The surface is illuminated with light whose wavelength is scanned using a monochromator so as to vary the absorption depth of the photons. The deeper in the semiconductor that carrier generation occurs, the fewer the number of minority carriers that will reach the surface and the smaller the photovoltage.
Light with wavelengths adjusted by a monochromator illuminates the surface, affecting how deep photons can penetrate into the semiconductor. Where carrier generation happens further within the semiconductor, fewer minority carriers will reach the surface, leading to a reduced photovoltage.