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Comparison Of Visible Light Sensor With Photosensitive Tube.
Apr 11, 2018

The application of optical sensors is very common. From the portable consumer market to the consumer television market to the medical, industrial and automotive markets, light sensors are ubiquitous. Some of these applications, such as barcode readers, laser printers and auto focus microscopes, use the reflected light of optical probes to sense the position. Other apps, such as digital cameras, mobile phones and laptops, use light sensors to measure ambient light.

The application of optical sensors is very common. From the portable consumer market (smart phone, PDA, desktop PC and portable music players, etc.) to the consumer television market (including LCD, plasma, rear projection and CRT TV, etc.), to the medical, industrial and automotive markets, light sensors is everywhere. Some of these applications, such as barcode readers, laser printers and auto focus microscopes, use the reflected light of optical probes to sense the position. Other apps, such as digital cameras, mobile phones and laptops, use light sensors to measure ambient light.

Optical sensors include photosensitive resistors, phototransistors or photodiodes. Among them, the simplest light sensor is photosensitive resistor. Low - end photosensitive resistors are manufactured from CdS(cadmium sulfide) materials, while more expensive photosensitive resistors are made by GaAs materials. The band gap of GaAs is small. It absorbs the low energy photons in the infrared light and makes the electrons jump to the conduction band. Its illumination range is from 1 lux to 100 lux.

The complexity of photodiodes is higher. Photons bombard the semiconductor junction to produce an electric current. To apply reverse bias to the photodiode. Large reverse bias can improve the sensor speed and linearity, but also improve the sensor's dark current and shotnoise. The photon bombards the semiconductor junction and produces a forward current, reducing the reverse bias current. In the design, the external circuit can be added to the photodiode to linearize the i-v curve.

The general characteristics of the photoelectric transistor are the same as that of the photodiode, but the amplification function is increased. It requires a larger bias current, but the noise associated with the current forces the sensor's sensitivity toward a higher lux range, i.e., 1000 lux ~ 100, 000 lux. The detection response time of photoelectric transistor is similar to that of photodiode, and can be adjusted by bias current. The offset current can also be changed with the detected signal level. Phototransistor can roughly determine the ambient light levels, such as indoor/outdoor, day/night as well as the bright light/shadow and so on, therefore, need external calibration circuit of the output signal.