The terms accuracy and precision are often used interchangeably. Yet they are not synonyms. In fact, they have completely different meanings.
Before we explain the difference between accuracy and precision, let's take a look at how a thermal camera takes measurements.
Suppose that in a laboratory we put a thermal camera on a black body (or black radiator). This object absorbs all electromagnetic radiation falling on it, so reflections do not interfere with the measurement. In addition, the lab has a constant temperature of 37 °C. When the camera takes a measurement every second for two hours, we get the graph below:
Most data points are between 36.8 °C and 37 °C, with outliers up to 36.6 °C and 37.2 °C. Thus, there are several (small) deviations from the real temperature.
Accuracy tells how well a camera can measure something. It shows the deviation between the (average) value from a series of measurements and the real value. We express this margin of error in ±1% or 1 °C (whichever is greater).
Suppose the actual absolute value of an object is 35°C , then the thermal imager is within specifications when the temperature is displayed as follows (taking into account correct settings such as emission, reflection and transmission, among others)
At a temperature of 170°C
Accuracy is the degree to which individual measurements differ from one another. The smaller random variations (such as the peaks in the graph above), the greater the accuracy. High-accuracy thermal cameras can pick up even the smallest thermal variations.
Accuracy is expressed in NETD (Noise Equivalent Thermal Difference)
The NETD is the final sum of inaccuracies that comes from influences of the detector, electronics, lens,...
The difference between accuracy and precision will become immediately clear when we compare this to a shooting exercise.
You fire four bullets toward a target. The goal is to hit the bullseye.
The distance between the bullets and the rose shows the accuracy. The spread of the bullets shows the accuracy.
The example below has low accuracy (the bullet holes are far from the rose), but high accuracy (the cluster of bullet holes is close together).
Below we see a different story, namely high accuracy (bullet holes close to the bullseye), but low accuracy (large distance between bullet holes).
