Determine the ratio of the total hemispherical emissivity to the normal emissivity for a nondiffuse surface if the intensity of emission varies as the cosine of the angle measured from the normal.
GIVEN
• A nondiffuse surface
• Intensity of emission varies as the cosine of the angle measured from normal
FIND
• The ratio of the total hemispherical emissivity (?) to the normal emissivity (?n)
SKETCH
the intensity of emission varies with cos?, then the emissivity must be proportional to cos?
The average hemispherical value is
The ratio ?/?n is given by
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