Distance to Spot Size Ratio

The distance-to-spot size ratio (D:S ratio) is a commonly used metric to describe the optical resolution of temperature measurement instruments, such as thermometers or thermography cameras. It is calculated by dividing the measurement distance (focal distance) D of the target by the measurement spot size. The D:S ratio value can be used to determine the measurement spot of the device at a specific distance. For accurate temperature measurement, the target size should be at least equal to the size of the measurement spot, ensuring that 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, fixed focus devices, such as compact pyrometers, provide the D:S ratio for a certain focal distance. Shifting the target out of focus decreases the D:S ratio, leading to a larger measurement spot and potential temperature deviations.

The D:S ratio is related to the focal length F and detector size d of the measurement device from an optics point of view. A longer focal length results in an enhanced D:S ratio, but also a typically larger dimension of the measurement device. Compact devices often have more moderate D:S ratios, which can be useful in some applications where the target object is large and the average temperature measurement over the total size of the object is required.

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