An optical calorimeter is a well known primary power standard in the field of basic optical measurements. An optical calorimeter accurately measures optical power by the DC substitution method after absorbing the optical energy as heat in a blackbody. Reflection at the absorbing cavity and room temperature change during measurements are directly related to the measurement uncertainty of an optical calorimeter. An excellent optical absorber, a Nickel-Phosphorus (NiP) alloy, has been successfully developed in addition to an optical calorimeter in which the effects of ambient temperature variations are minimized by shortening the measurement time through the use of a unique forecasting control method. The spectral reflectance of NiP at 0.4 to 1.8 m is 0.1% to 0.2%. This can be cut to 0.05% or less by using a cylindrical absorbing cavity. The time required for 10 measurements is 10 to 15 minutes. High-accuracy measurement at low levels is possible and ±1% measurement uncertainty at power levels of 100 W to 20 mW has been achieved.
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