  A challenge for sensors used in ultraviolet-C (UV-C) decontamination
protocols of N95 respirators is validation that the entire N95 surface receives
the minimum acceptable dose. Photochromic indicators (PCIs) can accurately
measure UV-C dose on nonplanar surfaces, but often saturate below doses
required to decontaminate porous, multilayered textiles such as 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
when UV-C is uncollimated. Through an analytical model, we show that tuning
attenuator refractive index, attenuation coefficient, and thickness can extend
dynamic range, but compromises ideal angular response unless the attenuator is
an ideal diffuser. To demonstrate this tradeoff empirically, we pair PCIs with
model specular (floated borosilicate) and diffuse (polytetrafluoroethylene)
attenuators, characterize the angular response, and evaluate on-N95 UV-C dose
measurement accuracy of each PCI-attenuator stack in a UV-C decontamination
system. While both borosilicate and polytetrafluoroethylene increase PCI
dynamic range >4$\times$, both attenuators introduce angle-dependent
transmittance, which causes location-dependent underestimation of UV-C dose.
The PCI-borosilicate and PCI-polytetrafluoroethylene stacks underreport true
on-N95 dose by 1) 14.7% and 3.6%, respectively, on a surface near-normal to the
array of source lamps, and 2) 40.8% and 19.8%, respectively, on a steeply
sloped location. Overall, we demonstrate that while planar optical attenuators
can increase PCI dynamic range, verification of near-ideal angular response is
critical for accurate UV-C dose measurement.
