Types and measurement principles of illuminometers
The types and measurement principles of illuminometers (also known as luxmeters) are specialized instruments for measuring luminosity 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 illuminometer is usually composed of a selenium or silicon photocell and a microampere meter.
Measurement principle of illuminometer:
Photocell is a photoelectric element that directly converts light energy into electrical energy. When light strikes the surface of a selenium photocell, the incident light passes through the metal film 4 and reaches the interface between the semiconductor selenium layer 2 and the metal film 4, generating a photoelectric effect at the interface. The magnitude of the potential difference generated is proportional to the illumination on the surface of the photocell receiving light. At this point, if an external circuit is connected, a current will pass through, and the current value will be indicated on the microampere meter with a scale of Lx. The magnitude of photocurrent depends on the strength of the incident light and the resistance in the circuit. The illuminometer has a shift device, so it can measure both high and low illuminance.
Type of illuminometer:
1. Visual illuminometer: inconvenient to use, low accuracy, rarely used
2. Photoelectric illuminometer: The composition and usage requirements of commonly used selenium photocell illuminometer and silicon photocell illuminometer:
1) Composition: microampere meter, shift knob, zero adjustment, terminal post, photocell, V（ λ) Correct the composition of filters and other components. Commonly used selenium (Se) photocell or silicon (Si) photocell illuminometer, also known as lux meter
2) Usage requirements:
① Photocells should use selenium (Se) photocells or silicon (Si) photocells with good linearity; Maintains good stability and high sensitivity even after prolonged work; When using high E, choose photocells with high internal resistance, which have low sensitivity and good linearity, and are not easily damaged by strong light irradiation
② Internal payment with V（ λ) Correct the filter, suitable for using the illumination of a heterochromatic temperature light source, with small error
③ The reason for adding a cosine angle compensator (milky white glass or white plastic) in front of the photocell is that when the incident angle is large, the photocell deviates from the cosine rule
④ The illuminometer should operate at or near room temperature (the drift of the photocell changes with temperature)
Calibration of illuminometer:
Let Ls vertically irradiate the photocell → E=I/r2, change r to obtain the photocurrent values under different illuminances, and convert the current scale into the illuminance scale based on the corresponding relationship between E and i.
Calibration method:
By using a light intensity standard lamp and changing the distance l between the photocell and the standard lamp at an approximate working distance of a point light source, the readings of the ammeter at each distance are recorded. The illuminance E is calculated using the inverse distance square law E=I/r2. From this, a series of different illuminance photocurrent values i can be obtained, and the variation curve between photocurrent i and illuminance E can be drawn, which is the calibration curve of the illuminometer. This can be compared to the dial of the illuminometer, which is the calibration curve of the illuminometer.
