Measuring Principles, Types and Calibration of Illuminance Meters
1. Measuring principle of illuminance meter:
Photovoltaic cells are photoelectric components that directly convert light energy into electrical energy. When light hits the surface of the selenium photovoltaic cell, the incident light passes through the metal thin film 4 and reaches the interface between the semiconductor selenium layer 2 and the metal thin film 4, generating a photoelectric effect on the interface. The magnitude of the generated potential difference has a certain proportional relationship with the illuminance on the light-receiving surface of the photocell. At this time, if an external circuit is connected, a current will flow, and the current value will be indicated on the microammeter with lux (Lx) as the scale. The magnitude of the photocurrent depends on the intensity of the incident light and the resistance in the circuit. The illuminance meter has a shifting device, so it can measure high illuminance or low illuminance. Types of illuminance meters: 1. Visual illuminance meter: inconvenient to use, low accuracy, rarely used 2. Photoelectric illuminance meter: Selenium photocell illuminance meter and silicon photocell illuminance meter are commonly used
2. Types of illuminance meters:
1. Visual illuminance meter: inconvenient to use, low precision, rarely used
2. Photoelectric illuminance meter: commonly used selenium photocell illuminance meter and silicon photocell illuminance meter
The composition and use requirements of the photoelectric illuminance meter:
1. Composition: Microammeter, shift knob, zero point adjustment, terminal, photocell, V(λ) correction filter, etc.
Commonly used selenium (Se) photocell or silicon (Si) photocell illuminance meter, also known as lux meter
2. Requirements for use:
① Photovoltaic cells use selenium (Se) photocells or silicon (Si) photocells with good linearity; they can still maintain good stability after long-term work, and have high sensitivity; when E is high, choose high internal resistance photocells, which have low sensitivity and good linearity , not easily damaged by strong light
② There is a V(λ) correction filter inside, which is suitable for the illumination of light sources with different color temperatures, and the error is small
③Add 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 rule
④ The illuminance meter should work at room temperature or close to room temperature (photocell drift changes with temperature)
3. Calibration of the illuminance meter:
Calibration principle:
Let Ls vertically irradiate the photocell → E=I/r2, change r to get the photocurrent value under different illuminance, and convert the current scale into the illuminance scale according to the corresponding relationship between E and i.
Calibration method:
Using the light intensity standard lamp, under the working distance of the approximate point light source, change the distance l between the photocell and the standard lamp, record the readings of the ammeter at each distance, and calculate the illuminance E by the inverse square law of distance E=I/r2, and then calculate the illuminance E by This can get a series of photocurrent values i with different illuminance, and make the change curve of photocurrent i and illuminance E, which is the calibration curve of the illuminance meter. From this, the calibration of the illuminance meter can be done by dividing the dial of the illuminance meter.
Factors affecting the calibration curve:
The photocell and galvanometer need to be re-calibrated when they are replaced; the illuminance meter should be re-calibrated after a period of use (generally 1-2 times within a year); the high-precision illuminance meter can be calibrated with a light intensity standard lamp; The calibration range of the illuminance meter can be changed from the distance r, and different standard lamps can also be selected, and a small-range ammeter can be selected.
