For pharmaceutical manufacturers, maintaining the cleanliness of their facilities, especially their stainless-steel process vessels, is Job #1. A UV light for your sight glass can help, as UV light can be used to inspect and verify SIP and CIP procedures have done an adequate job.
Vessels must be cleaned and sterilized thoroughly between each product batch. This is important to prevent contamination from previous batches or residual cleaning agents from adulterating the product. Depending on the type of drug being produced, an inadequately cleaned vessel could potentially force the facility to quality reject millions of dollars’ worth of compromised product.