incident light frequency, exogenous photocurrent is proportional to light intensity.

When light shines on the object, it can cause the resistivity to change or generate photo-emf which is called the internal photoelectric effect. The internal photoelectric effect is divided into photovoltaic effect and photoconductivity effect. Photovoltaic effect is the phenomenon that makes objects have a certain direction of the force under the influence of light. Optoelectronic devices based on the effects are photosensitive diodes, transistors, photovoltaic cells; photoconductive effect is that under the influence of light, electrons absorb the photon energy from the bonding state of transition to a free state, which leads to material changes in conductivity. Optoelectronic devices based on this effects are photosensitive resistor and so on.

3 The Classification of Photoelectric Sensor

Photoelectric sensor classification is varied. Photoelectric sensing devices based on external photoelectric effect are photoelectric tubes and photomultiplier tubes. Photoelectric sensors based on photoconductive effects are photoresistors. Photoelectric sensors based on barrier effects are based on photodiodes and phototransistors. Photoelectric sensors based on reverse side of the photoelectric effect are reverse photodiodes. Photoelectric sensors can be classified into analog optical sensors and digital optical sensors by signal forms. Photoelectric sensors also include fiber optic sensors, solid image sensors and so on. Figure 2 shows common photoelectric sensors.

Fig. 2. Common photoelectric sensors

4 The Application of Photoelectric Sensor

With the development of technology, the use of photoelectric sensors is more and more. Photoelectric sensors have a growing number of applications in everyday life, industrial production and information technology.

4.1 The Application that Light Source Is the Measured Object

The unit of light illumination E is lx (lux), which is one of the units commonly used in photometry, it means the degree of exposure of the illuminated physical object, it can be used to measure the illuminance meter. 1)The infrared radiation thermometer

Infrared radiation thermometer has many applications in non-contact temperature measurement. Infrared radiation thermometer can be used for both high temperature measurement and below freezing temperature measurement, this is also the trend of radiation thermometer. Commercially available infrared thermometer temperature is range from -30 ℃ ~ 3000 ℃, the middle is divided into several different specifications, you can select the appropriate model according to the need. Figure 3 shows the shape of the infrared radiation thermometer. 2) Pyroelectric sensor

Pyroelectric infrared sensor can detect infrared light transmitted by person or animal and output corresponding electric signal. Pyroelectric infrared detection devices have been widely used in infrared spectroscopy, infrared remote sensing and radiation detectors.

Fig. 3. The shape of the infrared radiation thermometer

It is also used in human detection, alarm, which can produce farinfrared radiation, such as security doors, hotel lobby automatic doors, automatic light control. For example: it will automatically turn off air conditioners, drinking fountains when no one is in the room; TV can judge that no one is watching or person has to sleep and automatically shutdown the circuit. Figure 4 is the shape of the pyroelectric sensor.

Fig. 4. The shape of the pyroelectric sensor

4.2 The Application That the Measured Object Absorbs Light Energy

In this application, the measured object can absorb light energy, according to the weakening degree of light energy to achieve the target analyte measurement. 1) Photoelectric turbidimeter

Photoelectric turbidimeter is based on the principle that the transmission attenuation and scattering attenuation of infrared light are relate to the suspended sediment

concentration to achieve the concentration of sludge and suspended solids measurements. Different types of sludge and suspended matter have different attenuation to infrared light, we can adjust the transmission frequency and use different algorithms to meet different applications. 2) Smoke alarm

No smoke, a photosensitive element receives a constant infrared LED transmitter. In the event of fire, smoke comes into the testing room and blocks some of the infrared light, the phototransistor output signal is weakened, it is judging by the threshold circuit, an alarm signal is occured. Figure 5 is smoke alarms.

Fig. 5. Smoke alarms

4.3 The Application That the Measured Object Reflects Flux

This sensor uses the principle of diffuse reflection. In such sensors, emitter and receiver mounted on the same device. The light emitted by the emitter is reflected by the target object, the reflected light is in all directions, part of the reflected light reflects into the receiver, so that can detect the target object. 1) Reflective smoke alarm

In the absence of smoke, due to infrared tube is perpendicular to the interior and painted black smoke absorption material, so infrared LED infrared light can not reach the infrared phototransistor. When the smoke comes into the smoke chamber, solid particles of the smoke produce diffuse reflection towards infrared light, so that part of the infrared light reaches the phototransistor and light current output. 2) Photoelectric tachometer

Photoelectric tachometer is reflective photoelectric sensor, which can be tens of millimeters away from the measured object outside and measure the speed without contact. 3) Color sensors

Color sensors are used to detect a specific color or spot on the object, it measures the color by comparing with the zone without color, rather than direct measures color.

4.4 The Application That Measured Object Blocks the Light

The shading photoelectric sensor needs two separate cases, light-emitting device is installed in a chassis, receiver is installed in another chassis. The light emited from emitter injects to the receiver, when the target object blocks the light, the receiver's output will change.

The photoelectric detector of strip deviation. When the strip goes wrong road, the edge often collides with the sending machine, resulting in waste. When the strip is in the correct position (center position), the amplifier output voltage is zero; when the strip goes left-side, the shading area is reduced, the output voltage reflects the deviation of strip direction and size. The photoelectric detector of strip deviation is shown in Figure 6.