Controller and control method

A controller for controlling a device in which an output from a region thereof including a first portion changes according to a first state of the first portion, the controller comprising:

• a first hold unit that performs first sample-and-hold processing that includes changing a first output thereof according to an intensity of a monitor signal inputted thereto and thereafter holding the changed first output, the monitor signal changing according to the output;
• a second hold unit that performs second sample-and-hold processing that includes changing a second output thereof according to an intensity of the monitor signal inputted thereto and thereafter holding the changed second output, the second hold unit differing from the first hold unit;
• and
• a control unit that causes the first hold unit to perform the first sample-and-hold processing while holding the first state of the first portion constant and alters the first state of the first portion after the first sample-and-hold processing, then causes the second hold unit to perform the second sample-and-hold processing while holding the altered first state of the first portion, and furthermore controls the first state of the first portion based on a difference, between a first intensity of the first output held by the first hold unit and a second intensity of the second output held by the second hold unit, obtained by subtracting one of the first and second intensities from another thereof.