A photoresistor is an electronic component whose resistance decreases with increasing incident light intensity. It can also be called a light-dependent resistor (LDR), or photoconductor.
A photoresistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.
A photoelectric device can be either intrinsic or extrinsic. In intrinsic devices, the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities added, which have a ground state energy closer to the conduction band - since the electrons don't have as far to jump, lower energy photons (i.e. longer wavelengths and lower frequencies) are sufficient to trigger the device.
Applications
Photoresistors come in many different types. Inexpensive cadmium sulfide (CdS) ones can be found in many consumer items such as cameralight meters, clock radios, security alarms and street lights. At the other end of the scale, Ge:Cu photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy.
An organic photoconductor 100 is mounted on the drum and is stretched tight by stretchers 99.
Following transfer of the toner image to substrate 130, photoconductor 100 is engaged by a cleaning roller 50, which typically rotates in a direction indicated by an arrow 52, such that its surface moves in a direction opposite to the movement of adjacent surface of photoconductor 100 which it operatively engages.
According to one embodiment of the invention, organic photoconductor 100 is mounted on a stretcher 120 and tensioned to a strain of 3 Kg per cm of width of photoconductor 100.
A method of manufacturing a photoconductor is also disclosed, which includes forming a photoconductive layer by vapor deposition on a conductive substrate and thermally treating the photoconductive layer at a temperature between 100 and 200 degrees Celsius for a period between 30 and 80 minutes.
In an exposure section 2, the photoconductor 10 is exposed to light in a pattern corresponding to the image to be produced.
The period of time between when the photoconductor is first exposed to light and when the potential of the photoconductive layer surface drops is determined by the migration period of the negative charge carriers.
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