In one exemplary embodiment, apparatus is provided with a plurality of laser diodes, sensing means and control means. During normal operation, the laser diodes produce a mixed light. The sensing means measures light that is output by the laser diodes; and the control means 1) compares the measured light to a spectral reference, and 2) sets drive signals of the laser diodes in response to the comparison. In a related and exemplary method, 1) a mixed light is produced using a plurality of laser diodes of different colors, 2) a measurement of light produced by the laser diodes is compared to a spectral reference, and 3) drive signals of the laser diodes are automatically set in response to the comparison.