The SMTIR990X Infrared Sensors

PRODUCT DESCRIPTION

The Smartec infrared sensors SMTIR9901 and SMTIR9902 are sophisticated full silicon infrared sensors and comprise of so called thermopiles. Thermopiles are based on the Seebeck effect, which is a long time standard for conventional thermocouples. The application of thin film technology allows the production of miniaturised and low cost sensor elements.

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The sensors can be used in measuring the radiation temperature without any contact. For different radiation temperature ranges various filters are available. The sensor type SMTIR9902 contains a temperature sensor for measuring the temperature of the sensor itself. The temperature range of the sensor-element is between -40 to 100 °C. The sensor is available in a standard TO-05 encapsulation and and is equipped with a 5.5 micrometer filter.

FUNCTIONAL DESCRIPTION

The output voltage is generated by a thermopile, formed by a series connection of thermojunctions deposited on a silicon substrate. The hot junctions are thermally isolated from the cold junctions on the substrate by etching an extremely thin membrane. A black absorbing layer on the hot junctions transform the incoming radiation into heat. A voltage proportional to the radiation is generated by the thermoelectric effect.

SMTIR9902 responsiveness as function of the incoming wavelength

As you can see from the graph above, this version is sensitive for IR radiaton with wavelength longer than 5,5µm. Visible light has wavelengths ranging from 400 nm to 800 nm, or 0,4 to 0,8 µm.

The used thermopiles are processed on 400 mm silicon substrates using BiSb and NiCr for the thermojunctions. For different radiation spectra various filters are available to get the optimal solution.

Easy and accurate measuring of the sensor body temperature can be done with a built-in temperature sensor (only for type SMTIR9902).

OUTPUT SIGNAL

The output signal is a DC voltage, which increases with the temperature difference between the sensor body and the temperature of the surface in view. Realistic output voltages for many applications are in the range of tens to hundreds of microvolts. Amplifiyng this output signal correctly is not a simple thing and requires dedicated electronics, one of the reasons being the output impedance of the sensor (50kOhm), which renders the input bias current of most amplifiers a significant source of inaccuracy.

PHYSICS INVOLVED

The generated voltage is actually determined by the resulting radiation equilibrium. Important physical laws involved are Plancks law and Stefan-Boltzmanns law. You find very good information on the physics of infrared sensors on the Everett Infrared and Electro-optic Technology website. A highly recommended article is in their detector.pdf document.

ANGLE OF VIEW

The standard Smartec infrared sensor has an angle of view of 90°. A version with a silicon lens, which has an angle of view of 7°, is available. This version has a high pass filter that starts at 2µm.

SUPPORT

All relevant documentation and files (data sheet, application notes, software etc.) for our products can be downloaded from the SUPPORTSHOP page.

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