As part of the development of the Sunwatch product, it was necessary to design and build solar simulators both for development use and for production testing. Following conventional practice, xenon arc lamps were used. For Sunwatch testing, it was important to achieve UV and visible/IR power ratios that represented a range of solar altitudes. This was done using a combination of short wavelength stop filters and a supplementary quartz halogen filament lamp. By choice of the filter stop wavelength and glass thickness and by adjusting the power to the filament lamp, a variety of solar conditions could be accurately simulated.