Infrared (IR) sensor is most often used in wireless technology that includes remote control functions and detection of surrounding objects or obstacles. It is a simple electronic device that emits and detects IR (thermal) radiation, so that it can detect objects or obstacles in its range. He has the ability to recognize heat and movement. IR sensors use infrared radiation of wavelengths between 0.75 µm and 1 mm, which is located between the visible and microwave regions of the electromagnetic spectrum. The infrared area is invisible to the human eye, but can be felt on the skin, like a feeling of warmth.
One of the classifications of IR radiation is that of the electronic senses that have a response in these areas:
Near Infrared (NIR) – 0.7 μm to 1.0 μm; the area from the end of the human eye sensation to the silicon response
Shortwave Infrared Area (SWIR) – 1.0 µm to 3 µm; from the silicon response to the “atmospheric window” area; covered by the InGaAs semiconductor at a wavelength of about 1.8 µm
Mediumwave Infrared Area (MWIR) – 3 µm to 5 µm; “atmospheric window” area, covered by InSb, HgCdTe and partially PbSe semiconductors
Longwave Infrared Area (LWIR) – 8 µm to 12 or 7 µm to 14 µm: covered by HgCdTe semiconductors and microbolometers
Very Longwave Infrared Area (VLWIR) – 12 dom to 30 µm: covered with silicon with impurities
Infrared sensors work on three basic laws of physics:
- Planck’s law of radiation: Any object whose temperature is not equal to absolute zero (0 K) emits radiation
- Stephan Boltzmann’s law: Total energy emitted by a black body at all wavelengths is related to the absolute temperature
- Wein’s law of displacement: Objects of different temperatures emit spectra whose peak is at different wavelengths that are inversely proportional to the temperature
The key elements of the Infrared Detection System are IR transmitter, transmission medium and IR receiver.
IR transmitter acts as a source of infrared radiation. According to Planck’s law of radiation, any object of temperature above 0 K is a source of IR radiation.
Transmission medium allows the passage of radiation from the IR transmitter to the IR receiver. That can be vacuum, atmosphere and optical fibers.
IR receiver can be photo diode and photo transistor. It can detect infrared radiation, which is why an IR receiver is also called an IR detector.
The selection of IR transmitters and receivers is made according to the appropriate parameters, such as photosensitivity or sensitivity, power equivalent to noise and detectiveness.
How the IR sensor works
The IR source (transmitter) emits radiation of the required wavelength. This radiation reaches the object and is reflected back. The reflected radiation is then detected by the IR receiver. The radiation detected by the IR receiver is further processed according to intensity. However, the output signal of the IR receiver is generally small and therefore amplifiers are used to amplify the detected signal.
Types of IR sensors
Based on the presence of IR sources, IR sensors can be classified as active and passive.
Active IR sensors include both a transmitter and a receiver. The LED or laser diode serves mainly as a source, with the laser diode being used for the image and the LED for the non-image IR sensor.
The active IR sensor works by radiating the energy that the detector receives and detects and is further processed by the signal processor, in order to obtain the necessary information. Active IR sensors are a beam break sensor and a reflection sensor.
The passive IR sensor has only detectors. No transmitter component. They use the object as a source and transmitter. The object radiates energy that IR receiver detects. The signal processor interprets the signal in order to obtain the necessary information. Passive IR sensors are bolometer, thermocouple, pyroelectric detector, etc.
There are two types of passive IR sensors: thermal and quantum. Thermal IR sensors, wavelength independent, use heat as an energy source and are slow with detection time and response time. Quantum IR sensors depend on wavelengths and have high detection time and response time. They require frequent cooling for accurate measurement.
Application of IR sensors
IR sensors are used in most of today’s equipment:
- Radiation thermometers – their operation depends on the temperature and type of object. They have a faster response and simpler measurements, the sample and measurement can be performed without direct contact with the object.
- Human body detection – the alarm system senses the human body temperature. If the temperature is higher than the limit value, the alarm goes off.
- Item counter – constant radiation is maintained between the transmitter and the receiver. As soon as the object intersects the radiation, the object is detected and the number increases. The same number appears on the display.
- Burglar alarm – works similarly to an item counter, the transmitter and receiver are on either side of the door frame. Constant radiation is maintained between the transmitter and the receiver, and when the object crosses the path, an alarm is activated.
- Proximity sensor – used in smartphones to find the distance of an object.
- Other – in IR recording devices, night vision devices, humidity analyzers, optical power meters, sorting devices, missile guidance, remote sensing, etc.
Advantages and disadvantages of IR sensors
Some of the advantages of IR sensors are:
- low power requirements, which makes them suitable for most electronic devices (laptops, phones, PDA…)
- can detect movement with and without the presence of light with almost the same reliability
- no need to contact the detection object
- they are not affected by corrosion or oxidation
- high noise resistance
Disadvantages of IR sensors are:
- the need for a line of sight
- blockade by various obstacles or facilities
- limited range
- the influence of various environmental conditions (rain, fog, dust …)
- slow data rate