Lens Selection for 10 mm Fuse-Clip Terminals

The 25° lens has an IFOV of 1.3 mrad, so at 2.5 m the spot diameter is 0.0013 × 2500 = 3.25 mm. A 10 mm target is therefore about 10 / 3.25 = 3.1 pixels across. A common measurement rule is that the target should fill at least a 3×3 pixel array, meaning the spot should be no more than about one-third of the target width, or roughly 3.3 mm here. The calculation is right at the boundary, but the target is a fuse clip with adjacent metal and air, and alignment through a window is difficult. In practice, 3.1 pixels across is not a robust measurement; it leaves little margin for movement, focus error, or off-center placement. The 25° lens at 2.5 m is therefore marginal and should not be used for a severity call on a 10 mm terminal. The 12° lens gives 0.0013? No: 0.00065 × 2500 = 1.63 mm, about 6.1 pixels across, which meets the requirement with margin.

The thermographer should use the 12° lens or reduce the distance so the target fills at least a 3×3 pixel array. The window transmission of 0.85 must be entered in the camera; otherwise the camera will compute a temperature that is too low because it assumes more target radiance reaches the detector than actually does. The window emissivity and reflected temperature should be checked, and the camera should be angled to avoid reflections from the enclosure. Focus must be on the terminal, not the window frame. Digital zoom only enlarges pixels after detection; it does not improve the optical spot size, the IFOV, or the detector resolution. A 2× zoom on a 3-pixel target produces a larger but still unresolved image. The thermographer should also confirm steady load, compare the same terminal on adjacent phases, and use a contact probe for any high reading. A close-up lens may be an alternative if the window geometry permits.

The situation shows a gap between the thermographer’s level and the task difficulty. A Level II may perform the survey, but the program should define minimum spatial-resolution requirements and require Level III review when the target is near the IFOV limit. The thermographer needs training on window transmission, reflected temperature, focus, and the difference between optical and digital zoom. The program should specify approved lens-and-distance combinations for common targets such as fuse clips, lugs, and breaker terminals, and should require that the measurement parameters be recorded with each image. Procurement should ensure that IR windows have adequate optical quality and transmission data. Quality assurance should include periodic image audits and a check that the reported spot size is no more than one-third of the target width. The thermographer should also be qualified for the electrical environment and permitted to work within the cabinet’s safety boundaries. Without these controls, the survey may produce precise-looking numbers that are not measurement-grade.