Emissivity

Introduction

Emissivity defines an object’s ability to emit infrared energy. The energy emitted is connected with the object’s temperature. It varies from 0 to 0.95. Most organic, painted, or oxidized surfaces have emissivities close to 0.95. Most Fluke Process Instruments sensors allow for variable emissivity, so accurate readings can be made on other materials, like shiny metals.

The three main methods for measuring infrared emissivity are calorimetric, radiometric, and reflection. The calorimetric method is based on heating the sample and analyzing the dissipated Stefan–Boltzmann power. Radiometry measures the surface temperature using an infrared thermometer. In reflective emissivity measurements, the intensity of the radiation reflected from a surface is measured by infrared (FTIR) and Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy or by Attenuated Total Reflection (ATR).

What is Emissivity of a Material?

The emissivity of a material in thermal dynamics measures its ability to radiate thermal energy compared to a perfect black body. The Stefan Boltzmann law is computed as the ratio of thermal radiation from the surface to thermal radiation from an ideal blackbody at the same absolute temperature and wavelengths under the same viewing conditions. It is a dimensionless number, ranging from 0 (for a perfect reflector or white body) to 1 (for a black body). Its value depends on the material, the nature of the surface, and the wavelength.

Scope:

The emissivity of a material determines its capability for the emission of heat radiation compared with that of an ideal blackbody at a given temperature. It is one of the main properties in thermal analysis, describing how energy leaves a material surface by radiation. We use it explicitly in thermal imaging, energy efficiency, and temperature measurement applications. Additionally, knowing a specific material’s emissivity helps us calibrate thermal sensors accurately and optimize heat transfer in various industrial processes. In general, it affects both the design and performance of systems related to thermal radiation.

Limitations, Advantages, Uses & Application of Emissivity

The most common uses are non-contact temperature measurements using Infrared thermometers, pyrometers, and thermal imaging. Infrared thermometers are essential for the thermal engineering of furnaces, heat transfer equipment, and building and equipment insulation calculations. They have also been used to determine planetary temperatures and for space vehicle material and insulation design. Earth surface emissivity is essential for thermal remote sensing in climatology, meteorology, and oceanography.

Conclusion

It measures how efficiently a surface emits heat radiation compared to a perfect blackbody at the same temperature. We cast a radioactive material with thermal conductivity (k) and emissivity as a solid sphere. It is a dimensionless quantity ranging from 0 to 1.

A perfect blackbody, which absorbs all incident radiation and emits the maximum radiation at any given temperature, has the value -1. The emissivity of 0 represents an ideal reflector that does not emit any thermal radiation. It plays a crucial role in heat transfer calculations, particularly in scenarios involving radiative heat transfer, such as thermal insulation, climate studies, and the design of thermal imaging systems.

FAQs