Sperry Instruments IRT200 Temp Check Gun Style Infrared ThermometerThe IR Thermometers allow for critical temperature measurements to be taken without having to touch an energized circuit. The D: S ratio or (Distance to Spot Ratio), is a rating used to state the distance, in feet, that the test unit should be from the target in order to measure a one foot diameter section. In other words, a 12: 1 D: S Ratio means that from twelve feet away, the test unit is measuring a one foot diameter section of the target.