Introduction to the testing principle and method of the illuminance meter
1. The principle of illuminance test
Illuminance is the areal density of the luminous flux received on the illuminated plane. The illuminance meter is an instrument used to measure the illuminance on the illuminated surface, and it is one of the most widely used instruments in illuminance measurement.
2. Structural principle of illuminance meter
The illuminance meter is composed of two parts: a photometric head (also known as a light receiving probe, including a receiver, a V (λ) pair filter, and a cosine corrector) and a reading display.
Measurement steps and methods
In a work room, the illuminance should be measured at each work location (eg desk, workbench) and then averaged. For an empty room or non-working room without a confirmed working location, if general lighting is used alone, it is usually selected
The illuminance is measured on a horizontal plane at a height of 0.8m. Divide the measurement area into grids (or close to squares) of equal size, measure the illuminance Ei at the center of each grid, and its average illuminance is equal to the average of the illuminance at each point, that is
Where Eav - the average illuminance of the measurement area, lx;
Ei ——the illuminance at the center of each measurement grid, lx;
N - the number of measuring points.
Uniformity of illuminance refers to the ratio of the minimum illuminance to the average illuminance on the specified surface, namely:
In the formula, Emin——refers to the minimum illuminance on the surface to be measured, lx.
In this experiment, the surface of the measuring point arranged in the room can be used as the designated surface, and the minimum illuminance can be regarded as the minimum illuminance value in the measured point.
Measure the side length of each square in the room as lm, and a large room can be
Where Eav - the average illuminance of the measurement area, lx;
Ei ——the illuminance at the center of each measurement grid, lx;
N - the number of measuring points.
Uniformity of illuminance refers to the ratio of the minimum illuminance to the average illuminance on the specified surface, namely:
In the formula, Emin——refers to the minimum illuminance on the surface to be measured, lx.
In this experiment, the surface of the measuring point arranged in the room can be used as the designated surface, and the minimum illuminance can be regarded as the minimum illuminance value in the measured point.
Measure the side length of each square in the room as lm, and a large room can take 2-4 m. In narrow and long traffic sections such as aisles and stairs, the measuring points are arranged along the centerline of the length direction, with an interval of 1-2 m
; The measurement plane is the ground level or a horizontal plane 150mm above the ground.
The greater the number of measuring points, the more accurate the average illuminance value obtained, but it also takes more time and effort. If the allowable measurement error of Eav is ±10%, the workload can be reduced by selecting the least measuring points according to the room shape index. The relationship between the two is listed in Table 1. If the number of lamps is exactly equal to the number of measuring points given in the table, the measuring points must be added.
Chamber index Kr Minimum number of measuring points Chamber index Kr Minimum number of measuring points
＜1 4 2～3 16
1～2 9 ≥3 25
In the formula, L and W are the length and width of the room, and hr is the height from the lamp to the measuring plane.
When local lighting is used to supplement general lighting, the illuminance of the working point should be measured according to the normal working position of the person, and the photocell of the illuminance meter should be placed on the working surface or the operating surface for visual operations.
An ideal lux meter should have the following conditions:
● Small size and light weight (Compact Size, Light Weight)
The opportunity to use the illuminance meter is very wide, and the timing of application is often in different places, so the first prerequisite for the portable illuminance meter is small size and light weight.
● Accuracy﹝Accuracy﹞
Whether the illuminance meter is good or not has an absolute relationship with its accuracy. Of course, it is also closely related to its price, so it is necessary to buy a lux meter with high accuracy at a reasonable price. Generally, the error should not exceed ±15%.
● Color Compensation﹝Color Compensation﹞
The types of light sources are all-encompassing. Some focus on red high-pressure lamps with longer wavelengths, or blue-violet ones with shorter wavelengths such as Daylight fluorescent lamps; there are also more evenly distributed ones such as incandescent bulb series. The sensitivity of the same illuminance meter to different wavelengths may be slightly different. Therefore, appropriate compensation is necessary.
● Cosine Compensation﹝Cosine Compensation﹞
We all know that the brightness of the illuminated surface is related to the incident angle of the light source. In the same way, when measuring with a light meter, the incident angle between the sensor (Sensor) and the light source will naturally affect the reading value of the light meter. Therefore, whether a good illuminance meter has the function of cosine compensation cannot be ignored.
