The current biodosimetry, a primary fluence validation method for ultraviolet (UV) reactors, is confronting some serious challenges (e.g., the high cost and the lack of accurate monitoring of the lamp output), thus greatly limiting its practical application. In this study, a monitored tunable biodosimetry (MTB) was developed to validate the fluence of UV reactors with a lower cost but a higher accuracy. A novel micro-fluorescent silica detector (MFSD), used to monitor the real-time fluctuation of lamp output, was inserted into the gap between the lamp and the quartz sleeve. An adjustable ballast was utilized to tune the lamp output by adjusting the input voltage so as to export a desired reduction equivalent fluence. Fluence measurements were conducted on a commercial annular UV reactor with two challenge microorganisms (Bacillus subtilis spores and Escherichia coli). Results show that the MFSD could monitor well the lamp output during the validation process, and the viable number of B. subtilis spores could be greatly reduced by as much as 218 times. E. coli, a UV-sensitive microorganism usually inapplicable for fluence validation in the conventional biodosimetry, became applicable in the MTB process. Moreover, the dependence of fluence on water temperature could be readily determined. This MTB process cannot only increase the accuracy but also reduce the cost for UV fluence validation in comparison to the conventional biodosimetry. (c) 2013 Elsevier B.V. All rights reserved.