The quality of a dry film lubricant coating or molybdenum based undercoat coating provided on the root of a gas turbine fan blade is determined by a method which includes providing an image of the coating taken under controlled conditions which include diffusing light through a translucent screen. The present invention discloses a compound represented by the following formula or a salt thereof. A first converter converts the present image data into a first sub image data and a second sub image data, and a second converter converts the previous image data output into a third sub image data and a fourth sub image data. Events on different links are uncorrelated. At the back end of the head, an electrical terminal is located on the fixed layer and electrically insulated from the upper shield and a current-supply circuit is electrically coupled to the terminal and the lower shield.