An instrument is described which measures the spectral dependence of the radiometric (photophoretic) force on a micron-sized particle in a static configuration. This spectrometer consists of a servo-stabilized Millikan chamber which can be used as a photophoretic balance over the spectral range from 200 nm to 1000 nm. Spectra may be taken in a vacuum as small as 10-4 torr. The action spectrum of the photophoretic force on a crystallite of CdS is used as an example. The pressure dependence of the force at 500 nm is consistant with a radiometric mechanism.
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