How to tell if UVC LEDs work: LED Dosimeter Cards

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Germicidal UVC light is an effective tool for eliminating pathogens on surfaces, but how do you know that the UVC dose is effective?
This tri-color LED UVC dosimeter™ for general use is a safe and efficient tool that can be used to verify UVC exposure of a surface, or an instrument, in a variety of environments, from hotels to restaurants to food and beverage manufacturing and processing facilities.
General purpose dosimeters verify you are delivering a UVC dosage high enough to reduce the microbial load on packaging materials, food products, and other contaminated surfaces targeted for disinfection. These UVC measurement and verification cards also provide you with real-time results that can be recorded, documented, and kept for future verification/validation purposes.
The technology consists of a photo-chromatic ink that changes color when it is stimulated by certain UVC radiation (260 to 280 nanometers). The central circle on the measurement cards changes from yellow to orange and pink, based on the UVC dose received (includes instruction card).

[@sassylady2001]

Regency charges $150.00 for 25 of these one time use cards. That is $6.00 per use. Too expensive.

[@ryanhimmelblau534]

Is the color change linear between yellow, orange, pink, and mauve? If I was to examine the color of the central circle with my spectrophotometer and get Spin and Spex values and then compare with the outer colors, could I quantify the actual UV dose received?
I have a spectroradiometer that is sensitive around 254nm, but I don't have the probe for it that is sensitive to 260-280nm that I need for evaluating my UVC LEDs. These cards could be a nice solution for me if I could correlate UV dose to measured spectrophotometric data.