5.3 Reflected Apparent Temperature (RAT) Methods

Emissivity tells the camera how much of the collected radiance is treated as emission. Reflected apparent temperature (RAT) tells the camera what radiation field is being reflected. For Level II work on metals and other low-ε surfaces, RAT is not a secondary checkbox — it is half of the radiometric problem.

Defining RAT

Reflected apparent temperature (also called reflected temperature, apparent reflected temperature, or T_refl) is the temperature of a blackbody that would produce the same infrared radiance as the actual radiation incident on the target from the surroundings (in the camera’s spectral band and for the relevant geometry).

In simpler field language:

RAT is the IR “brightness temperature” of everything the target can see in reflection — walls, open panels, people, heaters, sky, lamps — packaged as one equivalent temperature the camera uses in its reflection term.

It is not automatically:

• The air temperature on a digital thermometer
• The thermostat setpoint
• The temperature of the target itself
• A single wall temperature if the target also sees a hot furnace door on one side and a cold loading dock on the other

For opaque surfaces, the reflection weighting is ρ ≈ 1 − ε. Therefore:

Exam line: when ε is low, reflections dominate; measuring RAT carefully is mandatory for quantitative claims.

Direct vs Reflector Methods

Direct (background) method

Point the camera at a representative surface that approximates what the target sees — for example a wall facing the same way the bus “looks,” or a large cardboard sheet placed to represent the background — with ε set high (≈ 0.95–1.0) if that background is a high-ε diffuser, and read its apparent temperature. Enter that value as RAT.

Works best when:

• The background is fairly uniform
• The target is surrounded by similar high-ε surfaces (indoor rooms)
• You understand the target’s viewing hemisphere

Fails when:

• Specular metals see a hot lamp in a narrow angle while the wall you measured is cool
• Outdoor targets see cold sky in one direction and warm pavement in another
• You measure “air” with a contact probe and call it RAT without radiometric basis

Reflector method (preferred teaching standard for Level II)

Place a reflector so that it intercepts the same incident radiation field the target experiences, then measure the reflector radiometrically.

Crumpled aluminum foil method

1. Take household or industrial aluminum foil.
2. Crumple it, then gently open it so the surface is wrinkled / diffuse, not a flat mirror. Diffuseness averages many directions of the surroundings instead of locking onto one specular hotspot.
3. Place the foil near the target, oriented so it “sees” approximately the same surroundings the measurement surface sees (same side of the cabinet, same general direction).
4. Do not need to touch a dangerously hot surface — place the foil on a stick, cold area nearby, or fixture that shares the radiometric environment.
5. Set camera ε ≈ 1.0 (or 0.95–1.0 per procedure) and measure the foil’s apparent temperature.
6. Because the foil has very high reflectivity, that apparent temperature is essentially the RAT of the incident field.
7. Enter that value as the camera’s reflected temperature parameter for subsequent target measurements.
8. Then set the target’s true ε (or measure on tape) and read the target.

Why crumple (diffuse) instead of flat mirror foil?

Level II exams and courses emphasize the crumpled foil technique because it is field-practical and reduces specular “gotcha” reflections.

When Reflections Dominate Low-ε Targets

Classic field symptoms

• Polished stainless door shows a clear reflection of the inspector as a warm blob that moves when you move.
• Outdoor metal roof shows sky temperature patches that change with viewing angle.
• Open switchgear shows cool metal that is actually warm, because it reflects a cold wall — or the reverse near a heater.
• Rotating the camera or operator changes the “hot spot” location on shiny metal without load change.

These are reflection artifacts, not always real thermal faults. Quantitative work must separate them; qualitative work must not call every bright metal patch a failure.

Worked conceptual example

True metal temperature = 70 °C, ε = 0.10, surroundings (true RAT) = 20 °C.

If you leave RAT at 20 °C and ε at 0.10, the solution can approach 70 °C (idealized). If you wrongly enter RAT = 40 °C (you stood next to a warm panel when measuring background), the reflection term is wrong and the solved metal temperature shifts — error grows as ρ grows. If you also leave ε = 0.95, both terms are wrong and the reading may bear little relation to 70 °C.

Message: low ε multiplies the cost of RAT mistakes.

Measuring RAT Without Touching Hot Surfaces

Hot pipes, kiln shells, and energized bus create a practical problem: you need the reflection field, not a burn.

Safe Level II approaches:

1. Foil on an insulated handle or pole held in the same “view” the target has of the room — without contacting the hot surface.
2. Foil or diffuse reflector beside the line of fire, same hemisphere of incidence.
3. Direct measurement of the cold/hot walls, open doors, and heat sources that dominate the target’s view, then combine judgment when the field is nonuniform (document the choice).
4. For outdoor work, account for sky (often very cold in LWIR) vs ground — angle matters enormously on metals.

You are measuring radiation environment, not the surface contact temperature of the foil substrate. Thin foil quickly shows the reflected field when ε is set near 1 for the RAT reading; still avoid holding foil so it conducts from a hot pipe if that heats the foil structure into emission — crumple technique assumes reflection-dominated foil reading.

Integrating RAT with Tape/Paint Workflow

Recommended sequence for a quantitative metal measurement:

1. Observe geometry: what can the surface see?
2. Measure RAT with crumpled foil (ε ≈ 1.0 on foil).
3. Enter RAT in the camera.
4. Measure on tape/paint with ε for the reference emitter or use characterized metal ε.
5. Capture images documenting foil placement and measurement spots.
6. Re-check RAT if you move to a different room orientation or open a door (environment changed).

Common RAT Errors

Summary for Recall

RAT is the radiometric temperature of the radiation incident on the target for reflection correction. Because ρ ≈ 1 − ε, low-ε metals are reflection-dominated and demand careful RAT. The crumpled aluminum foil method creates a diffuse reflector: measure it with ε ≈ 1.0 to obtain RAT, then measure the target with the correct target ε (often on tape). Direct background methods work when the scene is uniform. You can obtain RAT without touching hot surfaces by placing the reflector in the same radiometric environment. Level II quantitative metal work that skips RAT is incomplete even if emissivity was “set to something low.”