Definition and Influence of emissivity in non-contact temperature measurement

In non-contact temperature measurement, the infrared or thermal radiation emitted by the measuring object is detected by a pyrometer. The pyrometer calculates the temperature from the received radiation according to Planck's radiation equation. The level of radiation is largely dependent on the emissivity of the measuring object. But what does emissivity actually mean and how does it affect practical measurements? How can the emissivity be determined and what does it depend on? What errors can occur with an incorrectly set emissivity and how can measurement errors be minimized? These and other questions are discussed in the following article.

 

The level of infrared/heat radiation depends not only on the temperature but also on the measuring object itself. The ability of a measuring object to emit the heat radiation it has absorbed is described by the emissivity. An ideal or so-called "blackbody" emits all the radiation it absorbs. A real body radiator emits less radiation at the same temperature than a "blackbody radiator". The emissivity ε is the ratio of the infrared radiation of a real measuring object Φr to the radiation of a "blackbody" Φs.

ε = Φr / Φs

The emissivity is therefore a dimensionless physical quantity between 0 and 1 or 0 and 100 %.

Radiation that hits a measuring object from the environment is reflected to a greater or lesser extent depending on the degree of reflection of the measuring object. Thermal radiation follows the same radiation laws as visible light. In the case of transparent objects (glass, films), additional heat radiation can come from the inside of the measuring object and from the background. The transmittance indicates the percentage of radiation passing through an object. The total radiation ΦΣ detected by a pyrometer is made up as follows.

ΦΣ = ε * ΦO + ρ * ΦU + τ * ΦH

ε = emissivity
ρ = degree of reflection
τ = transmittance
ΦO = object radiation
ΦU = ambient radiation
ΦH = background radiation

The radiation coefficients are linked via the formula:

1 = ε + ρ + τ

The transmission component does not apply to non-transparent objects.

1 = ε + ρ

The emissivity of a measuring object is largely dependent on the material or the surface of the material. Non-metallic and non-transparent objects are usually good heat emitters with an emissivity of > 80 %. The emissivity of metals can vary between 5 and 90 %. The shinier the metal, the lower the emissivity.

Furthermore, the emissivity can change depending on the wavelength. This property is particularly pronounced in metals. The radiation capacity of metals increases as the wavelength becomes shorter. A short-wave pyrometer is therefore recommended for selection.
 

Transparent objects such as glass, plastic or gases have specific wavelength ranges in which they have good radiation properties. Pyrometers with special sensors and filters that are sensitive to this wavelength must be selected to measure the temperature of these materials.

The radiation behaviour of metals and glass also changes depending on the temperature. Oxidation of the surface of metals and the change from solid to liquid significantly changes the emissivity.

The emissivity of metals increases as the temperature rises. With glass, the measuring depth of the pyrometer increases with the temperature and thus the proportion of radiation from the inner area.

In practice, external radiation from the environment can occur. A classic example is the measurement of a cold sheet metal inside a hot heating furnace. In addition to the object radiation, the pyrometer also detects the wall radiation of the furnace reflected on the sheet metal. The closer the object temperature approaches the furnace temperature, the smaller the measuring error.

Water-cooled sighting tubes must be used to record the true object temperature. These are used to shield the interfering radiation from the oven walls. The tube diameter should be at least 6 times the measuring distance to the object in order to create a sufficiently large shadow.

Information on the emissivity of various substances can be found in the literature or operating manuals. However, this information should be treated with caution. It is important to know for which wavelength and temperature the specified value is valid. In addition, these are values that apply under ideal measuring conditions.

Under real conditions, the radiation detected by the pyrometer can also result from the ambient radiation reflected or transmitted by the object. If the pyrometer is set to the idealized literature value, it will indicate a temperature that is too high.

To display the correct temperature, set the emissivity on the pyrometer to a higher value. This is referred to as an artificial increase in emissivity. The actual emissivity to be set can be determined by comparative measurement with a contact thermometer. Of course, the measuring error also depends on the accuracy of the contact measurement.

Alternatively, a sticker with a defined emissivity can be affixed to the measuring object at temperatures up to approx. 250 °C.
 

Firstly, the true temperature on the sticker is determined (Fig. 2). A comparative measurement is then carried out directly next to the sticker and the emissivity is set on the pyrometer so that the previous measured value is displayed again. As the influence of emissivity increases with temperature, this comparative measurement should be carried out at higher temperatures.

In the case of high object temperatures or inaccessible measuring objects, e.g. in a vacuum furnace, a comparative measurement with a very short-wave pyrometer is recommended, as for physical reasons the measuring error decreases with a shorter measuring wavelength.

An intensity comparison pyrometer (Figure 3) is ideal for this purpose. The measuring principle of these devices is based on an optical colour comparison at a wavelength of 0.67 μm. In addition, the measuring principle works regardless of the size of the measuring object.

The effects of a change in emissivity or incorrect pyrometer settings are shown in the diagram in Figure 4.

A few years ago, pyrometers came onto the market that detect radiation at two wavelengths simultaneously. The quotient of these two radiations is proportional to the temperature. If the received radiation of the two measuring channels changes due to a change in the emissivity, the quotient and thus the temperature still remain constant. However, this only applies if the emissivity change is identical for both channels. In practice, a change in metals is not constant. Two-colour pyrometers can then even produce considerably larger measuring errors than one-colour pyrometers. Therefore, the often quoted "emissivity-independent" measurement with two-colour pyrometers should be avoided.

A  two-colour pyrometer has measuring advantages if, for example, the radiation energy of both channels is weakened to the same extent by dirty inspection glasses or dust in the field of view. The temperature is still displayed correctly.

For critical measuring conditions, it is recommended that the two spectral temperature values and the quotient temperature are analyzed in parallel. Depending on the result, the pyrometer can be set to the better measuring method.

When selecting a pyrometer, great attention is paid to the measurement uncertainty specified in the brochure. However, with non-contact temperature measurement, the measuring error that occurs essentially depends on the metrological properties of the measuring object and the ambient conditions. The device-specific measuring error only has a minor effect. Therefore, the correlations described above must be taken into account both when selecting the pyrometer and when determining the measuring point.