Abstract: An apparatus, a method of servicing, and a method of measuring a process parameter for a coating process having an isolatable viewport are disclosed. The apparatus includes a viewport, a device positioned for selectively isolating the viewport from a process chamber, and a second chamber positioned between the viewport and the device. The device includes a first position to selectively isolate the viewport from a process chamber and to selectively maintain a predetermined process pressure in the process chamber, the device includes a second position to selectively permit line of sight measurement of the process chamber from the viewport, and the second chamber maintains the predetermined process pressure in the process chamber when the device is in the second position.