Optical Attenuators Extend Dynamic Range but Alter Angular Response of Planar Ultraviolet-C Dosimeters

Effective ultraviolet-C (UV-C) decontamination protocols of N95 respirators require validation that the entire N95 surface receives sufficient dose. Photochromic indicators (PCIs) can accurately measure UV-C dose on nonplanar surfaces, but often saturate below doses required to decontaminate porous, multilayered textiles like N95s. Here, we investigate the use of optical attenuators to extend PCI dynamic range while maintaining a near-ideal angular response-critical for accurate measurements of uncollimated UV-C. We show analytically that tuning attenuator refractive index, attenuation coefficient, and thickness can extend dynamic range, but compromises angular response unless the attenuator is an ideal diffuser. To investigate this tradeoff empirically, we stack PCIs behind model specular (floated borosilicate) and diffuse (polytetrafluoroethylene) attenuators, characterize the angular response, and evaluate on-N95 UV-C measurement accuracy within a decontamination system. Both attenuators increase PCI dynamic range >4×, but simultaneously introduce angle-dependent transmittance, which causes location-dependent underestimation of UV-C dose. PCI-borosilicate and PCI-polytetrafluoroethylene stacks underreport true on-N95 dose by (1) 14.7% and 3.6%, respectively, when near-normal to the source lamp array, and (2) 40.8% and 19.8%, respectively, in a steeply sloped location. Overall, we demonstrate that while planar attenuators can increase PCI dynamic range, verifying near-ideal angular response is critical for accurate UV-C measurements.