is area of enclosure, and  are temperatures of gas and surfaces respectively.  is emissivity of surface and  is emissivity of gas.

If the gas is a mixture of say  and  then

 
 or can be determined as follows.

• Determine the partial pressure of  or  in the gaseous mixture
• Determine the value of L by using equation 15.
• Make the  either in  or
• Use the standard chart for emissivity of  and  vs. temperature. In these chart  or  is plotted against temperature for different  value.
• Determine  by adding the emissivities of the individual gas.