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 equipment. This" Δ T 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.
Note 4 : It is proposed that the black body temperature be varied in steps as follows :
6.2. It is recommended that the MRTD be measured with horizontal and vertical orientation of the bars. Any other orientation is optional.
6.3. The criterion to be used for resolution is that it should be possible to see four bars (not just some modulation) on the display, although it is not necessary that all of each of the four bars be visible at the same time.
6.4. Measurements shall be taken at a minimum of four (4) spatial frequencies distributed approximately uniformly over the useful range of the imager. A minimum of three (3) measurements shall be taken for each pair of contrasts, at each spatial frequency, in each orientation, by each observer. Measurements shall be taken until stable results are observed. A single observer may be used, although multiple observers are recommended.
