# Minimum Resolvable Temperature Difference (MRTD)

The MRTD is the Minimum Resolvable Temperature Difference which can be detected by the IR camera under test. Subjective MRTD measurements are performed according STANAG 4349 (Edition 1, 1995 m.) standart.

Standart technical conditions:

• Desired spatial frequencies (range from 0.19 cy/mm to 3.23 cy/mm), defined in table on right.
• Desired target orientation (horizontal, vertical or both).
• Minimum 3 observers and 3 measurements for each observer.

The MRTD is calculated from four temperatures: \Delta T_1 , \Delta T_2 , \Delta T_3 , \Delta T_4 .
These four temperatures are determined by the observer.

\Delta T= \Delta T_\text{init}, the target bars are not detected by the camera under test.
\Delta T= \Delta T_1 , the target bars appear in positive contrast on the screen of the camera under test .
\Delta T= \Delta T_2 , the target bars disappear in positive contrast on the screen of the camera under test.
\Delta T= \Delta T_3 , the target bars appear in negative contrast on the screen of the camera under test.
\Delta T= \Delta T_4 , the target bars disappear in negative contrast on the screen of the camera under test .

Results of the MRTD test allow us to determine the distances at which objects are detected, recognized, and identified (DRI).

The DRI range is the longest distance at which it is possible to detect (respectively recognize or identify) an object with an optronic system with known MRTD. These ranges depend on the object size, the differential temperature of the object and the scene, the atmosphere absorbing coefficient, and the desired probability of detection (respectively recognition or identification).
DRI test is not a measurement made on images from a camera but it uses the results of an MRTD measurement.

Calculations are done by the formula below.

\text{DRI distances} = \frac{\text{Object size}}{\text{coef. depending on the probability of detection}} × \text{Spatial. freq. coef.}