Why Spot Size Matters

A lens receives the emitted infrared energy from a measuring spot and focuses it onto a detector. Measurements based on this technology can only be correct, if the measuring object is bigger in size than the detector spot.

For Optris infrared pyrometers and cameras, there are typically two types of optical configurations available. Standard optics cause the beam path to diverge. Therefore, with a bigger distance to the sensor, the measurement point is getting larger. Far-field optics are a specialized set of standard optics with an exceptionally narrow beam path, enabling the measurement of a small spot size to be maintained even when the sensor is mounted several meters away from the target, and are therefore optimized for long distances.

Close focus is designed with a converging beam geometry, meaning the spot size narrows to a minimum at a certain distance and then expands again. This results in a “waist” or focus point, where the optical system achieves the smallest spot size and highest spatial resolution.

Many infrared thermometers are equipped with fixed-focus optics, meaning they are pre-adjusted for a specific measurement distance. The user must select a model with a focal distance that closely matches their application. While accurate temperature readings are still possible outside the preset focus distance, the optical resolution (D:S ratio) decreases significantly, since optimal focus—and thus the smallest possible spot size—is only achieved at one fixed range.

In contrast, infrared thermometers with variable (adjustable) focus allow the user to adjust the focus to match the desired measurement distance continuously. This ensures optimal optical resolution at all times, as the D:S ratio is maximized for any selected distance. As a result, variable-focus devices are particularly suitable for applications that involve changing measurement distances or varying target sizes, offering greater flexibility and precision across a broader range of scenarios.

Distance to Spot Size Ratio for Pyrometry

For pyrometers, the Distance-to-Spot Size Ratio (D: S ratio) defines the relationship between the measurement distance and the diameter of the spot being measured by an infrared pyrometer. Conventionally, in pyrometry, the distance-to-spot size ratio is the diameter of a black body at which the radiation signal has decreased by 10% compared to the signal of a sufficiently large black body.
It indicates how small the measurement spot is relative to the distance from the sensor to the target. A higher D:S ratio corresponds to a smaller spot size at a given distance, enabling more precise temperature readings of small or distant objects. As the D:S ratio increases, the optical resolution improves. Conversely, a lower D:S ratio requires the sensor to be positioned closer to the target to ensure the spot is filled, minimizing background influence and measurement errors.
A key performance metric is the distance-to-spot size ratio D:S, which relates to the measurement distance z to the effective spot diameter [math]d_m[/math].

[math]D:S=\frac{z}{d_{m}}[/math]

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 only 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 spot size is defined as the diameter of a blackbody target that results in a signal drop of 10% compared to a reference signal from a sufficiently large blackbody. This definition takes into account spherical and chromatic aberrations, the effective detector area, and scattering effects within the optical path. A geometric estimation of the D:S ratio, based on the lens parameters and detector size, is given by the effective aperture, respectively, the lens diameter [math]D[/math] and their f-number [math]\ f[/math] and the sensor area of the detector [math]A_S[/math].

[math]D:S=\frac{f\ \cdot\ D\ }{\sqrt{A_S}}[/math]

For systems with coated IR lenses and detector areas much larger than the wavelength, this geometric estimate closely approximates the effective D:S ratio. In most cases, it is advantageous to have a small spot size (and therefore a large D:S ratio). The pyrometer can detect small targets even at larger distances. However, precise alignment of the device is required, often realized by laser or video sighting, handling the advanced optical resolution.

Typical D:S ratios vary depending on the temperature range and optical design. For low-temperature applications (−50 °C to 150 °C), moderate D:S ratios are used. For high-temperature applications (>150 °C), where detectors operate at shorter wavelengths, D:S ratios from 20:1 up to 300:1 is common.

Measurement Field of View for Thermal Cameras

Unlike pyrometers, which measure infrared radiation from a single spot, infrared cameras use two-dimensional focal plane arrays to generate thermal images. However, the resolution of these arrays is limited by the small pixel size, which is often on the same scale as the infrared wavelength. As a result, optical resolution is fundamentally constrained by diffraction, setting a physical limit on how finely details can be resolved in the image.

