Fuse Clip at 3 m: When Digital Zoom Lies
A thermographer must scan 12 mm wide fuse clips inside a 600 mm deep panel. The camera has a 160 × 120 detector and a 25° horizontal lens; the closest accessible position is 3.0 m from the clips. The camera displays a 4× digital zoom, which the thermographer uses to fill the screen with the clip. The clips are rated 100 A and carry 70 A; one clip shows 71 °C while the others show 52 °C. Emissivity is set to 0.95 for the tinned clips. Ambient is 24 °C. The report is due the next day and the panel cannot be opened further without a shutdown.
1. Calculate the IFOV and measurement spot at 3.0 m. Is 71 °C a valid clip temperature under a 3 × 3 pixel criterion?
Read the worked answer
IFOV = (25° × π/180) / 160 = 0.4363 / 160 = 2.73 mrad. Spot at 3.0 m = 2.73 mrad × 3.0 m = 8.2 mm.
The 12 mm clip is only about 12 / 8.2 ≈ 1.5 detector pixels wide. A 3 × 3 pixel criterion requires a spot no larger than 12 / 3 = 4 mm, which at this lens means a distance of about 4 mm / 2.73 mrad ≈ 1.47 m. Therefore 71 °C is not a valid clip temperature: the measurement area includes cooler background, so the reading is a weighted average and the true clip temperature is likely higher. The 19 K difference is at best a qualitative indication of an anomaly, not a quantitative temperature.
2. The thermographer says the 4× digital zoom makes the clip fill the display, so the measurement is fine. Explain why this is wrong and propose a compliant setup.
Read the worked answer
Digital zoom interpolates the existing detector pixels; it does not reduce the instantaneous field of view or collect more optical information. The measurement spot remains 8.2 mm and the clip still fills only about 1.5 detector pixels. To make the measurement valid, change the optics or the geometry:
- Move to ≤1.5 m if access allows; or
- Fit a narrower lens. A 12° lens gives IFOV = 0.2094 / 160 = 1.31 mrad and a 3.9 mm spot at 3 m - just about 3 pixels across the 12 mm clip. A 6° lens gives about 0.65 mrad and a 2.0 mm spot, which is comfortably compliant.
- Keep the target in focus and use a tripod; verify the spot with a known-size target.
Also verify the emissivity of tinned clips rather than assuming 0.95.
3. How would you word the finding in the report given the resolution limit?
Read the worked answer
Use wording that separates the location of the anomaly from the unreliable temperature magnitude, for example:
'Anomaly: line-side fuse clip, phase L2. Apparent temperature 71 °C versus 52 °C on similar clips at 70 A load. Measurement spot was 8.2 mm on a 12 mm target, below the 3 × 3 pixel criterion, so the temperature magnitude is not validated and the true clip temperature may be higher. Recommended action: re-scan at ≤1.5 m or with a telephoto lens, verify emissivity, and confirm with contact measurement or clamp current check before corrective work is scheduled.'
This preserves the useful information without presenting an invalid absolute temperature as fact.