Characterization of infrared systems

Infrared radiation receiving systems are generally used for remote sensing of thermal phenomena.

These systems, which usually produce a one- or two-dimensional image, have to satisfy a number of criteria that vary in stringency depending on the application.

Some equipment is designed for purely qualitative analysis of thermal phenomena.

This is usually a simple visualization of the thermal distribution of objects not directly perceptible to the eye. In this case, the system provides an image whose visible luminance contrast is proportional to the thermal contrast of the object observed. Clearly, the more interesting the observation, the more the system is able to discriminate between small temperature differences. This is where the notion of thermal resolution comes in.

Two other very important criteria must be added: spatial resolution, which defines the size of object details at the perceptual limit, and noise equivalent illuminance, which represents the minimum energy to be supplied to the system for it to deliver a signal at least equal to its own noise.

Thus, an object with an angular dimension greater than that corresponding to the spatial resolution of the system emitting infrared energy, such that the illumination is greater than the system noise, has every chance of being rendered correctly if the apparent temperature difference between itself and its surroundings is greater than the thermal resolution.

(The apparent temperature is that which a black body placed in a vacuum at the same location as the object would have, producing the same effect on the radiation sensor).

When it comes to quantitative measurement, the system used must also satisfy all the properties of measuring instruments: proportionality, fidelity, reliability...

There is then a unique correspondence between the electrical signal delivered and the object's apparent temperature (system noise is a cause of deviation from linearity, as it adds a random voltage to the signal, so the same cause can produce different effects).

Of course, there are other criteria that can be used in the radiometric characterization of infrared systems, such as spectral response, which defines operating wavelengths, but it soon becomes apparent that each new criterion chosen is closely related to those mentioned above.

Thus, the optical transfer function is directly related to spatial resolution, and in turn thermal resolution depends on the optical transfer function.