6.1 The Five Required Camera Parameters

Level II thermography is quantitative radiometry with a temperature scale attached. The camera does not “know” surface temperature by magic; it infers temperature from collected infrared radiance using a model that needs object properties, background radiation, and path properties. Modern radiometric imagers expose that model as a short list of user parameters. For free-path measurements (no IR window), certification curricula and camera manuals converge on five required inputs:

1. Emissivity (ε)
2. Reflected apparent temperature (RAT) — also called reflected temperature, T_refl, or background temperature
3. Atmospheric temperature (T_atm)
4. Relative humidity (RH)
5. Distance to the target (object distance)

Master these five before severity codes, trending, or report writing. Wrong parameters produce confident but incorrect temperatures — the worst kind of Level II failure.

Why Parameters Exist: The Radiance Budget

Radiation reaching the detector is not pure emission from the target. A simplified radiometric budget for a graybody target and a participating atmosphere is:

L_total ≈ τ_atm · [ε · L_bb(T_obj) + (1 − ε) · L_bb(T_refl)] + (1 − τ_atm) · L_bb(T_atm)

In words:

The imager measures L_total (band-limited). It then inverts the model to report T_obj. If you feed wrong ε, RAT, or path terms, the inversion still produces a number — just not the true surface temperature.

Free path vs windowed path

The five parameters assume clear air (plus ordinary atmosphere) between lens and target. When an IR inspection window sits in the path, you also enter window transmittance (τ_window) — covered in Section 6.3. Do not confuse window τ with atmospheric τ; both reduce signal, but they are separate corrections.

Parameter 1: Emissivity (ε)

Emissivity is the fraction of blackbody radiation the surface emits at the camera’s spectral band and viewing geometry. It is the primary object property for quantitative work.

Level II rule: Prefer a measured or reference-emitter ε (tape, paint, contact cross-check from Chapter 5) over a blind handbook number for critical quantitative readings. Enter the ε that applies to the spot you measure, not a global “panel average.”

What wrong ε does

Emissivity errors are often the largest single source of quantitative mistake on short indoor paths where atmosphere is nearly transparent.

Parameter 2: Reflected Apparent Temperature (RAT)

RAT (reflected apparent temperature) is the radiometric temperature of the radiation field that the target reflects toward the camera. It is not always equal to room air temperature on a thermometer.

Sources that set RAT in practice:

• Walls, ceilings, and open doors in the target’s hemispherical view
• Hot bus work, transformers, or process equipment facing a shiny surface
• Cold sky on outdoor low-ε metals
• Your own body heat if you stand in the specular reflection path of a polished panel

How to set RAT (field methods overview)

For high-ε painted equipment, a 5–10 °C RAT error may move reported temperature only slightly. For ε = 0.1 metal, the same RAT error can dominate the reading. Level II exams repeatedly test the idea that low ε amplifies RAT mistakes.

Parameter 3: Atmospheric Temperature

Atmospheric temperature is the air temperature along the line of sight used by the camera’s atmosphere model. It helps estimate how much the path emits and, together with humidity and distance, how much the path transmits.

Practical guidance:

• Use a trusted thermometer or the facility’s environmental reading for the inspection space
• Outdoor long-path work: air temperature may differ from surface temperature of pavement or equipment — enter air, not the sun-loaded metal temperature
• Do not confuse T_atm with RAT; they can be similar indoors but are different physical quantities

Parameter 4: Relative Humidity (RH)

Relative humidity controls water-vapor content, the main variable absorber in common LWIR atmospheric windows for industrial survey distances. Higher RH → more absorption → lower atmospheric transmittance for a given path length and temperature.

Enter RH as a percent from a hygrometer or reliable environmental station. Guessing “50%” in a steam plant or outdoor tropical survey is not Level II practice when quantitative claims are at stake.

Section 6.2 expands when RH errors matter most. For this section, remember: RH is not optional trivia; it is an input to τ_atm.

Parameter 5: Distance

Distance is the optical path length from the camera (or front of lens, per manufacturer convention) to the measurement plane on the target. The atmosphere model uses distance to scale attenuation. Distance also couples to spatial resolution (IFOV and spot size — Section 6.4), but as a radiometric parameter its first job is path correction.

Many cameras default distance to 1 m and atmosphere to “standard” conditions. Leaving defaults on a 15 m substation shot silently assumes almost no atmospheric loss relative to the true path.

Why All Five Must Be Correct Together

It is tempting to “fix only ε” because it is famous. Level II quantitative accuracy is a system:

Compensation myth: You cannot reliably cancel a large ε error by tweaking distance, or fix a wrong RAT by changing RH. Each parameter maps to a different physical term. Exam distractors often suggest “increase distance to fix low emissivity” — that is incorrect.

Interaction with temperature span and focus

Parameters do not replace optical basics:

1. Focus — Soft focus mixes neighboring radiances; parameters cannot unmix blur
2. Range / calibration interval — Wrong temperature range clips or coarsens data
3. Spot size / IFOV — Undersized targets fail spatially even with perfect ε (Section 6.4)
4. Spectral filters / windows — Extra optical elements need their own τ

Think of the five parameters as the radiometric solution inputs after you have a sharp, on-range, adequately resolved image.

Parameter Entry Workflow (Field Sequence)

Use a repeatable workflow so nothing is left at factory default for quantitative captures:

Step-by-step

1. Plan the measurement — Identify target material, access, distance, indoor vs outdoor, and whether an IR window is present.
2. Stabilize the camera — Power on, complete NUC/shutter as required, select correct temperature range and lens.
3. Measure environment — Record air temperature and RH with a thermometer/hygrometer; note wind and precipitation for outdoor work (affects heat transfer even when not direct camera params).
4. Determine distance — Tape, laser rangefinder, known bay dimensions, or careful pacing; enter the path length to the target surface.
5. Determine RAT — Use foil/reflector method or justified ambient estimate appropriate to ε; recheck if surroundings change (doors open, heaters on).
6. Determine emissivity — Table only when justified; prefer tape/paint/contact method for metals and critical severity calls.
7. Enter all five on the camera (and window τ if used) before the radiometric snapshot used for analysis.
8. Acquire — Focus, compose, capture radiometric image; store parameters with the file when the camera embeds them.
9. Verify plausibility — Compare to contact reading on a suitable surface, to sister phases, or to expected process temperatures; investigate outliers before writing P1 priorities.
10. Document — Report ε, RAT, T_atm, RH, distance (and window τ) with the finding so QA can reconstruct the measurement.

Workflow table for route-based surveys

Factory Defaults and Common Exam Traps

Linking to the Rest of Level II

• Chapter 5 builds how to get ε and RAT right.
• Section 6.2 deepens atmosphere, RH, and distance physics and traps.
• Section 6.3 adds window transmittance as an optical layer.
• Section 6.4 ensures the target is large enough for the IFOV so the radiance you correct is actually from the intended surface.
• QA/QC chapters later will audit whether Level I work left defaults in place.

Summary for Recall

Quantitative IR cameras invert a radiance model. The five free-path parameters — emissivity, RAT, atmospheric temperature, relative humidity, and distance — supply object emission, reflected background, and atmospheric transmission/emission terms. All five must match the actual spot and geometry at the moment of capture; defaults and single-parameter “fixes” create systematic temperature bias. Use a disciplined entry workflow, document every value, and add window transmittance when a viewport is in the path. Level II professionalism is as much parameter discipline as pattern recognition.