LIGHT SENSORS
Light sensors are devices used to detect and react to different level of light in
different appliances. For some light sensors, they are more commonly known as
photoelectric devices because they could convert light energy into electricity.
Types of light sensors:
1. The Photoconductive Light Sensor
2. Photojunction Devices
3. Photovoltaic Light Sensor
1. Photoconductive light sensor
Different from other light sensors, the photoconductive light sensor is not a device for
producing electricity but changes its physical properties such as resistance when the light
irradiates on the sensor. The most common type of photoconductive device is called
photoresistor which changes its resistance when light intensity varies. Photoresistor sensors
are also known as light-dependent sensors. They are widely used for gauging and
responding to light levels.[2]
When incident light on a photoresistor exceeds a certain frequency, photons absorbed by
the semiconductor give bound electrons enough energy to jump into conduction band. The
resulting free electrons conduct electricity, therefore lowering the resistance of photoresistor.
[4]The unit of measurement for photoresistor is Ohm.
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Figure 1. Photoresistor
Applications: Photoresistors can be used in camera light meters, street lights, clock
radios, alarm devices, night lights, outdoor clocks, solar street lamps and solar road studs.
They are also used in some dynamic compressors together with a
small incandescent or neon lamp, or light-emitting diode to control gain reduction.
2. Photojunction Devices
Photojunction devices are made from silicon semiconductor PN-junctions which are sensitive
to the light and could detect both visible and infra-red light. Photojunction devices are
specifically made for sensing light and basically two types sensors are included which are
photodiode and phototransistor.
2.1 Photodiode
The structure of photodiode is quite similar to the PN-junction diode except that the outer
casing has a transparent lens to focus the light on the photodiode. The current-voltage
characteristic of photodiode in the dark mode is very similar to a normal rectifying diode. The
same as for a normal diode, when the photodiode is forward biased, the current shows an
exponential increase. When the reverse bias is applied, a reverse saturation current causes
an increase of depletion region.
When light falls on the junction, more hole/electron pairs are produced and the leakage
current increases when the light intensity increases. As seen from the figure below, the
photodiode current is proportional to the light intensity falling on the PN-junction. This
photodiode can be used for the accurate measurement of light intensity. By measuring the
current (A) through the photodiode, the intensity of light can be measured. [2]
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Figure 2. Photodiode
Applications: Photodiodes are often used in electronics devices such as CD players, smoke
detectors and the receiver for remote control devices. Because of the high light sensitivity,
photodiodes are widely used in medical applications, such as detectors for computed
tomography.
2.2 Phototransistor
Phototransistor can be considered as a photodiode with amplification. Thus phototransistor
operates the same as photodiode except for providing current gain and more sensitive than
photodiode. With no light on the junction normal leakage, when light falls on the base, more
electron/hole pairs are formed and current is amplified by the transistor. [2]
The applications of phototransistor are very similar to the photodiode. For the photosensitive
element in the digital camera, which is the CMOS, it consists of thousands of phototransistor.
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Figure 3. Phototransistor
3. Photovoltaic light sensor
By converting solar energy into electricity power, the usage of photovoltaic cells is similar in
many ways to a battery because they supply DC power. Thus photovoltaic light sensor is
always considered as power supply in the circuit rather than measurement. Usually, solar
cell is the most common photovoltaic light sensor. [1]
The working principle of the solar cell is the protons of light reach the solar cell and then
excite electrons into a higher state of energy. The excited electrons act as charge carriers for
an electric field. Thus the current is formed when flow of electrons is formed.
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Figure 4. Photovoltaic light sensor
Application: The main application of photovoltaic light sensor is used as solar panel. The
solar cells are extremely essential for powering space vehicles such as satellite and
telescope. They provide a very reliable and economical way of powering space vehicles
instead of using expensive fossil fuel. Because of its stability and economic efficiency,
photovoltaic light sensor is widely used as the battery of toy, calculator and etc.
Further Reading:
[1] Stephen Fonash (2012), “Solar Cell Device Physics”, Elsevier Science.
[2] Law, Kock Yee (1993), "Organic photoconductive materials: recent trends and
developments", Chemical Reviews, American Chemical Society
[3] Working Principles of photodiodes:
http://ecee.colorado.edu/~bart/book/book/chapter4/ch4_7.htm
[4]Light Sensors: http://www.electronics-tutorials.ws/io/io_4.html
Reference:
Figure 1,2,3 source: http://www.electronics-tutorials.ws/io/io_4.html
Figure 4 source: http://www.intechopen.com/books/global-warming-impacts-and-futureperspective/alternative-resources-for-renewable-energy-piezoelectric-and-photovoltaicsmart-structures
[1] http://www.chm.bris.ac.uk/webprojects2003/ledlie/uses_of_solar_cells.htm
[2] http://www.electronics-tutorials.ws/io/io_4.html
[3] http://www.education.rec.ri.cmu.edu/content/electronics/boe/light_sensor/1.html
[4] http://www.electroschematics.com/6355/ldr-light-dependent-resistor-photoresistor