objective measurement value, by the space environment, The light intensity and direction determine factors, which is the meaning of brightness itself.

But the brightness of the reaction in our eyes is called the brightness. The brightness is a subjective evaluation of the observer. It indicates how much of the reflected visible light enters the human eye and is perceived. The brightness depends largely on the exposure. The brightness of the surface, but also related to other factors, such as the age of the observer, the overall brightness distribution in the observer's field of view, even two surfaces with the same brightness may produce different brightness (grey in the picture) The squares look darker against the backdrop of a black background, but in fact the brightness is the same).

At the same time, the brightness also has two brightnesses of incident and outgoing, for example, the brightness of the surface of a fluorescent tube is about 1.0 cd/m2, and the brightness of the working surface using the fluorescent lamp as the illumination device may be greater than 100 cd/m2.

What is the application of illuminance and brightness?

Illuminance is mainly used to measure the level of lighting quality and is the minimum requirement that people can see. Brightness is mainly used to judge the atmosphere of the lighting space, which is an amount that people can see comfortably, but not the only amount. In the case of the same illuminated surface, the illuminance is determined by the luminous flux, which can be measured with an illuminometer, but must be greater than 3 cd/m2 under the conditions of bright vision, ie the surface to be tested.

The brightness is measured using a luminance meter, which is related not only to the luminous flux, but also to the spectral light visual function, that is, to the composition of the spectrum contained in the visible light.

All indoor space lighting needs to assess the horizontal illumination of the working surface, but pay attention to the adaptability of illumination and color temperature. The lower the general illumination, the lower the color temperature, such as the high illumination of the stadium, the high color temperature, not only the comfort, but also the broadcast. The requirement, the illuminance of the camera at the same time, is actually the application of the semi-cylindrical illumination.

Road lighting and facade lighting need to assess the surface brightness, the key is to highlight the details, but also need to see more clearly in the middle of the visual situation.

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