Emissivity & it’s Measurements

EMISSIVITY

The relative ability of a material’s surface to emit heat by radiation is measured by its emissivity (ε). All objects at temperatures above absolute zero emit thermal radiation. However, for any particular wavelength and temperature. The ratio of energy radiated from an object’s surface to the energy radiated from a blackbody at the same temperature and wavelength is known as emissivity.

EMISSIVITY (ε) = Radiant energy of an object surface at a given temperature

                       Radiant energy of a blackbody at the same temperature

The values of emissivity can range from 0 to 1 for perfect radiator (Blackbody) value is 1 while for perfect reflector (Whitebody) value is 0. Because they release a portion of their maximum possible blackbody radiation at a given temperature, most natural objects are classified as “graybodies.”

Emissivity of Materials

The emissivity of a particular material depends on its specific chemical makeup and surface characteristics. Smooth, shiny surfaces, for example, tend to have higher reflectivity and thus, low emissivity. Materials including cloth, rubber, plastics, ceramics, water, and organic materials have high emissivity. The emissivity factor materials broadly categorized as metal and non- metal and its emissivity standard values illustrated

Table 1: Metal & Non- Metal Materials Emissivity

Factors Affected the Emissivity of a Material

There are several factors which affected the emissivity of a material. We need to be aware of their effect on emissivity values

• Effect of Materials or Surface Condition

In the case of metallic materials, emissivity will decrease with polishing and increase with surface roughness. Metals which have been subject to a high temperature industrial process normally have a heavy oxide layer and have a high and stable emissivity values.

• Effect of Wavelength

Emissivity will usually vary with wavelength – for example, the emissivity of polished metals tends to decrease as wavelength becomes longer and the emissivity of roughness of metal tends to increases. Material properties usually depend on wavelength. Figure 1 the emissivity of metals usually decreases non-metals can show increases as well

Figure 9: Captured image through TEMPSENS handheld camera model Thermeye640

Captured image through TEMPSENS handheld camera model thermEye640 and its industrial application for instance, if you want to measure a human body temperature you would set the emissivity to 0.98 as per standard emissivity value suggested for the human skin and the reflected temperature to the environmental temperature 68°F/20°C indoors, and your camera will compensate.