Comparison of the SECM images (a, b) and the optical micrographs (50 ×, c, d) of a T24 cell. The scanning area in SECM images was (a) 80 × 60 μm and (b) 80 × 80 μm. The acquisition time of each SECM image was about 3 min (256 × 256 pixels). The color profile of each image indicates the profile of the detected current in the scanning area. To keep all the images in proper contrast, every image uses its own color scale. The inset of Figure 2a is the cross section line extracted from the red dashed position in the SECM image. The background tilt was adjusted. In cross section lines, lower current indicates higher height of a certain spot.