The light flux can be measured exclusively in laboratory conditions, otherwise it is simply impossible to determine the level of illumination. However, the devices for each room already have a certain marking, on the basis of which it is possible to calculate the degree of illumination of the room, expressed in lighting units. The main conditions are to know the area and have the skills of elementary calculations.
Content
- Basic characteristics of light
- Calculation features
- Measuring device
Basic characteristics of light
Light is a physical quantity, which has properties such as luminous flux power, luminosity, luminous intensity, illumination and brightness.
The power of the luminous flux, that is, the visible light radiation, can be estimated by the perceived effect of light on human vision. The luminous flux is measured in lumens.
A conventional incandescent lamp of 100 watts has a power rating of 1350 lumens, while a lamp with a fluorescent glow of the same "wattage" shows much more - 3200 lumens.
More specifically, one point source having a luminous intensity of 1 candela emits a luminous flux of 1 lumen. The solid angle in this case has a value of 1 steradian.
The next characteristic of light is its strength, which characterizes the flux density, this value is measured in candelas. In a very generalized way, it was previously believed that 1 cd is equal to the light of 1 candle, this unit was even equated to 1 W. You can increase the light intensity by installing a concave mirror reflector on one side of the light bulb.
With regard to brightness, this value is expressed in terms of the ratio of candela per square meter. m, that is, the projection of the strength of the luminous flux onto a flat illuminated surface at a right angle. Luminosity, or luminosity, reflects the density of the light flux in relation to the area of the illuminated surface - 1 lumen per square meter. m.
A characteristic of light such as illuminance demonstrates the surface density of light illuminating a specific surface area. This value is expressed through the ratio whether / sq. m. In physics, there are special units in which the illumination is measured - lux.
Calculation features
When calculating the degree of illumination of any room, it is necessary to take into account the law of additivity. It works when there are several light sources affecting a certain area.
The law of additivity is expressed in the summation of the illumination produced by each light object separately:
∑ E = E1 + E2 +… + En.
The illuminance rating is only used for surfaces that reflect light, and do not glow with their own. For example, the moon, walls, floor and any other planes. Room illumination is measured as follows:
- For a room with dimensions of 3x3x3 m, subject to the presence of a five-watt LED with a power of 100 lumens in it, it is necessary calculate the total area of all illuminated surfaces - floor, ceiling and walls, and then divide the luminous flux by square.
- This makes 100 lumens / 9 sq. m * 6 = 100/54 = 1.85 lux - units of measurement of illumination.
- If the light source is equipped with a special lens, thanks to which on one of the surfaces a circle with a diameter of, for example, 1 m will be brighter illuminated (the area of the circle, respectively, will be equal to 0.78 sq. m), then the illumination of this area will have a value of 128 lux.
There are also fairly standard indicators - under certain conditions, the light shows approximately the same values, so such situations can be generalized. Some examples of illumination include the following:
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The illumination of the earth's surface on a clear day, when the sun is at its zenith, is equal to 100 thousand. OK. In the shade at the same time, 10-25 thous. lx In a room near a window, the value reaches 100 lx. - On a cloudy day, depending on the degree of cloud cover of sunlight, the average value is approximately 1,000. OK.
- Clear weather on a full moon gives 0.2 lux.
- The illumination of the moon is 135 thousand. OK.
- For normal reading, in which the eyes are not strained too much, an average of 40 lux is needed.
- Most plants need 500 lux for normal life, but different species require a different spectrum ratio.
Measuring device
There is a special device for measuring the level of illumination of a surface - light meter. His device includes a photocell that captures light. The mechanism of operation of the luxmeter can be both digital and analog - in both cases, the measurement accuracy is quite high. GOST assumes a maximum error of about 10%.
In many designs, the area containing the photocell is connected to the rest of the area using an elastic twisted wire for work in hard-to-reach places. The device is equipped with light filters, using which you can adjust the measurement process, taking into account the special nuances of the terrain.
When working with the device, it should be placed horizontally - any deviations from this plane can negatively affect the measurement accuracy. And also it is necessary to avoid the influence of random shadows. A detailed methodology for each type of lighting is described in the corresponding state standard.
