The paper gives a brief review of calculation methods for the effective emissivity of a blackbody radiator. A new equation is deduced from the integral equation method to estimate the effective emissivity of an isothermal cavity, and an approximate equation which is derived from this equation, can be used for evaluating the effective emisivity of a blackbody cavity. The difference of calculated results between the new approximate equation and the integral equation is less than 0.4% when the material emissivity is 0.7 and the closeness factor of the cavity is 0.995.
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