Principle and application of illuminance meter

　　Principle of illuminance meter

　　An ordinary pocket electronic illuminance meter uses a photoresistor as a photoelectric conversion device, which includes a DC power supply, a voltage conversion circuit, a switch, a photoresistor, a test circuit, an A/D converter, a decoder, a display driver and a display connected in series at the output end of the DC power supply.

　　The utility model avoids the use of a photometric head, and the DC power supply only uses a 3V battery. The decoder, display driver and display are integrated on an integrated circuit card, making it simple in structure, small in size, easy to move, and also has the advantages of sensitive response.

　　Illumination test principle

　　Illuminance is the surface density of the luminous flux received on the illuminated plane. Illuminance meter is an instrument used to measure the illuminance on the illuminated surface and is one of the most commonly used instruments in illuminance measurement.

　　The structural principle of the illuminance meter

　　The illuminance meter consists of two parts: a photometer head (also called a light-receiving probe, including a receiver, a V(λ) pair filter, and a cosine corrector) and a reading display.

　　An ideal pocket illuminance meter should have the following conditions:

　　Small size and light weight

　　Lux meters are widely used and often used in different places, so the first prerequisite for a lux meter is that it should be portable, small in size and light in weight.

　　Accuracy

　　The quality of a light meter is absolutely related to its accuracy. Of course, it is also closely related to its price. Therefore, it is necessary to buy a light meter with high accuracy at a reasonable price. Generally, the error should not exceed ±15%.

　　Color compensation

　　There are many types of light sources. Some are more focused on red high-pressure lamps with longer wavelengths, or shorter blue-violet wavelengths such as Daylight fluorescent lamps; there are also more evenly distributed ones such as the incandescent bulb series. The sensitivity of the same illuminometer to different wavelengths may be slightly different, so appropriate compensation is necessary.

　　Cosine compensation

　　As we all know, the brightness of the illuminated surface is related to the incident angle of the light source. For the same reason, when using a light meter for measurement, the sensor and the incident angle of the light source will naturally affect the reading of the light meter. Therefore, whether a good light meter has a cosine compensation function cannot be ignored.

　　Lux meter precautions

　　① Photocells should use selenium (Se) photocells or silicon (Si) photocells with good linearity; they can maintain good stability and high sensitivity after long-term operation; when E is high, high internal resistance photocells should be used, which have low sensitivity and good linearity and are not easily damaged by strong light.

　　② It has a built-in V(λ) correction filter, which is suitable for illumination with light sources of different color temperatures and has a small error.

　　③ Add a cosine angle compensator (opalescent glass or white plastic) in front of the photocell because when the incident angle is large, the photocell deviates from the cosine law.

　　④ The illuminance meter should operate at room temperature or close to room temperature (photocell drift changes with temperature).