If you are a thermographer for hire, a customer is the entity that has hired you to perform the infrared inspection.  This could be the company where the survey is performed; a contractor you are working for at a site; or an owner of a residence for a home inspection.

If you are an in-house thermographer, a "customer" could be a cost center, a specific location or department that engages your services. Think of how you would want to search reports in the future.