What is the lux?

LED flood light intensity refers to the energy of visible light per unit area, referred to as illuminance, unit lux (Lux or Lx). A physical term used to indicate the intensity of illumination and the amount by which the surface area of an object is illuminated. In photometry, “luminosity” is the density of luminous intensity in a given direction, but is often misunderstood as illuminance. The international unit of luminosity is the candlelight received per square meter (China, Hong Kong and Macao called Candela).

Illumination/illuminance on a surface illuminated by light is defined as the luminous flux that illuminates a unit area. Let the luminous flux on the bin dS be dΦ, then the illuminance E on this bin is: E=dΦ/dS. 1 lx = 1 lm/m2. An object that is uniformly illuminated by light has an illuminance of 1 lux when the luminous flux obtained on an area of 1 square meter is 1 lumen. Lumens are the units of luminous flux.

Light intensity and photosynthesis intensity curve

Light intensity and photosynthesis intensity curve

A point light source having a luminous intensity of 1 candle, the luminous flux emitted in a unit solid angle (1 steradian) is “1 lumen”. Candlelight (Candela), transliteration “Candela”. The concept of candlelight was first invented by the British, and it is the unit of Luminous intensity. At that time, the British used a pound of white wax to create a light that was emitted by a one-foot candle to define the candle unit. Today’s definition has changed: heating with a cubic centimeter of black illuminator until the illuminant melts into a liquid, 1/60 of the amount of light emitted is the standard source, and candlelight is the standard source. The unit of light emitted.

How to caculate the Illumination?

The lighting calculation has the following calculation methods:

 1. Calculation of average illuminance (utilization coefficient method, estimated curve method) 

 2. Calculation of point source illuminance 

3. Calculation of line source illuminance 

4. Calculation of brightness of surface source illuminance

Indoor lighting is recommended to use the calculation of the average illuminance (using the coefficient method), the specific calculation method is:


Eav in the formula – the average illuminance on the work surface, lx;

Φ——light source luminous flux, lm;

N – the number of light sources;

U – utilization factor;

A——working area, m2;

K——maintenance factor of the luminaire.

The method takes into account the luminous flux that is directly projected onto the working surface by the light source and the luminous flux that is reflected by the indoor surface and then projected onto the working surface. The coefficient method is suitable for indoor general illumination with uniform arrangement of lamps, high reflection coefficient of walls and ceilings, and no large equipment for shielding. However, it is also suitable for outdoor illumination where lamps are evenly arranged. This method is more accurate.

The procedure for calculating the average illuminance using the coefficient method:

The first step is to fill in the original data;

The second step calculates the spatial ratio;

The third step is to find the effective ceiling space reflectance;

The fourth step calculates the average reflectance of the wall surface;

The fifth step is to check the maintenance factor of the luminaire;

The sixth step is to use the coefficient table to check the utilization factor (manufacturer sample or check the table in Chapter 4 of the Lighting Design Manual);

The seventh step calculates the average illuminance.

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