Types of illuminance meters and the principles of their measurement
Types of illuminometers and measurement principles An illuminometer (or lux meter) is an instrument that specializes in measuring photometry and brightness. The measurement of light intensity (illuminance) is the degree to which an object is illuminated, that is, the ratio of the luminous flux obtained on the surface of the object to the illuminated area. An illuminance meter usually consists of a selenium photovoltaic cell or a silicon photovoltaic cell and a microammeter.
Light meter measurement principle:
Photocells are optoelectronic components that convert light energy directly into electrical energy. When light strikes the surface of the selenium photovoltaic cell, the incident light passes through the metal film 4 and reaches the interface between the semiconductor selenium layer 2 and the metal film 4, producing a photoelectric effect on the interface. The magnitude of the potential difference generated has a certain proportional relationship with the illumination on the light-receiving surface of the photovoltaic cell. At this time, if an external circuit is connected, current will flow through, and the current value will be indicated on the microammeter with lux (Lx) as the scale. The size of the photocurrent depends on the intensity of the incident light and the resistance in the loop. The illuminance meter has a gear shifting device, so it can measure both high and low illuminance.
Types of illuminance meters:
1. Visual illuminance meter: inconvenient to use, not very accurate, rarely used
2. Photoelectric illuminance meter: Commonly used selenium photovoltaic illuminance meter and silicon photovoltaic illuminance meter
The composition and usage requirements of photocell illuminance meter:
1. Composition: microammeter, shift knob, zero point adjustment, terminal, photocell, V(λ) correction filter, etc. Selenium (Se) photovoltaic cells or silicon (Si) photovoltaic cells are commonly used as illuminance meters, also known as lux meters.
2.Usage requirements:
① Photovoltaic cells should use selenium (Se) photovoltaic cells or silicon (Si) photovoltaic cells with good linearity; they can maintain good stability for a long time and have high sensitivity; when the E is high, use photovoltaic cells with high internal resistance, which have low sensitivity and good linearity. , not easily damaged by strong light exposure
② There is a V (λ) correction filter inside, which is suitable for illumination of light sources with different color temperatures and has small errors.
③Add a cosine angle compensator (opalescent glass or white plastic) in front of the photovoltaic cell. The reason is that when the incident angle is large, the photovoltaic cell deviates from the cosine law.
④The illuminance meter should work at room temperature or close to room temperature (photocell drift changes with temperature changes)
Calibration of light meter:
Calibration principle:
Let Ls illuminate the photocell vertically→E=I/r2. By changing r, the photocurrent values under different illumination can be obtained. The current scale is converted into the illumination scale based on the corresponding relationship between E and i.
Calibration method:
Use a light intensity standard lamp to change the distance l between the photovoltaic cell and the standard lamp at an approximate working distance of the point light source, record the readings of the ammeter at each distance, and calculate the illuminance E according to the inverse square distance law E=I/r2, as This can get a series of photocurrent values i with different illuminances, and draw the change curve of photocurrent i and illuminance E, which is the calibration curve of the illuminance meter. From this, the dial of the illuminance meter can be graded, which is the calibration curve of the illuminance meter.
Factors affecting the calibration curve:
Photocells and galvanometers need to be recalibrated when replaced; the illuminance meter should be recalibrated after being used for a period of time (generally it should be calibrated 1-2 times a year); high-precision illuminance meters can be calibrated with light intensity standard lamps; expand The calibration range of the illuminance meter can change the distance r, or different standard lamps can be used, and a small-range galvanometer can be used.
