Thermopiles

Thermopiles

A thermopile is composed of multiple thermocouples connected in series or parallel. The thermopile is not a component that measures the absolute temperature. It outputs only the voltage proportional to the local temperature difference or temperature gradient.

Photosensitive area

This represents the area of the absorbing film that absorbs infrared rays.
Increasing the absorbing film area increases the amount of infrared radiation received, which in turn increases the output voltage. On the other hand, the time constant becomes larger and the response time slower.

Output voltage

It represents the output voltage of the thermopile under certain conditions. Since the output voltage varies depending on the temperature of the object to be measured, there are two types of output voltages, one representing the rating and the other representing the output voltage for the conditions of the application. A higher output voltage places a smaller burden on the amplifier circuit and thus should reduce the cost of circuit components.

Measurement condition 1

This measurement condition is generally used to describe the output of infrared sensors. This evaluation method is limited by the aperture, which is the percentage of the 500k blackbody furnace to the total field of view of the thermopile. Therefore, even if there is a large temperature difference between the thermopile and the reference unit (blackbody furnace), the amount of infrared radiation received by the thermopile will be small, resulting in a small output voltage. However, this is not a particularly optimal evaluation, as the standard is originally intended to express the characteristics of pyroelectric infrared sensors, etc.

Measurement condition 2

It is used as a condition for expressing output voltage in specific applications such as ear thermometers. This evaluation method is based on the temperature of a blackbody furnace (e.g., 37℃) being placed in the full field of view of the thermopile, which increases the amount of infrared radiation received and results in a large output voltage. In this way, a more realistic output voltage can be determined.

Thermopile resistance

The total resistance value of thermocouples connected in series is called the thermopile resistance. In general, resistive elements always generate thermal noise (Johnson noise), which increases in proportion to the square root of the resistance value. Therefore, the higher the resistance value, the higher the noise voltage. In addition, when constructing an amplifier circuit, the higher the resistance value, the higher the induced noise, so the thermopile resistance should be as low as possible.

Time constant

The time required for the infrared power to enter the thermopile sensor and for the output signal to increase to 63.2% of its maximum value. The smaller this value is, the faster the response speed of the sensor.

Infrared rays

A generic term for electromagnetic waves whose wavelength is longer than visible light and shorter than microwaves. Here it represents the infrared radiation that is naturally emitted from all objects having a temperature. Thermopiles use these to provide non-contact temperature sensing.

Viewing angle

This is the angle at which the sensor range is capable of detecting infrared radiation and is 50% of peak output. For our thermopile, it is ±50°.

Absorbing film

The film is designed to absorb infrared radiation. It is built into the sensor and absorbs only light of a certain wavelength.

Transmittance wavelength band

The thermopile is equipped with an infrared transmittance filter, which represents the wavelength band of infrared rays transmitted by the filter. Infrared rays emitted from the human body are 8 to 12 μm, and these rays must be transmitted for non-contact temperature measurement.

Aperture diameter

When measuring thermopiles using a blackbody furnace, this dimension represents the blackbody aperture. If the blackbody aperture is large relative to the viewing angle of the thermopile, the aperture diameter is not considered.

Blackbody furnace

A blackbody is an idealized object that can completely absorb all external, incoming electromagnetic waves over all wavelengths and can also radiate heat. The heat source when measuring the output characteristics of the sensor is temperature with blackbody paint.

Table data

Data representing the output characteristics of thermopiles. The table shows the correspondence between object temperature and sensor temperature.

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