Light Sensors

Also known as photo detectors or optical detectors, these are transducers which convert light energy into an electrical quantity. Chemical detectors like photographic plates use the energy of a photon to start a chemical process. In this website, we would discuss the different types of light sensors used in the industry. Light sensors have lots of applications, from line following robots, optical mark readers, scanners and even digital cameras. Discrete light sensors today use semiconductors like silicon, germanium, gallium arsenide and other semiconductors. In the old days, light sensors resemble the vacuum tubes where they are big, bulky and brittle because of their glass envelopes. Modern light sensors are very compact and they are available in different specifications and cost. Some light sensors have built in signal conditioners and built in amplifiers to increase the sensitivity of the photodetector.

Light Sensor Technologies

1. Photoresistors or Light Dependent Resistors (LDR) – these are light sensitive devices that changes its resistance depending on the light intensity. Photoresistors are typically made up of CdS or Cadmium Sulfide cell. CdS decreases its resistance whenever it is hit by light. Whenever a photon hits the surface of the CdS cell, an electron gets knocked off from the semiconductor crystal structure, this electron is known as a free electron and it is responsible for the increase in conductivity of the cell. LDRs can be found on automatic street lights and camera light meters.

Lead sulfide (PbS) and indium antimonide (InSb) LDRs (Light Dependent Resistor) are typically used for detecting light energy in the mid-infrared region.

Ge:Cu photoconductors are used in infrared astronomy and spectroscopy. They are the best infrared detectors available.

2. Photodiodes – are similar in construction with regular semiconductor diodes except for the fact that they are designed to be exposed to light energy. To facilitate exposure to light, a glass or plastic window is provided on the casing of the photodiode. The glass window allows light to penetrate the semiconductor where a voltage is generated or a change in resistance is exhibited by the photodiode. Photodiodes operate in 2 modes.

Photovoltaic mode – the photodiode acts like a solar cell and it generates a voltage whenever it is struck by light.

Photconductive mode – the photodiode acts like a variable resistor, reducing its resistance whenever it is hit by light.

A variation of the PN junction photodiode is the PIN Photodiode which offers greater light sensitivity and faster response times. Below are some of the materials used in manufacturing photodiodes and also the wavelength range in which they are sensitive.

Silicon 190–1100
Germanium 400–1700
Indium gallium arsenide 800–2600
Lead(II) sulfide

3. Photovoltaic cells or a solar cell – the construction of solar cell is similar to photodiodes. In fact, a solar cell is just a giant photodiode. Solar cells used for generating power can be as large as a house, while solar cells used for sensing light comes in packaging as small as a pea.

4. Avalanche Photo Diode – this is a highly sensitive photodiode which uses a high reverse bias voltage to generate as much current as possible for a given amount of photon striking its surface. The avalanche photo diode is the semiconductor equivalent of the photomultipliers used during the vacuum tube years.

5. Phototransistors – this is a photodiode with gain. This is basically a transistor housed in a transparent case, as all transistors will acts as phototransistors. Regular transistors are housed in a “black casing” to prevent them from becoming phototransistors. When light strikes the phototransistor surface, the electrons generated on the base-collector junction goes into the base where it is amplified by the transistor.

6. Charge-coupled devices (CCD) – CCDs are well known for its use in digital photography, digital cinematography and image recording for digital astronomy. CCDs can be thought of as a photo capacitor, the charge on the CCD is dependent on the amount of light that falls on its surface.

Light Sensor Applications

1. ambient light sensor – these are light sensors specifically designed to detect the amount of ambient light in a room. This type of light sensor is designed in such a way that it has the same spectral light sensitivity as the human eye. The human eyes is more sensitive to the Yellow-Green spectrum of light compared to either blue or red light. Ambient light sensors are usually built with a signal conditioner and amplifier.

2. automatic light sensor – sensors with built in chips that triggers other devices. Automatic light sensors can switch a camera, a door or a light source. Automatic light sensors are used for switching street lamps whenever the skies becomes darker.

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