Understanding Emissivity for Infrared Temperature Measurement

What Emissivity Is and Why It Matters

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 radiation arriving at its sensor and calculates a temperature using settings such as emissivity, reflected apparent temperature, distance, atmosphere, and optics. If those assumptions are wrong, the calculated temperature can be badly misleading. Low-emissivity metals are especially vulnerable because reflected radiation can dominate the signal.

Representative tables are useful screening aids, but critical readings still need a verified target condition, reference target or contact probe, appropriate instrument settings, safe-work controls, and qualified thermography review.

Emissivity by Material Type

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: Many non-metallic coatings are high-emissivity targets in the long-wave IR band, but exact values depend on product, finish, thickness, aging, temperature, and wavelength. Treat table rows as screening prompts until the surface is verified.

How Surface Condition Changes Emissivity

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.

Setting Emissivity on Your Instrument

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 representative value only as a starting point, then verify it against a reference target or contact measurement when the result matters.
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 radiation from nearby objects that reflects off the target and reaches the sensor. Around strong radiant sources, direct sun, hot equipment, shiny metal, or inspection windows, treat reflected temperature as a source of error that needs a deliberate procedure rather than a default.

Techniques for Measuring Low-Emissivity Surfaces

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

Electrical tape method: Apply a known high-emissivity tape compatible with the target temperature and allow it to reach thermal equilibrium. Measure the tape with the appropriate setting, then compare nearby bare-surface readings. The tape product, adhesive temperature limit, target temperature, and contact quality still matter.

High-emissivity paint or coating: For permanent or high-temperature applications, use a coating with documented infrared properties and temperature limits. Do not assume every black paint has the same emissivity.

Contact thermocouple verification: A contact probe can anchor the comparison, but probe attachment, surface gradient, thermal mass, calibration, and safety controls all affect the result. Record the full setup, not just the emissivity number.

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.

Common Measurement Errors

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.