10.8: Focus, Range and Distance (FoRD) Errors
- Page ID
- 57923
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Overview of FoRD
Focus, Range, and Distance (FoRD) represent three fundamental physical constraints in thermography:
- Focus – Optical sharpness of the image
- Range – Selected temperature measurement range
- Distance – Separation between camera and target
FoRD errors are especially critical because they permanently affect the data at the moment of capture and cannot be corrected through thermal tuning, software adjustment, or post-processing.
Focus Errors
Improper focus reduces spatial resolution and alters temperature readings.
Causes:
- Failure to manually focus
- Relying on autofocus without verification
- Changing distance without refocusing
Impact:
- Averaging of hot and cold areas
- Reduced apparent temperature differences
- Masked small anomalies
Key Principle:
An out-of-focus image cannot be fixed later.
Prevention:
- Manually focus at inspection distance
- Verify focus visually
- Re-focus whenever distance changes
Range Errors
Selecting an incorrect temperature range reduces measurement resolution.
Causes:
- Using overly wide ranges
- Failing to adjust range for different components
Impact:
- Reduced sensitivity
- Flattened thermal contrast
- Increased noise
Key Principle:
Thermal tuning adjusts display appearance, not sensor resolution.
Prevention:
- Select the narrowest appropriate range
- Re-adjust range when conditions change
Distance Errors
Distance directly affects spatial resolution and measurement accuracy.
Causes:
- Excessive standoff distance
- Inspecting small targets from too far away
- Ignoring IFOV and GSD limits
Impact:
- Pixel averaging
- Missed small anomalies
- Invalid temperature readings
Key Principle:
If a target does not fill enough pixels, no amount of software correction can recover the data.
Prevention:
- Reduce distance when possible
- Use higher-resolution sensors or appropriate lenses
- Document distance limitations
FoRD vs. Thermal Tuning
Thermal tuning (level, span, palette changes) affects image appearance only.
|
FoRD Factor |
Can Thermal Tuning Fix It? |
|
Out-of-focus image |
❌ No |
|
Excessive distance |
❌ No |
|
Incorrect range resolution |
❌ No |
|
Poor contrast |
⚠️ Appearance only |