Usually a thermopile has four pins, two of which give a voltage across a series of thermocouples and the other two pins which are used to measure thermistor resistance. The sensor generates a voltage proportional to the incident infrared (IR) radiation power. One can compute the object’s temperature from the sensor signal because every object emits IR radiation with a power which is a strict function of its temperature. The total radiation power Pobj emitted by an object of temperature Tobj can be expressed as:
with s being the Stefan-Boltzmann constant and e the so-called emission factor (or emissivity) of the object. In an ideal case, e has the values ‘1’ and ‘0’. For most substances, the emission factor lies in the range 0.85 to 0.95. Equation 1 is called the Stefan-Boltzmann law.
This heat-balance equation relates the net power Prad received by the sensor to two temperatures: Tobj and Tamb. In most cases the instrument’s temperature Tobj equals (or is near to) the temperature of the ambient Tamb. Therefore, refer to this value as Ta, the ambient temperature. The total heat power Prad received from the object at temperature Tobj is given to:
The power of incident radiation Prad also depends on the angular measure of the cone opening from which the sensor receives radiation Ø. Here the emissivity of the object and sensor are taken equal to e. Sensor generates a voltage Vtp which is proportional to the power of incident radiation Prad.
Where s is sensitivity k is calibration constant, and e is emissivity.
Usually, the sensitivity of the thermocouple is in the range of microvolts. If the ambient temperature is fixed, an empirical relation between Vtp and Tobj or a look up table from the sensor manufacturer gives the object temperature. From Equation 2, it is evident that Vtp changes according to changes in ambient temperature (Ta). This ambient temperature needs to be compensated to get the correct object temperature.
The output of a thermopile sensor is usually in the order of microvolts, so amplification by a very low noise, very low offset amplifier is needed. The analog signal from the amplifier can be digitized using an ADC and stored as VTP when amplified. From Vtp, uncompensated temperature can be found using a look up table or sensor characteristic.
A voltage divider is constructed using the thermistor and a known precise reference. The value of thermistor resistance can be calculated using Kirchhoff’ law through a look up table or the Steinhart equation provided by the sensor manufacturer. Ambient temperature Ta can be found from the thermistor resistance. The object temperature is found by adding the uncompensated temperature from sensor to Ta, the ambient temperature. Figure 5 depicts the overall block diagram of the subsystem. Peripheral functions like LCD display, memory, keypads, and a USB port will also be part of any IR thermometer.
In this example, the overall bill of materials includes two precision amplifiers, LCD driver chip, MCU with ADC, USB controller, segment LCD, voltage reference, EEPROM, key pads , and so on.
With new next-generation System on Chip (SoC) architectures, all of the above functionality can be implemented on a single chip. As an example the Cypress’ PSoC integrates a microcontroller core, amplifiers, filter blocks, configurable ADC, LCD driver, capacitive sensing for touch buttons /proximity detection, internal storage memory, full speed USB 2.0 interface, and various other functions. Figure 6 depicts the complete implementation of an infrared thermometer.
Overall, an IR thermometer has a competitive edge over conventional thermometers in several aspects. This article has discussed in brief the principles of IR thermometry and design methodology and can serve as a starting point for your own IR thermometer design.
About the Author
Sanjeev Kumar has a bachelor's degree in Electronics and Communication from the College of Engineering, Guindy, Chennai, India. He currently works on PSoC-based applications at Cypress Semiconductor. Previously he designed medical electronics equipment at HD Medical Services Ltd.