5.2.4. Any variation in temperature across the useful area of the test pattern small be so shall as not to be detectable on the image by the observer.
5.2.5. The measurement temperature is that of the test pattern, which should be 20 +/- 2 ℃ unless otherwise specified.
5.3. The transmission losses between the test pattern and the imager and the emissivities of the test pattern and the black body should be known and taken into account when results are calculated.
5.4. In carrying out the measurements the observer may optimise results by altering both the illumination level in the room and any settings on the imager, adjusting the distance of his eye from the display, and slightly altering the position of the test pattern within the field of view. If a monitor is used, it must be of the type normally used with that imager, and it is recommended that, once set up, it should not be adjusted during measurement.
MEASUREMENTS
6.1. Starting from an invisible test pattern, the method is based on the determination of the temperature difference ( + or-) required to make the test pattern visible. Owing to limitations in the test equipment, the indicated temperature differences may not be the true temperature differences between test pattern and black body. Therefore the following procedure is recommended to establish the MRTD : starting with the temperatures of the test pattern and black body approximately equal (see Note 2), the temperature of the black body shall increase 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 test pattern 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 test pattern and the black body of 0.8 . Δ T, both for positive and negative contrast.
