In an image forming method, an electrostatic latent image formed on a surface of a photoreceptor is developed into a developer image with a developer, and a printing paper transported on a transfer belt is brought into contact with the surface of the photoreceptor so as to transfer the developer image onto the printing paper. The back wall includes: a first region located between the four outer edges and the outer peripheral wall; and a second region which does not include the first region and a region facing the back surface of the display device, and is located between the corner section and the outer peripheral wall. In certain embodiments, the height of the base is less than the height of each of the test strips, such that a portion of each of the test strips extends beyond the distal or top edge of the base. The same invention can be used, in some embodiments, to allow magnetic stripe cards or biometric measurements to become authorized to operate or allow access to the system or device or service. The said PSD determination comprises storing temporal samples of said incident signal received during a predetermined duration and performing a first Discrete Fourier Transform processing on said stored samples.