Emissivity: Why Shiny Surfaces and Soot Give Odd Readings

The same spot can read differently on clean stone, soot or metal even when its real temperature hasn't changed. The reason is emissivity, and it's the main thing a gun can't see for itself.

What emissivity is

Emissivity is a measure of how well a surface gives off heat as thermal radiation. It's a ratio. The radiation from a surface is compared with that from an ideal black surface at the same temperature, and the ratio varies from 0 to 1 for ordinary surfaces¹. A value near 1 means the surface radiates freely. A value near 0 means it gives off little.

It isn't a fixed property of a material name. The emissivity of a surface depends on its chemical composition and geometrical structure¹. Rough and smooth versions of the same material can differ. So can fresh and weathered ones.

Why this matters to a gun

An infrared thermometer converts the radiation it receives into a temperature. To do that it has to assume an emissivity. Most surfaces have high emissivity, and most guns rely on that simplifying assumption, but reflective surfaces have lower emissivity than non-reflective surfaces².

If the real surface radiates less than the gun assumes, the gun gets less radiation and reports a number that's too low. Reflections add a second problem. A shiny surface can bounce radiation from a hotter body, such as the fire, into the gun.

Emissivity can even move on one surface. Emissivity was found to change, often drastically, with surface roughness, bulk and surface composition, and even the temperature itself³. An oven floor changes with use, so the number can drift from week to week.

What you'll see in an oven

Here is how the common surfaces tend to behave. This is a guide to likely trouble, not a set of values.

An infrared picture of a cold beer can shows vastly different (and incorrect) temperature values depending on the surface material¹. The same trap applies to a floor with mixed patches.

What you can do about it

Some sensors have an adjustable emissivity setting². If yours has one, read its manual for the right setting. Don't guess a number.

If it has no setting, a shiny target can be measured by applying a non-reflective paint or tape, with some loss of accuracy². On an oven floor, don't put tape or paint in the chamber. Check the oven maker's manual first, and don't apply anything the manual doesn't allow.

A safer habit is to choose the same kind of spot every time:

1. Pick a clean, dull part of the floor.
2. Avoid shiny metal parts, like a door edge, as targets.
3. Measure the same spot each session.
4. Compare readings only between spots of the same kind.

Comparing against a second thermometer

A sensor with an adjustable emissivity setting can also be matched to a surface. With a surface temperature known from a contact thermometer, the setting can be matched to it. The result applies to similar surfaces only². That's a method for a user who has a trusted contact reference and a gun with the setting. Most home setups won't have both.

For checking whether your gun is off for other reasons, read Checking Whether Your Thermometer Is Accurate. For which spots on the floor suit a gun, see Where to Aim.