Infrared sensors are electronic instruments which are used to sense certain characteristics of the surrounding by emitting and detecting infrared radiation. Infrared sensors are capable of measuring the heat that is emitted by an object by detecting its motion.
Theory of Infrared Sensors
Infrared waves are invisible to the naked eye. Infrared radiation can be found between the visible and microwave regions in the electromagnetic spectrum. Infrared waves usually have wavelengths between .75 and 1000 µm. infrared technology is applied in many everyday products. Infrared detectors are used to interpret signals sent from a remote control to a television. Their low power requirement and simple circuitry are just a few key benefits of infrared sensors.
The Types of Infrared Sensors
Infrared sensors can be classified into 2 main types:
- Thermal infrared sensors – this type of sensor uses infrared energy as heat. Their photo sensitivity in very dependant of the detected wavelength. Thermal detectors do not require cooling at all but their response time is very slow and they have extremely low detection capabilities.
- Quantum infrared sensors – this type of sensors provide higher detection performance and has faster response speed compared to thermal infrared sensors. Their photosensitivity in very much dependant on wavelength. Quantum detectors have to be properly cooled in order to obtain precise measurements.
The Key Applications of Infrared Technology
- Night vision devices
Night vision equipments have implemented infrared technology because there is insufficient light available to see unaided. Before converting them back to visible light, night vision devices convert ambient photons of light into electrons and amplify it using either electrical or chemical process.
- Infrared tracking
This is also known as infrared homing. It is a missile guidance system which operates using the infrared electromagnetic radiation which is emitted from a target in order to track it. Infrared is strongly radiated by hot bodies such as people, aircraft and vehicles therefore these missile systems are more often than not known as “heat-seekers”.
- Art history & restoration
Infrared reflectography is used by art historians all over the world to reveal hidden layers in paintings. This technique is used to determine the originality of the painting.
- Hyperspectral imaging
Hyperspectral imaging accumulates and processes information from across the electromagnetic spectrum. It can also be used to track nanoparticles inside large living organisms.
The most common data logging applications are temperature measurements as they are found across a wide variety of industries such as machine monitoring, medical monitoring, cold chain integrity and many more. There are 4 types of temperature sensors that are commonly used with data loggers and it can be somewhat confusing and difficult to choose the right type.
Thermocouples are the most commonly used as well as the cheapest temperature sensor available in the markets. A thermocouple is just 2 distinct metal wires of very specific alloys which are fused together at one point. Thermocouples produce an output voltage which is closely related to the temperature. The voltage is measured by the data logger and then a calibration equation is applied to convert the voltage into temperature. A cold junction reference is also incorporated into the data logger for any offset voltage that happens at the connections between the thermocouple wires and data logger.
A thermocouple temperature measurement typically involved multiple voltage measurements of the thermocouple and reference junction and scaling the voltage to temperature and compensating for the reference temperature. You should consider thermocouples when you want a simple low cost device that is very simple to use, yet efficient and effective at the same time.
- Semiconductor sensors
Semiconductor sensors incorporate a solid state device, like a diode reference, that has an established voltage versus temperature characteristic alongside signal processing electronic applications to generate voltage output that is directly proportional to the temperature. Semiconductor sensors only require a simple voltage measurement and linear calculation to scale the signal to temperature. You can consider semiconductors if you have a specific temperature range in your mind and still want to stick to your budget.
- RTD sensor
RTD sensros are a device in which resistance increases linearly with temperature. RTD commonly consists of a fine platinum wire wound around a cylinder. In some cases, nickel and copper wire are also used instead of platinum wire. RTD is made to have specific resistance at 0°C therefore the resistance vs. temperature curve has a very specific slope, with its most common value standing at 100 Ohm. To measure the temperature, the data logger will source a known voltage through the RTD sensor and measure the resulting current from which it can automatically calculate the resistance. The data logger will then calculate the temperature by using the slope of the resistance vs. temperature curve. RTD sensors are usually more stable and precise compared to thermocouples, but at the expense of a more limited range. You might want to consider RTD sensors if you need measurements of very high accuracy for a narrow temperature window.