Why are polished metals difficult to measure accurately with infrared thermography?
A) They absorb too much heat
B) They have very high emissivity
C) They reflect infrared radiation strongly
D) They produce electrical interference
Correct Answer: C) They reflect infrared radiation strongly
Infrared thermography measures surface temperature by detecting infrared radiation emitted by an object. The key factor here is emissivity - how well a surface emits thermal radiation.
- High emissivity → Accurate temperature reading
- Low emissivity → Poor accuracy
Behavior of Polished Metals
Polished metals (like aluminum, stainless steel, copper) have:
- Very low emissivity (typically 0.02–0.1)
- Very high reflectivity
This means:
- Instead of emitting their own heat, they reflect infrared radiation from surrounding objects.
Why This Causes Measurement Errors
When you point a thermal camera at polished metal:
- The camera doesn’t see the true temperature of the metal
It mostly detects reflected temperatures from:
- Nearby equipment
- Human bodies
- Sunlight or hot objects
Result: False readings / misleading thermograms
Practical Example (Real Inspection Case)
In an electrical panel inspection:
- A polished busbar may appear cool or hot incorrectly
You might:
- Miss a real fault
- Diagnose a false overheating issue
Why Other Options Are Incorrect
A) Absorb too much heat
- Polished metals actually reflect more, absorb less
Very high emissivity
- Opposite is true (they have low emissivity)
D) Electrical interference
- Not related to thermography measurement accuracy
How to Solve This Problem (Field Tips)
To get accurate readings on polished metals:
- Apply black tape or paint (high emissivity surface)
- Use emissivity correction settings in camera
- Change viewing angle to reduce reflections
- Measure indirectly (compare with similar components)
Key Takeaway
- Polished metals are difficult to measure because they behave like mirrors for infrared radiation—not true emitters.