Their spatial resolution is typically described by the instantaneous field of view [math]IFOV[/math] —the angular size of a single pixel:

[math]IFOV=\frac{FOV}{Number\ of\ Pixels}[/math]

Values range from 0.3 to 15 milliradians, depending on the optics and resolution of the camera. However, due to the small pixel dimensions, which are often comparable to the infrared wavelength, the resolution is fundamentally limited by diffraction.

The Measurement Field of View (MFOV) defines the smallest measurable area that a single pixel of an infrared camera can resolve at a given distance. It describes the relationship between the distance to the target and the pixel footprint on the object surface. A smaller MFOV indicates higher spatial resolution, allowing the camera to detect finer temperature differences in smaller areas. As the distance to the object increases, the projected area per pixel grows, reducing the precision of localized temperature measurements. To ensure accurate readings, the target must cover multiple pixels—typically at least 3×3—to minimize averaging effects and background influence.

The distance-to-spot size ratio can be estimated using the following formula, which enables an effective comparison between the performance characteristics of cameras and pyrometers:

[math]D:S=\frac{1\ }{MFOV}[/math]

Spot Size for Ideal Temperature Measurement in Infrared Pyrometry and Thermal Imaging

Measurements based on infrared devices can only be correct if the measuring object is bigger than the measurement spot size of the infrared device. For accurate temperature measurement, the target size should be at least equal to the size of the measurement spot. This ensures that the measurement device detects at least 90% of the target’s energy if the target is exactly the size of the measurement spot.

Ensuring the object is larger than or at least equal to the measurement spot is essential. Only ratio pyrometers can measure objects smaller than the measuring spot without causing inaccuracies, leading to a signal attenuation of 80%. To ensure accurate measurement, always verify the measurement spot size of a pyrometer at the specified distance.

To ensure reliable and accurate measurements, the target should completely fill—and ideally exceed—the measurement spot.

This is particularly critical in the 8–14 µm range, where sensitivity to background influence is highest due to the broad spectral response. The relationship between the target size and the measurement spot size has a significant impact on accuracy. When the target fills or exceeds the measurement spot, the sensor captures only the target’s radiation, and no temperature deviation occurs. However, if the spot size exceeds the target dimensions, the sensor also detects radiation from the surrounding, typically cooler background. This results in a mixed signal, leading to temperature underestimation. The extent of the measurement error depends on the temperature contrast between the target and its surroundings, as well as the proportion of background within the spot.

Figure 3 illustrates how incomplete target coverage within the measurement spot leads to significant temperature errors when using infrared sensors operating in the longwave infrared (LWIR) band. When the target fully fills the spot, the sensor captures only the target’s radiation, resulting in accurate temperature readings. However, if the measurement spot exceeds the target size, the sensor also detects radiation from the surrounding, cooler background. The resulting signal is a weighted average, leading to underestimated temperatures.

The extent of this deviation depends strongly on the temperature difference (ΔT) between the target and the background, as well as the proportion of the target within the spot. Larger ΔT values and smaller target coverage lead to increasingly severe errors. As shown, even modest underfilling can introduce deviations exceeding −30 %, especially in scenarios with large temperature contrasts.

Figure 4 illustrates how measurement accuracy deteriorates when the infrared measurement spot exceeds the target size. As the ratio of target size to spot size decreases, the sensor increasingly detects radiation from the cooler background, resulting in a significant underestimation of the actual temperature. The effect varies depending on the device’s spectral sensitivity. Long-wave sensors (e.g., 8–14 µm, labeled “LT”) exhibit the highest deviation, while short-wave sensors (e.g., 0.5–1 µm range) show better resilience to partial spot filling. For accurate readings, the measurement spot must be substantially smaller than the target, especially for long-wavelength devices.