In most applications the pre-set threshold temperature of probes with predetermined threshold levels and the temperature of the hot surface (of the medium) are not exactly the same. Rather, the threshold temperature of the infrared sensor is always set lower than it would need to be for the detection of the hot surface.
This makes sense for the reason that temperature oscillations or oscillating emissions from the metal surface often occur which would cause the infrared detector to trigger when it was not wanted. Experience shows that the threshold temperature of the infrared detector should therefore be chosen 50 - 100 degrees Celsius lower than is required. In other applications, materials must be detected which span a large temperature range (300 - 600°C). Then the lowest occurring temperature must still be detectable, which implies that the threshold temperature of the infrared detector must be chosen to be very low. Therefore, there is always a difference between the medium temperature and the threshold temperature of the infrared detector. This is the differential temperature.
The connection between differential temperature and the achieved angular field is represented in diagram 1.
In order to determine the actual angular field, one selects the circle with the desired or estimated differential temperature and looks for the intersection points with the radiation diagrams of the A or B optics. Once one has found these intersection points, one must only read off which angular radius runs through these points.
Example: Differential temperature 100 degrees, 4° optics (B), the intersection point of the differential temperature circle and the radiation diagram is at angular radius of ± 1.2 degrees. The actually achieved angular field is therefore 2.4 degrees. Due to the characteristics of the photovoltaic cells used in the infrared detector and the infrared optics, the actually achieved angular field is not constant, but is dependent on the temperature of the medium. This effect is comparable to the overexposure of a photograph.
If the hot surface is smaller than the field of view of the infrared detector, not so much energy enters into the opening of the infrared detector as would be possible under full illumination. Therefore the temperature will be falsely determined. This can be corrected when it is known what percentage of the field of view is covered by the hot surface.
If the illumination is not 100 %, the threshold temperature of the infrared detector must be lowered in order to detect the hot surface. (Diagram 2)
Illumination (%) = objekt surface area
detector visible surface
For infrared detectors with spherical optics, the field of view is always circular. For specific optics (50, 100 mm focal length) there is a constant angular field ( Cos Phi ). At a predetermined distance (A), the infrared detector "sees" a circular area that is called the visible surface (B). If the hot surface is as large as the field of view or even larger, the illumination is 100 % (Diagram 3).
B = 2 x A x tan Cos Phi
The energy emitted by a hot surface at temperature T is distributed throughout the entire surrounding space. The further the infrared detector is from the hot surface, the less the energy is that can enter into the optics of the infrared detector. Since the temperature measurement in the infrared detector succeeds through conversion of energy into temperature, the infrared detector measures an increasingly smaller temperature the further away it is removed from the hot surface. The larger the separation therefore, the more the threshold temperature of the infrared detector must be lowered.
It is assumed in diagram 4 that the field of view of the infrared detector is always fully illuminated.