# Contrast Enhancement for Topographic Imaging in Confocal Laser Scanning Microscopy
## 6 Experiment  
For a systematic investigation of the PDM the setup is aligned properly using the 50 \(\upmu\)m (10 AU) pinhole and an image of the test target is recorded in focus. A depth scan of the target along the optical axis proves the focal plane to be in the maximum of the axial intensity distribution. In the next step, the upper lens (L2) is moved further away from the pinhole which results in a change of the focal spot in relation to the pinhole and thus a decrease of the recorded intensity. By reviewing the axial intensity distribution, it is evident that the focal plane shifted to the left slope of the depth scan, this shift of the lens is called _PDM far_. For all pinhole displacements, the lens is moved until the intensity drops to 76% of the in-focus intensity at this point. The process and the resulting depth scans (along the optical axis) are shown in Figure 7. Accordingly, a shift of the lens closer to the pinhole results in a change of the focal plane to the right slope of the depth scan and is designated _PDM close_.  
This sequence of acquiring an in-focus, _PDM far_ and _PDM close_ image is repeated for all pinhole sizes. During the experiments it was evident that the images for the displaced pinhole show a higher  
Figure 6: Design of the back end of line of the ASIC, the width of the lines of the writing is 10 \(\upmu\)m.  
Figure 7: Z-scan of the sample with maximum intensity in focal plane (black curve), _PDM far_ (blue) and _PDM close_ (red) using the 10 AU pinhole.  
contrast than the in-focus images. In Figure 8 an image improved with the _PDM far_ is shown along with the corresponding in-focus image for the 10 AU pinhole.  
Both images are captured in the focus plane, thus the sample's z-position is always the same. In both images the structures of 300 nm size can be distinguished from each other. However, the enhanced contrast, achieved by the PDM, simplifies the recognition of single structures.  
For further analysis we also perform a line scan with a fixed x-position on the images. The line scans are shown in Figure 9.  
To investigate the diffraction limited performance of the system, we analyzed the 250 nm structures as shown in Figure 10.  
Figure 8: (**a**) In-focus image using a 10 AU pinhole. (**b**) Image with _PDM far_ using a 10 AU pinhole.  
Figure 9: Line scans from the 300 nm specimen of the test target at a fixed x-position with a 10 AU pinhole. (**a**) In-focus. (**b**) With _PDM far_.  
We achieved the best performance with the 1 AU pinhole and the PDM, the clear separation of the structure demonstrates a diffraction limited imaging system.  
In the next step, a processed ASIC is placed under the confocal system. The images were taken with a configuration of 10 AU pinhole in-focus and _PDM far_. They are shown in Figure 11.  
For emphasizing the characteristics of the PDM a line scan of the ASIC is shown for the in-focus and the _PDM far_ case in Figure 12.  
Figure 11: (**a**) In-focus image. (**b**) Image with _PDM far_ using a 10 AU pinhole.  
Figure 10: Image of the resolved 250 nm structure using a 1 AU pinhole and the _PDM far_.  
Especially single characteristics or irregularities in the features of the line have a much higher contrast.