#57: Measuring objects smaller than the field-of-view

For some non-contact temperature monitoring tasks, the object to be measured is too small to adequately fill the field-of-view of one of the IRt/c models. The monitoring can still be successfully performed if two conditions are met:

The object size and distance from the IRt/c are constant.

The area surrounding the object within the field-of-view of the IRt/c has a repeatable temperature.

The signal produced by the IRt/c represents the average temperature within its view. Accordingly, the signal can be represented by the equation:

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where T is the output signal, Tt the target object temperature, At the target object area, Ts the surroundings temperature, As the surroundings area as seen by the IRt/c, and A the total area seen by the IRt/c. For example, to measure the temperature of a thin rubber strip 2,5 mm (0.1”) wide moving continuously 25 mm (1”) away from an IRt/c.2, at a temperature expected to be about 93°C (200°F), and a surrounding temperature at 27°C (80°F). At 25 mm (1” ) distance, the IRt/c.2 spot size will be approximately 13 mm (.5”).

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Computing the results for the equation gives:

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This result shows that the average signal will be 17°C (31°F) above the surroundings temperature, compared to an actual object temperature of 67°C (120°F) above surroundings, or approximately one-fourth, which is the ratio of object area to surroundings area measured. Therefore, if the surroundings are expected to be repeatable to ,6°C (1°F), the IRt/c signal will be repeatable to 2°C (4°F). For the final display on a controller, or other read-out device, calibrate in standard fashion by using the available offset adjustment. If the object is to be controlled over a wide range of temperatures, calibrating with a span adjustment will yield greater accuracy.

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If the target temperature falls within the range of one of the LoE models, the LoE model should be used, even if the target is not metallic. Since a small target results in the same radiation mathematics as low emissivity, a LoE model will reduce errors due to size change and positioning by a factor of approximately 4. See Tech Note #59.

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