Minimum Spot Size

The minimum spot size is a common parameter used to describe the optical resolution of temperature measurement devices like thermometers or thermography cameras. It refers to the smallest target size at which a temperature can be accurately measured from a specific distance. Since the spot size changes with focal distance, it’s typical to calculate the distance to spot size ratio (D:S ratio).

The D:S ratio is computed by dividing the measurement distance (focal distance) D by the measurement spot size. This ratio is crucial for determining the spot size at various distances to ensure accurate temperature measurements. For accurate temperature measurement, the target size must be at least the size of the measurement spot. The minimum spot size is defined as the diameter of the spot size when the measurement device detects 90% of the target’s energy. This limit value sets the size of the minimum spot size that must be guaranteed.

In devices with adjustable focus, a single D:S ratio is applied for the entire measurement range. Although the spot size increases with distance, the ratio remains constant. Conversely, in fixed focus devices, such as compact pyrometers, the D:S ratio is given for a specific focal distance only. Shifting the target out of focus decreases the D:S ratio, resulting in a smaller 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 optical standpoint. A longer focal length enhances the D:S ratio but also increases the overall size of the measurement device. Therefore, compact devices often have more moderate D:S ratios, which can be useful in some applications where 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 minimum spot size is also relevant. Each pixel in the detector array represents a potential measurement spot, defining the instantaneous field of view (IFOV). Since 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 field of view (MFOV) and usually encompasses 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.