Understanding Emissivity for Infrared Temperature Measurement

Every object above absolute zero emits infrared radiation. The amount of radiation emitted depends on two things: the object's temperature and its emissivity. Emissivity is a dimensionless number between 0 and 1 that describes how efficiently a surface emits thermal radiation compared to a theoretical perfect emitter (a blackbody, which has emissivity = 1.0).

An infrared thermometer or thermal camera measures the IR radiation arriving at its sensor and uses an internal algorithm to calculate the object's temperature. That algorithm assumes a specific emissivity value. If the assumed emissivity is wrong, the calculated temperature will be wrong. This is not a small error. A polished stainless steel surface (emissivity around 0.15) measured with an IR camera set to the default 0.95 will read dramatically lower than the actual temperature. On a 400-degree surface, the error can exceed 200 degrees.

For most non-metallic surfaces - painted metal, wood, concrete, plastic, rubber, paper, fabric - emissivity is above 0.90 and the default setting of 0.95 produces reasonably accurate results. The problems arise when measuring bare metals, polished surfaces, or materials with unusual surface properties.

Non-metals (emissivity 0.85 to 0.97): Most organic and inorganic non-metallic surfaces have high emissivity. Concrete, brick, wood, asphalt, rubber, plastic, and painted surfaces all fall in the 0.85 to 0.97 range. These are the easiest targets for IR measurement because the default instrument setting works well.

Oxidized metals (emissivity 0.40 to 0.90): When metals corrode or develop a heavy oxide layer, their emissivity increases substantially. Heavily rusted steel can have emissivity above 0.85. Oxidized copper reaches 0.60 to 0.78. The oxide layer is a non-metallic coating that emits IR radiation more efficiently than the bare metal underneath.

Bare metals (emissivity 0.02 to 0.30): Polished and clean bare metals are the most challenging targets. Polished aluminum can be as low as 0.04. Polished copper is around 0.03. Polished stainless steel ranges from 0.10 to 0.20 depending on the specific alloy and finish. These low values mean that most of the radiation reaching the IR sensor is reflected from the surroundings, not emitted by the target.

Coatings and paints (emissivity 0.90 to 0.97): Almost all paints - regardless of color - have emissivity above 0.90. Flat black paint is often cited as 0.97, but white paint is typically 0.92 to 0.95. The color of the paint has almost no effect on emissivity in the long-wave IR band (8-14 micrometers) used by most thermal cameras. This is counterintuitive but important: a white-painted pipe and a black-painted pipe emit nearly the same amount of IR radiation at the same temperature.

The same material can have dramatically different emissivity values depending on its surface condition. Consider copper:

• Polished copper: 0.02 to 0.05
• Machined copper: 0.07
• Slightly oxidized copper: 0.20 to 0.30
• Heavily oxidized copper: 0.60 to 0.78
• Black oxidized copper: 0.78 to 0.82

This range from 0.02 to 0.82 on the same base material illustrates why a single "copper" emissivity value is meaningless without specifying the surface condition. When looking up emissivity values, always match the surface condition to what you are actually measuring in the field.

Surface roughness also affects emissivity. A rough surface has more surface area per unit of projected area, which increases the effective emissivity. Sandblasted metal has higher emissivity than machined metal of the same alloy. This is one reason why rough castings are easier to measure with IR than machined surfaces.

Temperature itself can change emissivity. Many metals become better emitters as they get hotter because the oxide layer grows and the surface properties change. Published emissivity values typically specify the temperature range at which they were measured. Using a room-temperature emissivity value for a 600-degree surface may introduce errors.

Every IR thermometer and thermal camera has an emissivity setting, though on basic models it may be fixed at 0.95. Professional instruments allow you to adjust emissivity from about 0.10 to 1.00. Some advanced thermal cameras allow per-region emissivity settings so you can measure different materials in the same image.

To set emissivity correctly:

1. Identify the target material and its surface condition.
2. Look up the emissivity value in a reference table (such as the one in this tool). Match both the material and the surface condition.
3. Enter the value into your instrument before taking the measurement.
4. If the emissivity value is uncertain, use one of the verification methods described in the next section.

Some instruments display a "reflected apparent temperature" setting in addition to emissivity. This setting compensates for IR radiation from nearby hot objects that reflects off the target and reaches the sensor. In most field conditions, setting the reflected temperature to the ambient air temperature is adequate. In environments with strong radiant heat sources (near furnaces, boilers, or in direct sunlight), the reflected temperature may need to be measured with a crumpled aluminum foil technique.

When you must measure a bare metal surface with low emissivity, several practical techniques improve accuracy:

Electrical tape method: Apply a piece of black electrical tape to the target surface and allow it to reach thermal equilibrium (several minutes for most surfaces, longer for thick metal). Measure the tape temperature with emissivity set to 0.95. The tape temperature equals the surface temperature. This is the simplest and most reliable method for moderate temperatures (below about 200 degrees F, above which the tape adhesive fails).

High-emissivity paint or coating: For permanent or high-temperature applications, apply a thin coat of high-emissivity paint (flat black engine paint works well) to a small area of the target. Measure the painted spot with emissivity set to the paint's known value (typically 0.94 to 0.97).

Contact thermocouple verification: Place a contact thermocouple on the surface to get a true temperature reading. Then adjust the IR instrument's emissivity setting until the IR reading matches the thermocouple reading. Record the emissivity value for future measurements of that same material and condition.

Cavity effect: If the target has a hole, groove, or cavity, the effective emissivity inside the cavity is higher than the flat surface emissivity. Measuring inside a bolt hole or pipe opening can give better accuracy than measuring the flat surface. The deeper the cavity relative to its opening, the closer the effective emissivity approaches 1.0.

Using default emissivity on bare metal: The most common error. The default 0.95 setting is correct for painted surfaces but wildly wrong for polished or bare metals. Always check and adjust emissivity before measuring any metallic surface.

Measuring through glass: Standard glass is opaque to long-wave IR radiation. You cannot measure the temperature of an object through a glass window with a standard thermal camera. The camera sees the glass surface temperature and reflected radiation, not the object behind the glass. Special short-wave cameras and measurement windows exist for this purpose.

Ignoring reflected radiation: On low-emissivity surfaces, most of the radiation reaching the sensor is reflected from the surroundings, not emitted by the target. If a hot object (like a steam pipe) is near the target, its reflected radiation can make the target appear hotter than it actually is. Changing your measurement angle can sometimes reduce this effect.

Distance and spot size: IR instruments measure the average temperature within their field of view (spot size). As distance increases, the spot size grows and the reading averages over a larger area. If the target is smaller than the spot size, the reading includes background radiation and will be inaccurate. Most instruments specify a distance-to-spot ratio (D:S ratio) in their specifications.