What has been disclosed is a system and method for authentication of JPEG image data prior to transmission to an intended recipient which enables the recipient to ascertain whether the received image file originated from a known identified source or whether the contents of the file have been altered in some fashion prior to receipt. The first emitter and first receiver are directed towards a first target area while the second emitter and second receiver are directed towards a second target area. Based on the relative weights, automated scores may be determined for one or more first essay responses to a first prompt. The improved technique has advantage for dynamic gain control in a backward-pumped distributed RFA because the feedback signal may be detected right after the RFA.