IFOV vs. MFOV

Infrared cameras are often described using two different metrics for field of view, each addressing slightly different questions. Instantaneous field of view, or IFOV, relates to how much of the scene a single detector element can capture through the optics.

IFOV comes from the history of scanning thermographic systems. In matrix-detector cameras, it is calculated by dividing the horizontal or vertical field of view by the number of image elements. Since imaging lenses, especially wide-angle ones, don’t usually map angles across the entire sensor, the per-pixel IFOV depends on its position. Typically, a mean IFOV is reported for each axis. This idealized field serves as a useful way to estimate how much of a scene a pixel covers at a certain distance. However, it should not be confused with the smallest object size that can still produce a valid temperature measurement.

The idea that smaller is better does not hold because temperature readings depend on how the system measures radiance over a finite aperture. Optical blur and detector sampling also affect how that energy is distributed.

When designing optics for infrared cameras, it’s important to focus on the detail contrast that can be effectively represented in the image. This is defined by the modulation transfer function. Since thermal cameras are more concerned with thermal contrast than visual ones, this function is used along with the slit response function. The result shows how many pixels an object must fill to get an exact temperature measurement. In high-performance infrared optical systems, this is usually 3×3 pixels. In lower-quality systems, it may take as many as 10×10 pixels to capture 90% of the energy in some cases. A high-performance camera lens also enables a greater measuring distance while using the same number of pixels, allowing for exact temperature readings of smaller structures and objects.

MFOV is meant to address practical measurement limits. The measurement field of view, or Minimum field of view for quantitative temperature measurement—commonly referred to as MFOV—is a metric connected to how small a target can be before the indicated temperature deviates due to optical diffraction. There is a difference between what a thermal camera can nominally resolve in an image and what it can consistently measure as temperature within known uncertainty.

The measurement field of view is not a purely geometric measure. Instead, it refers to the field of view corresponding to the diameter of a circular target where the camera’s radiance-proportional signal drops to a specified fraction of a large-aperture reference as a diaphragm is gradually closed. Common fractions for this are 90%. This fraction must be stated alongside the measuring distance, lens, and temperature of the reference radiator.

While MFOV itself is not an uncertainty, selecting a target smaller than MFOV will create a systematic bias in the reading. This bias needs to be accounted for in the uncertainty budget or avoided through design. That’s one reason MFOV appears in many uncertainty statements as a condition, along with calibration documents outlining distance, source size, spectral band, and emissivity assumptions.

When you design or choose a system for a specific task, remember that IFOV describes how the scene is sampled while MFOV describes how the instrument averages energy across its optical and detector apertures. Real-world blurring and crosstalk can make the effective spot size larger than a single geometric pixel.

Using MFOV is a good idea to ensure that the important temperature features are large enough at the given distance and with the selected lens so that the indicated values fall within an acceptable bias.

To help simplify the process of selecting the right thermal camera, Optris has created an optical calculator. Optris’ Optics Calculator assists in choosing a thermal camera or pyrometer and lens by plotting the field of view and spot size against distance. It estimates the minimum measurable object size (IFOV/MFOV), working distance, and coverage to align optics with target size and distance.