Emittance Values for Real Bodies

Emittance, denoted by the symbol $\epsilon$, is a measure of a real body's ability to radiate thermal energy compared to a perfect blackbody at the same temperature. A perfect blackbody is defined as an ideal emitter with an emittance of 1.

Real bodies are not perfect emitters. They emit less radiation than a blackbody at the same temperature. Therefore, their emittance value is always less than 1.

However, real bodies do emit thermal radiation, meaning their emittance is greater than 0.

Combining these points, the emittance ($\epsilon$) for real bodies must fall within the range:

$0 < \epsilon < 1$

Monochromatic Emittance for Real Bodies

Monochromatic Emittance, denoted by $\epsilon_\lambda$, is the emittance of a body at a specific wavelength ($\lambda$). Similar to the total emittance, it compares the radiation emitted at that specific wavelength to that of a perfect blackbody at the same wavelength and temperature.

A perfect blackbody has a monochromatic emittance of 1 for all wavelengths ($\epsilon_\lambda = 1$).

Real bodies, when emitting radiation at a specific wavelength, emit less than a perfect blackbody. Thus, their monochromatic emittance is less than 1.

Real bodies also emit radiation at specific wavelengths, so their monochromatic emittance is greater than 0.

Therefore, the monochromatic emittance ($\epsilon_\lambda$) for real bodies is also within the range:

$0 < \epsilon_\lambda < 1$

Conclusion on Real Body Radiative Properties

For any real body, both its total emittance ($\epsilon$) and its monochromatic emittance ($\epsilon_\lambda$) are strictly between 0 and 1. These values quantify how efficiently a real surface radiates energy compared to the theoretical maximum defined by a blackbody.