The 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 12 ft away, the test unit is measuring a one foot diameter section of the target.