# Contrast Enhancement for Topographic Imaging in Confocal Laser Scanning Microscopy
## 7 Discussion  
The principle of contrast enhancement is visible both in the simulation and the experiments. It is evident, that in the focus at the maximum intensity, the contrast is nearly zero for height differences below about 80 nm, while moving out of focus the contrast increases, which is clearly recognizable in Figure 3 and also in the experiment. While the simulation of ideal lenses shows a symmetric intensity and contrast enhancement distribution around focus, the curves for the real lens are asymmetric. Also the depth scans measured in the experiment and shown in Figure 7 are slightly asymmetric, in contrast to the simulated depth scans of the ideal lenses in Figure 4. According to [PERSON] et al. [15] the asymmetry of the depth scans is caused by aberrations. Additionally, [PERSON] et al. [31] show the asymmetry of depth scans depending on the combination and severity of the aberrations. The increase of contrast is much higher in the simulation of the real lens than for ideal lenses, especially on the left side, due to the spherical aberrations which influence the simulation of the real lens. We assume, that the effect of aberrations is probably even higher in our experiments than in the simulation. In contrast to the simulation, other aberration types, such as astigmatism and coma, may also be present. Though, in the experiment, further effects occur such as diffraction from the steps, which are not covered in the simple simulation and probably even increase the contrast of the test target images. In summary, we simulated ideal lenses, to show the basic principle of contrast enhancement. In the next step, we replaced the lower lens L1 by a real lens in the simulation and got a higher increase of contrast, probably due to the spherical aberrations of the real lens. Then finally, we performed the experiment and achieved an even higher contrast improvement that we attribute to a certain extent to further aberrations in the experiment that are not covered in the simulations. However, the principal effect of contrast enhancement can already be seen in the simulations. For an evaluation of the PDM, we compare three characteristics: contrast, slope and intensity. For the intensity value, we take the maximum intensity of the acquired image. For calculating the slope, we take a line of the test target structures in vertical y-direction which is marked in Figure 8. The line scan in x-direction from the normalized image is displayed in Figure 9. For reproducibility, we calculate the maximum slope at every peak along the whole line and take the mean value. Along with the increase of contrast for the PDM, images feature a decrease of intensity of about 24%. In comparison, the contrast increase of the _PDM far_ to the in-focus case is higher than the one of a smaller pinhole. For emphasizing this, Figure 13a shows the comparison of the characteristics of the 10 AU pinhole in focus, with _PDM far_ and the 5 AU pinhole. Figure 13: Comparison of the image characteristics of the test chart. (**a**) For the 10 AU pinhole in-focus, with _PDM far_ and 5 AU pinhole in-focus and with _PDM far_. (**b**) For the 2 AU pinhole in-focus, with _PDM far_ and 1 AU pinhole in-focus and with _PDM far_. In Figure 13b the characteristics of the 2 AU and 1 AU pinholes are shown. The relative increase of contrast and slope is for bigger pinholes higher than for smaller pinholes. Thus the PDM is probably more efficient for bigger pinholes. Nevertheless, the absolute contrast and slope is the highest for the _PDM far_ of the 1 AU pinhole. Indeed, the image with the 5 AU pinhole in-focus has a higher intensity than the 10 AU pinhole _PDM far_ but still it demonstrates a higher contrast. The increase of contrast and slope can also be seen in the line scans in Figure 9. In case of non-destructive testing of materials the reduction of intensity can easily be compensated with an increase in laser power.