Spot Size Ratio

The distance to spot size ratio (sometimes referred as spot size ratio SSE) is a widely used metric to describe the optical resolution of temperature measurement instruments, such as thermometers or thermography cameras. It is calculated by the ratio between the measurement distance (focal distance) D of the target and the measurement spot size. Using D:S ratio value can be used to calculate the measurement spot of the device at a specific distance. If the target size is at least equal to the size of the measurement spot, highly accurate temperature measurement is guaranteed. That means the measurement device detects at least 90 % energy of the target if the target is exactly the size of the measurement spot.

In devices with adjustable focus, a single D:S ratio is applied for the full measurement range. Although the spot size increases with distance, the ratio remains constant. Conversely, fix focus devices, such as compact pyrometers, the D:S ratio is given only for a certain focal distance. Shifting the target out of the focus, the D:S ratio is decreasing, leading to a larger measurement spot and potential temperature deviations.
From the optics point of view, the D:S ratio is related to the focal length F and detector size d of the measurement device. A longer focal length results in an enhanced D:S ratio, but also in a typically larger dimension of the measurement device. That’s why compact devices have often more moderate D:S ratios. In some applications this can be useful because the target object is large and needs to be tested in its entirety, taking an average temperature measurement over the total size of the object.

For thermal imaging cameras, the concept of the minimum spot size is also common. Each pixel out of the detector array is representing a potential measurement spot, defining the instantaneous FOV (IFOV). Because the pixel size is often small and the diffraction-limit creates an Airy-disk rather than a sharp spot, more than one pixel is required to achieve high accuracy in temperature measurement. This spot size is known as measurement FOV (MFOV) and usually encompassing 3×3 pixels. In that case the MFOV is comparable to the measurement spot of a pyrometer and D:S ratio can also be calculated for thermography cameras.