6 JUNE 2016
Many common temperature measurements are made by bringing the temperature sensor into contact with the object being measured. Examples include sticking a thermometer into a turkey one is cooking, and gluing a thermocouple onto a piece of equipment being monitored. Sometimes, though, it can be inconvenient to try to measure temperatures via physical attachment -
Temperature and Radiation
The amount of radiation emitted by an object increases of the fourth power of its absolute surface temperature. An object with a surface temperature of 200K would therefor emit 16 times the energy as would be emitted by the same object at 100K. This fourth-
When referring to 'radiation' in the context of temperature measurement, we are generally concerned with electromagnetic radiation with wavelengths ranging from a few tens of microns (infrared) down to approximately half a micron (visible light). This range of wavelengths allows for useful measurements from roughly room temperature to a few thousand degrees C. While temperature measurements of exotic astronomical objects may require detecting x-
The Infrared Thermopile
An infrared thermopile is a sensor IC that is designed to measure the amount of incident radiation. The device is directed so that the 'window' on the top of the IC package is pointed at the object whose temperature is to be measured.
Melexis MLX90247 Infrared Thermopile
How the Thermopile Works.
The figure below shows a very simplified view of what is inside a thermopile sensor. On the right is the input window, which is also an optical filter that rejects visible light-
The micromachined diaphragm is critical to making this sensor work. Because it is very thin relative to the rest of the silicon die, it has a very low thermal mass, and heat is not conducted away to the case as quickly as it would be from the rest of the silicon die. This allows even small amounts of incoming radiation to create a temperature difference or gradient between the diaphragm and the rest of the die.
The actual sensing of this temperature gradient is performed by a thermopile -
Some Practical Considerations
While the simplified transducer described above can result in a sensor that is responsive to the temperature of remote objects, there are a number of issues that must be considered when attmepting to implement a generally usable sensor.
The first of these considerations is the use of the input filter. As mentioned before, this filter passes radiation around 10um. Why this wavelength? Although a hot object emits radiation over a range of wavelengths, the distribution of energy over these wavelengths is a function of temperature. More specifically, for any given temperature there is a wavelength at which meissions are highest. For example, an object at 1000C will emit most most of its energy in wavelengths around 2um, while one at 25C will emit most strongly around 10um, as shown in the plots below. Note, however, that the plots have been normalized to the peak emission for that temperature -
Because the thermopile element itself measures temperature gradients, the measurement is sensitive to the IC package's ambient temperature. Consider the case in which the package is at 25C, and the object being measured is at 50C. Radiation from the object will warm the transducer's diaphragm to some temperature slightly above 25C, creating a temperature gradient and an output voltage, which is what would be expected. Now consider the case where the IC package is at 50C -
To get around the problem of the transducer's ambient temperature, the thermopile may also incorporate a separate thermistor dedicated to measuring the die temperature. By combining the die temperature measurement with the measurement of the temperature gradient created by incoming radiation it now becomes possible to account for the effects of the transducer's ambient temperature.
A major application issue is that of field-
Finally, not all surfaces are equally efficient at emitting radiation. This emission efficiency is referred to as the surface's emissivity, and varies between 0 and 1, with 1 being a characteristic of the ideal case called a black-
One particularly successful application for infrared thermopile sensors is in medical thermometesr, specifically those which are inserted into a patient's ear for the temperature measurement. There are several reasons that make this application a good match for this sensor.
Although high precision (0.1C) is needed, the required measurement range is rather limited to about a +/-
Inserting the thermometer into one's ear effectively fills the transducer’s field-
The emissivity of human skin is typically very high (~1). Additionally, the ear canal is a cavity with a small opening. This makes it look like a cavity radiator, which behaves very closely to an ideal black body (emissivity =1) regardless of the material of which the cavity is made. The combination of skin's high emissivity and the cavity radiator effect combine to increase measurement consistency.
Since the thermopile element is tiny, it comes to thermal equilibrium in a matter of seconds, making for a fast measurement. In contrast, it takes a lot longer to get a good temperature reading with a traditional oral thermometer.