Emissivity measurements

LASUR carries out thermo-optical properties on all types of materials between -263 ° C and 3000 ° C for wavelengths ranging from UV to far infrared and angles of incidence from the normal up to 85° of inclination.
The thermo-optical properties (particularly the emissivity) are determining factors for energy exchange by radiation and for thermography measurements.

Do not hesitate to contact us for any inquiries.

LASUR carries out thermo-optical measurements on your materials :


  • Temperature range from -263°C up to 3000°C
    Devices: Induction furnace, Resistance heating and cooling by liquid helium
  • Wavelength: from UltraViolet (0.3 µm) to far Infrared (1000μm)
    Detector: MCT, InSb, Si, InGaAs, UV pyrometer, IR cameras, spectral bolometer
  • Directional measurement: from normal to 85°
    Integration with hemispherical spectral quantities and total hemispherical quantities
  • Error < 5% (in IR)


Specific system for high temperature measurements

Induction furnace heating

Temperature control by UV pyrometer (PYROPHOT)


Liquid nitrogen cooled enclosure for negligible wall radiation
High vacuum down to 10-7 mbar for negligible convection and oxidation



Please feel free to contact us for any inquiries.



Adding emissivity, as a multiplication factor to Planck’s Law which describes only blackbody radiation, must be used to characterize real bodies thermal radiation. For temperature measurements with a pyrometer, it is such a perturbing factor that there is no reliable data, especially concerning metals, on consequences of temperature and measurement wavelength variations on emissivity.

A systematic study would be very useful for a better understanding of how a body can be heated or cooled by radiation absorption or emission. One can study emissivity behavior at high and low temperatures as a function of the observation wavelength (spectral emissivity), of the observation angle (directional emissivity), or by integrating the overall possible values that these parameters can show (total emissivity).

Also some metallurgists have a special interest in studying the possible correlations between phase transitions and emissivity variations. It is well known that a solid-liquid transition results in an emissivity change. However, what is still to be established concerns the consequences on emissivity of metallurgical transformations after heat treatment.

Such variations can be observed on spectral as well as on total emissivity. LASUR offers an “emissivimeter-pyrometer” based on the principle that the radiated energy in the short wavelength range is highly sensitive to temperature, then, for large wavelengths, the sensitivity to emissivity is dominant. While the PYROPHOT pyrometer is measurement temperature at the shortest wavelength compatible with temperature levels, the spectral emissivity is simultaneously measured with maximum precision at a large wavelength.

Return to basics

To measure a surface temperature we use the radiation flux of thermic origin that all bodies emit. The physical law that governs this phenomenon is Planck’s Law that describes the behavior of an ideal entity called the blackbody. The set of curves in Figure 1 describes the spectral distribution of this radiation for several temperatures representative of those commonly encountered, particularly in metallurgy.




We see that the peaks of these curves are located in the infrared (λ>0,8µm) and that the hotter the body is, the more the curve shifts toward short wavelengths.
All equipment currently marketed work in the infrared, meaning close to energy peak, which allows reducing the cost of their fabrication.


Emissivity gets in the way


Unfortunately the blackbody is a purely theoretical entity for, in nature, no body is perfectly black or even grey. Planck’s Law gives only the maximum radiant energy that a body can emit. This value must be weighed by an inclusive factor between 0 and 1 called emissivity and symbolized by the Greek letter ε. To complicate everything, this factor which depends on the nature of the body to be measured, varies according to the condition of the surface (oxidized or not), the wavelength and even the temperature! If controlling an industrial process is desired in following the evolution of the heating of a body, a constant and precise temperature measurement is necessary. However, as we have seen, the result obtained by measuring the flux depends also upon the variation in emissivity. Up to now, there has been no reliable and exhaustive experimental data available that would allow an analytical representation of this factor, or even a reliable interpolation with all the parameters concerned. In the low-cost equipment that can be found on today’s market there is a pre- defined emissivity value, usually 0.8. On the more sophisticated equipment the user chooses an emissivity value that remains unchanged until the end of the running process. As a direct consequence, the measurement result is false and the estimation of the error value is very unpredictable since we have no idea of the amplitude of variation of ε !


Signal variation 100 times greater in ultraviolet

Ultraviolet pyrometry and its generalization, ultraviolet thermography, bring the solution because the measurement is done in a spectrum range where the signal variation with temperature is so large that it hides the consequences of an emissivity variation. Due to the linear scale used in Figure 1, there is no evidence of the steep slope of the curves on the left of the energy peak. In Figure 2, with a logarithmic scale, we zoom toward the short wavelengths for the two temperatures 800°C and 1000°C.



It can be seen that at 0.3µm, in the UV, a 200°C variation results in a multiplication by 1000 of the energy signal, whereas it is only multiplied by 10 at 1µm. The drawback is that the energy levels in the UV are 10-10 times lower.

A reliable solution, highly sensitive and sturdy

Photomultipliers are detectors that have been designed to deal with these extremely weak energy levels. We use photomultipliers in the photon counting mode whose intrinsic noise is only 10 photoelectrons/second and a 100 photoelectron/second signal (that is to say 10-16 joules) is good enough to get a significant measurement result. Due to the extreme sensitivity of our detectors we can work with a very narrow bandwidth, almost monochromatically. Since the signal doubles every 20°C, we obtain very precise results. Another advantage of such a measurement process is that it is fully digital, from the captor to the display of the result. Thus we get rid of the drift problem that is the main flaw with analog devices and we get an excellent repeatability of results. Moreover, these photon counting devices have proven to be very sturdy and they can work continuously in a harsh environment for years.