The solution to the IRT’s parallax sighting problem was borrowed from Dr. Leitz’ TTL/SLR Leicaflex® Camera – sight through the same optics that the picture is taken through!  This completely eliminates parallax error at all working distances, focally and afocally.  To solve this Parallax Problem in an infrared thermometer, a visible light is projected through the (achromatic) lens system.  The visible sighting light beam and the invisible infrared field-of-view beam are co-incident everywhere, and so the visible light shows all of the geometrical nuances of the infrared field-of-view. 

Figure 1 shows one of the more fascinating possibilities enabled by the new TTL Sighting System.  That is measuring small objects which were previously next to impossible to measure.  With one inch of Parallax Error, the operator can’t even get in the ballpark of small electronic components.

Figure 1
Infrared Temperature Sensor Measuring 1 mm Spot Target

In Figure 1, the light sighted infrared thermometer is focused down to a target spot size of one millimeter (0.040”).   This enables non-contact temperature measurement of the tiny overheated resistor.  Notice the square image (actual size and shape) of the infrared detector projected onto the 0603-sized resistor. A second huge advantage enabled by the TTL Sighting is the ability to continuously vary the working distance and target spot size to match targets as small as the head of a pin or as large as a cotton field.  The target spot (defined as the planar intercept of the instruments’ field-of-view with the target surface) is always illuminated showing its exact size, shape and distortions under any focal conditions.