Understanding Calibration and Accuracy
It can be difficult for the average practitioner or technician to understand the relevance or implications of IR camera specifications when it comes to the difference between Accuracy and Sensitivity and the various terms used to describe these performance specifications. Here we outline some of the key factors that should be considered:
1. The very basic, ‘accuracy specification of ±2ºC or 2% of a measurement’ would apply to the wide range of temperatures being measured with a range of possible protocols and conditions that would affect the accuracy of the measurements when compared to a known ‘absolute temperature’.
2. An ‘absolute temperature’ is a specific temperature that has been calibrated against the same temperature that is known to be accurate (a calibrated black body temperature).
This is used to calculate the ‘partial errors’ for each variable of the temperature measurement equation.
3. The ‘Partial errors’ can result from any of several variables in the typical IR camera temperature measurement equation, including:
- Reflected ambient temperature
- Ambient temperature
- Camera response
- Calibrator (blackbody) temperature accuracy
The calculations and accuracy results are only valid for the camera performance while being tested in a controlled laboratory environment within all correct protocols and observing minimum standards.
THIS IS NOT GOING TO APPLY TO THE ACCURACY AND SENSITIVITY OF MEASURMENTS IN THE FIELD (hence the ±2ºC uncertainty of absolute).
In medical and clinical applications we are more interested in the camera specifications related to ‘comparative’ temperature accuracy and sensitivity relating to stability and repeatability over time for the detection of temperature changes and differentials that are specific to changed physiology rather than unstable camera performance.
With proper calibration and attention to factors such as emissivity, ambient temperature, reflective surfaces, focus and spot size, the possible margin of error for measuring ‘absolute temperature’ can be within 1ºC.
A cameras calibrated potential to measure ‘comparative temperature’ will be described as a Delta T (ΔT), which is the difference between two temperatures or the temperature ‘accuracy’ of measurement between two temperatures, this can be described as calibrated accuracy to within 0.1°c (1/10th) even though the temperature ‘sensitivity’ can be measured in comparative sensitivity to 0.01°c (1/100th)
No two cameras will have exactly the same calibration values, and these values can and do change throughout the life of the camera and its re-calibrations.
Specific parameters of accuracy are recorded on an individual camera’s calibration certificate.
The Meditherm range of cameras are built and calibrated for the specific medical and clinical applications that they are intended for.