AC/225(Panel VI)WP/61(3rd revise)
TAKING OF MEASUREMENTS
6.1. Owing to limitations in the test equipement +he indicated temperature differences may not be the true temperature differences between target plate and black body. Therefore the following procedure is recommended to establish the MRTD: starting with the temperatures of the target and black body approximately equal (see Note 2), the temperature of the black body shall increased until the test pattern appears in positive contrast (hot bars). When the observer can just resolve the test pattern the temperature difference (ΔT+) is recorded The black .body temperature is then reduced causing the test pattern to disappear, and then to reappear with negative contrast (cold bars). When the observer can just resolve the target again the temperature difference (ΔT-) is recorded (see Note 3). The two measurements must be taken without delay so that temperature drift is avoided (see Note 4).
NOTE 2: The procedure can be speeded up by starting with a temperature difference between the target plate and the black body of 0.8 ΔT, both for positive and negative contrast.
NOTE 3: If it can be established that the temperature drift is negligible, both positive and negative contrast measurement need be made only for the lowest spatial frequencies, in order to determine the "ΔT offset" of the test equipement. This " ΔT offset" is then calculated as the mean of the temperature differences for the positive and negative contrasts (that is," ΔT offset" = 0.5[ΔT++ΔT-]), and its value is then substracted from the temperature differences measured for a single contrast for each of the remaining spatial frequencies in the test run. Positive contrast measurements for these frequencies are recommended.
