ASNT Level III IR cheat sheet

All surfaces at T > 0 K emit infrared radiation; spectral exitance follows Planck's law, which peaks at a wavelength given by Wien's displacement law (lambda_max ~ 2898/T in micrometres).

Stefan-Boltzmann law: total radiant exitance W = epsilonsigmaT^4, so a small temperature error produces a much larger apparent radiant-flux error.

Kirchhoff's law: for an opaque body, spectral emissivity equals spectral absorptivity at each wavelength and temperature (epsilon_lambda = alpha_lambda).

For opaque targets, rho + alpha = 1; for a graybody, emissivity is constant with wavelength but less than 1.

Real targets are non-gray; apparent temperature depends on the spectral bandpass of the instrument because emissivity varies with wavelength.

Atmospheric transmission windows for IR thermography are roughly 2-5.6 micrometres (MWIR) and 8-14 micrometres (LWIR); water vapor and CO2 produce absorption bands.

Conduction, convection, and radiation act together; surface temperature seen by the camera is the result of internal heat flow, surface heat transfer, and radiative exchange with surroundings.

Thermal contrast from a defect depends on the thermal resistance of the defect and the imposed heat flow; deeper or lower-resistance defects produce smaller surface signatures.

Photon detectors (InSb, MCT, QWIP, strained-layer superlattice) require cryogenic or thermoelectric cooling; microbolometer thermal detectors operate uncooled.

Detector figures of merit include responsivity, detectivity D*, noise-equivalent temperature difference (NETD), and noise-equivalent power (NEP).

NETD is the temperature difference producing a signal equal to the RMS noise; lower NETD means better thermal sensitivity.

IFOV = detector element size divided by focal length; measurement FOV (MFOV) is typically taken as 3x3 pixels and the target should fill at least that area.

Spatial resolution is limited by the detector array; instantaneous field of view relates pixel size and lens focal length in milliradians.

Accuracy statement combines radiometric accuracy (typically +/-2 degC or +/-2 percent of reading) with repeatability and drift specifications.

Spectral bandpass, lens transmission, and atmospheric path attenuation must be accounted for when converting indicated to true temperature.

Emissivity and reflected-temperature settings in the camera software directly affect temperature output; incorrect settings bias every reading.

Active thermography applies external energy (flash, halogen, ultrasonic, eddy current, or laser) to reveal subsurface features; passive thermography observes natural thermal gradients.

Pulsed thermography analyzes cooling curves; pulsed phase thermography uses the phase image from the Fourier transform, which is less sensitive to emissivity variation and non-uniform heating.

Lock-in thermography uses periodic excitation and extracts amplitude and phase at the excitation frequency.

Pre-inspection steps include defining the target, selecting the spectral band, setting emissivity, distance, focus, and reflected temperature, and recording environmental conditions.

For building envelope surveys, a steady indoor-outdoor temperature difference of at least 10 degC and low solar load are generally required for meaningful results.

Electrical and mechanical inspections look for abnormal heating from resistance, friction, or load imbalance; loads should be at or near normal operating levels.

Documentation should record camera make/model, calibration date, ambient conditions, emissivity and reflected temperature settings, and inspection geometry.

Reference or baseline images of similar components in the same survey help distinguish normal from abnormal thermal patterns.

Interpret thermal patterns by comparing to expected behavior: uniform heating, geometry effects, and known thermal bridges must be separated from anomalies.

Emissivity differences, solar loading, wind, and reflections can create false indications; verify by changing viewing angle or repeating under different conditions.

A hot spot is not automatically a defect; evaluate magnitude, location, size, and correlation with component function and load.

Temperature rise criteria may be expressed as absolute limits, delta-T over ambient, or delta-T relative to similar components (comparative analysis).

Severity classification commonly uses pattern (uniform/stratified/localized) and delta-T thresholds; document the basis for each classification.

Phase or derivative images help suppress emissivity and non-uniform heating artifacts in active thermography evaluation.

Repeatability requires consistent camera settings, geometry, and environmental conditions between inspections.

False negatives can arise from low thermal contrast, deep defects, or insufficient excitation energy; state detection limits in the report.

ASTM E1934 provides standard guide for examining electrical and mechanical equipment with infrared thermography.

ASTM E1933 covers measuring and compensating for emissivity using infrared imaging radiometers.

ASTM E1186 addresses air leakage detection in building envelopes using infrared thermography.

ISO 18434-1 covers condition monitoring and diagnostics of machines using thermography; ISO 18436 addresses personnel certification categories.

NFPA 70B and NFPA 70E reference thermographic inspection of electrical equipment and electrical safety practices during such inspections.

ASTM E2582 covers infrared flash thermography of composite panels; ASTM D4788 covers bridge deck delamination detection.

Standards typically require documented procedures, calibrated equipment, qualified personnel, and reporting of limitations and assumptions.

Acceptance criteria should be traceable to a standard, manufacturer data, or engineering analysis; state which basis applies in the report.

ASNT SNT-TC-1A is a recommended practice for employer-written qualification and certification programs; ASNT CP-189 is a standard for such programs.

Level I performs calibrated inspections under supervision; Level II performs and interprets inspections and may supervise Level I; Level III establishes procedures, interprets standards, and administers the program.

Qualification includes training hours, documented experience, and examination (general, specific, and practical) as defined by the employer's written practice.

Level III certification requires a current ASNT Level III certificate or employer certification meeting the written practice criteria.

Recertification typically occurs every five years with evidence of continued satisfactory performance and, where applicable, reexamination.

Vision requirements include near-vision acuity and color perception adequate for the method, documented at initial and periodic intervals.

Records of training, experience, examinations, and certification must be maintained by the employer and available for audit.

The written practice defines method scope, levels, training/experience tables, examination content, and recertification intervals.