A dual wavelength spectrophotometer produces a relatively small, high power, high duty cycle light spot from a single relatively low power multi-chromatic light source. A Xenon arc lamp light source is focused by an ellipsoidal mirror onto a rotating partially reflective optical chopper. The chopper comprises a wheel having mirrored segments alternately separated by transparent segments. Light reflected by the mirrored segments passes through a first monochromator which produces a first monochromatic light beam. Light transmitted through the transparent segments passes through a second monochromator and emerges as a second monochromatic light beam having a wavelength different from the wavelength of said first monochromatic light beam. The first and second monochromatic light beams are recombined into a single dual wavelength light beam that is reflected through a sample to be analyzed. Reflective front surfaces are employed throughout the system in order to minimize power loss.