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ASNT Level III IR study materials
ASNT Level III IR study resources

ASNT Level III IR: Infrared Equipment, Detectors, and System Performance

ASNT Level III IR · Work through your exam, one topic at a time.

All guide sections

Infrared Equipment, Detectors, and System Performance

Detector Types and System Parameters

Infrared systems use detectors that convert IR radiation into electrical signals. Two main classes: thermal detectors (e.g., microbolometers, thermopiles) respond to temperature changes and are generally uncooled, while photon detectors (e.g., InSb, HgCdTe, InGaAs) require cooling (often to cryogenic temperatures) for high sensitivity and fast response. Key performance metrics include Noise Equivalent Temperature Difference (NETD), which is the temperature change that produces a signal equal to the noise level; smaller NETD means better sensitivity. Instantaneous Field of View (IFOV) is the angular resolution of a single detector element: IFOV = (detector size)/(focal length). The field of view (FOV) is the total angle viewed by the system. Spatial resolution is often expressed as the smallest detectable object size at a given distance, using the IFOV and distance. Modulation Transfer Function (MTF) describes how well the system reproduces contrast at different spatial frequencies. Frame rate, spectral response, and dynamic range are also critical.

Traps in Questions

  • Confusing NETD with accuracy; NETD is a sensitivity metric, not an absolute accuracy specification.
  • Misunderstanding IFOV: it is an angular measure, not a linear size; to find the smallest resolvable spot, multiply IFOV (in radians) by distance.
  • Assuming all detectors are cooled; microbolometers are uncooled and have lower cost but may have lower sensitivity.
  • Forgetting that lens focal length affects both FOV and IFOV.
  • Overlooking the need for regular calibration and the effect of ambient temperature drift on uncooled detectors.

Worked Example / Rule of Thumb

A common rule: To resolve a target, it should span at least 3 pixels (i.e., 3 × IFOV). For example, if a camera has IFOV = 1 mrad and the target is 10 m away, the smallest resolvable spot is 10 mm; to reliably detect a 30 mm feature, it should cover 3 pixels, so the feature should be at least 3 × 10 mm = 30 mm. This is the “3-pixel rule” for detection. Also, NETD is often given in mK; a NETD of 50 mK means the system can detect temperature differences of 0.05°C.