Thermal imaging camera — setting emissivity and reflected apparent temperature

The guide on thermographic inspection covers how infrared inspection is used as a supplement to NEN 3140 and which NTA 8220 fire-risk class follows from a found temperature difference. This article covers a condition that precedes a reliable measurement: a thermal imaging camera does not directly measure surface temperature, but calculates it from detected infrared radiation — and that calculation is only correct if the camera has been correctly set beforehand for the properties of the surface being measured.

What a thermal imaging camera actually measures

A thermal imaging camera detects infrared radiation falling on the sensor and then converts it into a temperature value based on a number of set parameters: the emissivity of the measured surface, the reflected apparent temperature of the surroundings, the measurement distance, and atmospheric conditions. The detected radiation is not exclusively radiation from the measured object itself: part of it is radiation from surrounding objects that is reflected off the measured surface. Without correctly setting these parameters — emissivity in particular — the displayed temperature can deviate substantially from the actual surface temperature.

Emissivity: how well a surface emits infrared radiation

Emissivity (ε, a value between 0 and 1) indicates how efficiently a surface emits infrared radiation relative to a theoretically ideal radiator (a "black body", ε = 1). Materials relevant to electrical inspections differ substantially in emissivity:

Oxidised or patinated copper/aluminium (such as a weathered busbar or contact surface): typically a relatively high emissivity in the range of 0.6 to 0.8.
Bare, polished metal (a new or cleaned copper or aluminium connection): a considerably lower emissivity, often only 0.1 to 0.3 or lower — such surfaces behave more like a mirror for infrared radiation than a radiation source in their own right.
PVC cable insulation and most painted or coated surfaces: a relatively high, stable emissivity in the range of 0.9 to 0.95, which makes these materials relatively straightforward and reliable to measure.

A bare metal contact surface — precisely the type of connection that is often the most interesting location during an electrical inspection — therefore has the lowest, hardest-to-measure emissivity of all the materials mentioned.

Why an incorrect emissivity setting causes large measurement errors

At a low emissivity, most of the radiation detected by the camera is reflected radiation from the surroundings, not radiation emitted by the object itself based on its own temperature. If the camera is set to an emissivity that is too high relative to the actual bare metal surface, the measured temperature is systematically displayed too low — an actually overheated contact point can therefore be missed, precisely at the location where detecting overheating is most relevant.

The practical solution: a reference material with known emissivity

A common practical technique for reliably measuring a bare metal contact surface is applying a small piece of matte black tape or paint with a known, high emissivity (typically around 0.95) directly next to or on the contact point to be inspected, before the installation is energised and loaded. Because the reference material is in close thermal contact with the surface being measured, the temperature of the reference tape gives a reliable indication of the actual temperature of the underlying metal — even though the bare metal itself is difficult to measure directly.

Reflected temperature: the "crumpled aluminium foil" method

Besides emissivity, the reflected apparent temperature of the surroundings must also be correctly set, particularly for surfaces with a lower emissivity where reflection plays a larger role. A common practical technique for determining this value: placing a piece of crumpled (and then partly unfolded) aluminium foil — which itself has a very low emissivity and thus a strongly reflective, diffuse surface — in the camera's field of view and setting the camera's emissivity to around 1.0; the displayed value then approximates the reflected temperature of the surroundings, which is subsequently entered into the camera's settings as the reflected temperature.

Practical relevance

For a NEN 3140 thermographic inspection of a distribution board with a mix of painted enclosures, insulated cables and bare copper or aluminium busbar sections, it is important to adjust the emissivity setting per measured surface type, or to apply a reference tape at a hard-to-measure bare metal contact surface — one fixed emissivity setting for the whole inspection produces unreliable results on mixed materials, precisely at the locations (bare contact surfaces) most relevant for detecting an overheated connection.

Common mistakes

Using one fixed emissivity setting for the entire inspection, regardless of whether the measured surface is painted, insulated or bare metal — this produces a systematically too-low temperature reading on bare metal contact surfaces.
Not setting the reflected temperature, or leaving it at the default value, in an environment with substantially different ambient temperatures (for example direct sunlight on an outdoor switchboard) — this mainly distorts measurements of low-emissivity surfaces.
Not applying a reference material with known emissivity at a critical bare metal contact point — without a reference, a reliable absolute temperature measurement on such surfaces is barely possible.
Comparing absolute temperature values between measurements taken with different emissivity settings — a meaningful comparison (for example during a re-inspection) requires the same settings, or the difference in setting must be explicitly accounted for in the interpretation.