In order to develop an optical absorber for use in the measurement of optical power, the authors investigated and evaluated ultra-black films of nickel-phosphorus alloy, which were plated and etched on substrates using an electroless plating process. The plating and etching bath components and conditions were examined. The spectral reflectances and surface morphologies of the films were measured precisely. An optical absorber with a spectral reflectance of about 0.1% to 0.2% and a low wavelength dependence in the wavelength range of 488 to 1550 nm has been developed. The absorber was exposed to two accelerated-aging environments: (a) 2000 hours in a dry (1% to 2% relative humidity) environment at 100 degrees C, and (b) 2000 hours in a damp (85% relative humidity) environment at 85 degrees C. After exposure (a), the spectral reflectance of the absorber increased slightly, but it retained an excellent level of 0.18% to 0.23% in the upper wavelength range. After exposure (b), the spectral reflectance and wavelength dependence showed almost no change.
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