Abstract: A UV radiation detector with a replaceable secondary window for that is easily replaced by an end user thereby increasing the accuracy of UV radiation measurements. The UV radiation detector with a replaceable secondary window generally includes a housing having a primary window, a UV radiation sensor within the housing to measure UV radiation passing through the primary window, a secondary window protecting the primary window from the fluid and a retainer member having an outer opening removably connectable to the housing to retain the secondary window. The secondary window can be easily replaced after becoming contaminated.