What is the primary factor that determines the accuracy of temperature measurement in infrared thermography?
A) Color palette used
B) Camera emissivity setting
C) Distance to the target
D) Size of the display screen
Answer: Camera emissivity setting
Explanation: Infrared Thermography Temperature Accuracy
Understanding Infrared Thermography
Infrared thermography is a non-contact temperature measurement technique that detects infrared radiation emitted by objects. Every object with a temperature above absolute zero emits infrared energy. Thermal cameras detect this radiation and convert it into temperature values and thermal images.
However, the accuracy of the measured temperature depends heavily on how the camera interprets the radiation emitted from the surface. This interpretation relies on a critical parameter called emissivity.
What is Emissivity?
Emissivity (ε) is a measure of how efficiently a surface emits infrared radiation compared to an ideal blackbody.
- The value ranges from 0 to 1.
- 1.0 represents a perfect emitter (blackbody).
- Lower values represent reflective materials.
Examples:
- Black paint have emissivity: 0.95
- Human skin have emissivity: 0.98
- Oxidized metal have emissivity: 0.70 – 0.85
- Polished aluminum have emissivity: 0.05 – 0.10
If the emissivity setting in the thermal camera is incorrect, the camera will calculate the wrong temperature.
Example:
- If the real emissivity of a surface is 0.90 but the camera is set to 0.50, the camera will display incorrect temperature values, sometimes with large errors.
Why Emissivity is the Most Important Factor
Thermal cameras do not measure temperature directly. Instead, they measure infrared radiation and then calculate temperature using emissivity and other parameters.
The basic radiation relation is based on Stefan–Boltzmann law, which shows that emitted radiation depends on emissivity and temperature.
If emissivity is incorrect:
- The camera interprets the radiation incorrectly.
- The calculated temperature becomes inaccurate.
Therefore, correct emissivity setting is essential for accurate thermography measurements.
Why the Other Options Are Incorrect
A) Color palette used
The color palette only affects how the thermal image is displayed visually.
Examples:
- Iron palette
- Rainbow palette
- Grayscale palette
Changing the palette:
- Does not change the measured temperature
- Only changes visual interpretation
Thus, it does not affect measurement accuracy.
C) Distance to the target
Distance can affect measurement slightly due to:
- Atmospheric absorption
- Spatial resolution (spot size)
However:
For most industrial thermography inspections, this effect is relatively small compared to emissivity errors. Therefore, distance is not the primary factor.
D) Size of the display screen
The screen size of the thermal camera or monitor has no influence on temperature measurement.
It only affects:
- Viewing comfort
- Image visualization
The temperature calculation happens inside the camera's detector and processing electronics, not the display.
Practical Example in Industrial Thermography
Consider inspecting an electrical busbar:
If the busbar is shiny copper with emissivity around 0.20 but the camera is set to 0.95, the camera may show:
- Temperature: 45°C
But the real temperature might actually be:
- 85°C
This could cause a serious fault to be missed which may lead to overheating or equipment failure. Therefore, thermographers often:
- Apply black electrical tape
- Use high-emissivity paint
- Adjust emissivity settings correctly
Importance for Thermography Certification
Understanding emissivity is a fundamental concept in:
- Level 1 Thermography Certification
- Level 2 Thermography
- Level 3 Thermography
- FLIR Infrared Training
- Condition Monitoring Programs
Many exam questions specifically test knowledge about emissivity and measurement accuracy.
