According to one embodiment, an image forming apparatus includes a photoreceptor, a charging unit, an exposure unit, a developing unit, a transfer unit, a fixing unit, an environment detecting unit and a control unit. In one embodiment, a plasma is ignited by subjecting a gas in a multi-mode processing cavity to electromagnetic radiation having a frequency between about 1 MHz and about 333 GHz in the presence of a plasma catalyst, which may be passive or active. Pre-positioning the clamp piston can include moving a stop member to an advanced position in the cylinder housing, and urging the clamp piston to bear against the stop member before the releasable locking step has been completed. The data memory is adapted to store a look up table including a plurality of position coordinates and calibrating rules f each corresponding to each of the position coordinates and can be used to convert actual signal value V0i of a basic contact area A0 to a standard signal value Vs. The screening methods of the invention are capable of efficiently screening for prophylactics andor therapeutic drug for various diseases resulting from LLPL deficiency.