The use of a bench scale apparatus (often referred to as a ‘collimated beam’) to apply fluences to water samples is common practice in disinfection research and in validating the performance of full-scale UV disinfection reactors. This study investigated the sources of potential experimental variations in the calculation of fluence when conducting polychromatic collimated beam exposures. Spectral variations associated with lamp operating conditions (e.g. cooling of the lamp), the angle of the spectroradiometer relative to the lamp when measuring the UV fluence rate, and the shape of the arc within the lamp are important to consider in order to achieve reproducible UV fluences when using a polychromatic collimated beam. Specific recommendations are provided to encourage greater experimental rigour and reproducibility in polychromatic UV disinfection studies, including taking spectral output measurements before and after UV exposures and monitoring the lamp voltage as an indication of lamp output stability.