Minimum resolvable temperature difference (MRTD) is a measure for assessing the performance of

infrared camera Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

s, and is inversely proportional to the

modulation transfer function The optical transfer function (OTF) of an optical system such as a camera, microscope, human eye, or projector specifies how different spatial frequencies are captured or transmitted. It is used by optical engineers to describe how the optics pro ...

. Typically, an operator is asked to assess the minimum

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...

difference at which a 4-bar target can be resolved. This minimum difference will change with the

spatial frequency In mathematics, physics, and engineering, spatial frequency is a characteristic of any structure that is periodic across position in space. The spatial frequency is a measure of how often sinusoidal components (as determined by the Fourier tr ...

of the bar target used. A curve of MRTD against

is obtained which characterises the performance of the imaging system. Modern infrared imaging systems can have low spatial frequency MRTDs of tens of millikelvins.

Manual test

A manual subjective test is implemented to determine the MRTD. An operator uses a series of 4-bar targets of different spatial frequencies. For each target he/she adjusts the

blackbody A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body a ...

, (source of Infrared radiation), temperature up and down until the pattern is "just resolvable." The positive and negative temperature differences are stored into a two dimensional

array An array is a systematic arrangement of similar objects, usually in rows and columns. Things called an array include: {{TOC right Music * In twelve-tone and serial composition, the presentation of simultaneous twelve-tone sets such that the ...

. The corresponding spatial frequencies used in each test are also stored into an array. The MRTD curve is a plot of these arrays (just resolvable temperature difference versus target spatial frequency). From the experimental MRTD data, a general

polynomial In mathematics, a polynomial is an expression consisting of indeterminates (also called variables) and coefficients, that involves only the operations of addition, subtraction, multiplication, and positive-integer powers of variables. An ex ...

best fit Curve fitting is the process of constructing a curve, or mathematical function, that has the best fit to a series of data points, possibly subject to constraints. Curve fitting can involve either interpolation, where an exact fit to the data is ...

is calculated and the result is the MRTD curve which gives direct insight into the quality of the image. i.e. the infrared camera's ability to resolve detail, in this case temperature.Electro Optical Industries, Inc.(2005). EO TestLab Methodology. In Education/Ref. .

Calculations

F(x) = \frac

, the MRTD curve ::

\Delta\,t(i)

= array of just resolvable temperature differences ::

f_s(i)\,\!

= array of spatial frequencies

Minimum detectable temperature difference

Minimum detectable temperature difference (MDTD), also called minimum detectable temperature (MDT), is not the same phenomenon as MRTD and is only subtly different. Like MRTD, it is a measure of the performance of infrared cameras. However, MDTD is a measure of visibility, not resolvability.

Manual test

The manual subjective test for MDTD is similar to the one for MRTD. A trained operator views a series of pinhole targets at different spatial frequencies. For each series of pinhole targets the operator ramps the blackbody (source of IR radiation) up and down until the targets are "just visible". The data at which the pinhole targets are "just visible" is stored into an array and plotted against spatial frequency, and a curve is fitted to the data. The MDTD curve is thus defined as the

\Delta\,

temperature versus spatial frequency. Here the relevant spatial frequency is ''f'' = 1/''W'' where ''W'' is the angular subtense of the target.

References

* An objective MRTD for discrete infrared imaging systems, A. H. Lettington and Q. H. Hong, Meas. Sci. Technol. vol. 4 pp. 1106–1110 (1993) {{DEFAULTSORT:Minimum Resolvable Temperature Difference Infrared imaging

Manual test

Calculations

Minimum detectable temperature difference

Manual test

See also

References