## 4 Discussion  
This study demonstrated ultrahigh resolution polarization sensitive OCT with a conical scanning pattern for imaging the human cornea in vivo. The improved axial resolution provides a clearer separation between corneal layers and allows for the visualization of very subtle changes, as for example, the thickening of the surface layer at the periphery of the cornea (transition between corneal and limbus epithelium, cf. Figure 2b). A key element for maintaining the high resolution over the entire field of view is the variable dispersion compensation. As the depth dependent dispersion within the cornea is negligible for our imaging wavelength band, a different and simpler approach was used than what has been introduced quite recently for visible light OCT [23]. In comparison with other ultrahigh resolution OCT systems that have been presented so far [21; 22; 24; 25; 26; 27; 28; 29; 30], our system provides a large field of view that covers the entire cornea from limbus to limbus. In addition, due to the conical scanning, a relatively high transverse resolution is maintained as there is no need to take care about the trade-off between a sufficiently large depth of focus (for imaging the spherical shaped cornea over the entire field of view) and high transverse resolution. However, in our current configuration, a relatively low sampling density in the slow scanning direction (y-direction) was used in order to keep the measurement time and thus, the motion artifacts low. Visualization of individual cells, such as endothelium cells or nerves in the en-face imaging plane, was not possible with the instrument because of the low sampling density in the y-direction and/or insufficient transverse resolution. We highlight that the imaging system uses rather low light power (1 mW) on the eye. According to the safety standards and considering that the beam is focused on the cornea, higher light powers may be used. This requires a more powerful light source and/or a different splitting ratio of the fiber beam splitter and may result in a 5-10 dB increased sensitivity. Thus, high quality scans as presented in Figure 3 may be obtained within shorter time and with less image averaging. Another advantage of the presented system is the additional contrast that is provided by the polarization sensitivity of the instrument. Specifically, the images recorded with the cross polarized  
Figure 6: Quantitative evaluation of stromal structure observed in the cross polarized channel. (**a**) Central horizontal autocorrelation function retrieved from en-face images. Black, red, green, blue and orange correspond to an imaging depth of 100, 200, 300, 400, 500 pixels (101, 202, 303, 404, 505 \(\upmu\)m) from the corneal surface, respectively. Purple corresponds to the autocorrelation function of residual noise (800 pixel depth); (**b**) Integrated autocorrelation coefficient (IACF) in dependence of imaging depth (the corneal surface is located at pixel 90); (**c**) Integrated intensity in dependence of imaging depth. Black subject 1, red subject 2. OCT channel show different structures (cf. Figure 3) that are not visible in standard OCT images. Previously, this information was used for improved layer segmentation [13]. Thereby, it was assumed that the depolarization of the backscattered light originates only from the stroma. The higher resolution that is provided by the new instrument allows a more precise assessment. As can be seen in Figures 3 and 4, the process of depolarization in the cornea can already be observed within [PERSON]'s layer. This leads to an observable signal within this layer in the image of the cross-polarization channel (cf. inset of Figure 3b). From a histological point of view, the similar optical behavior (i.e., depolarization) of [PERSON]'s layer and the stroma seems to be reasonable as [PERSON]'s layer is regarded as \"acellular condensate of the most anterior portion of the stroma\" [1].