## 3 Results
In order to quantify the change of these structures, this study calculated in the central area of the cornea (circular area with 4.2 mm in diameter) the normalized 2D autocorrelation of each image after applying an intensity threshold (in order to minimize the influence of noise all values below this threshold were set to zero). The threshold was set empirically and resulted only in a few visible pixels (due to noise) in areas where no signal was expected. The central point of the resulting autocorrelation function is eliminated (this part was set to zero) to avoid this exceedingly high signal. Figure 6a shows the central part (60 pixels) of the cross section (x-cross_corelation_coefficient plane) of the autocorrelation function. The different colors indicate the varying imaging depths and clearly show first a slow decrease in the total signal followed by an increase of the width of the autocorrelation function with depth. The signal decrease probably results from a decrease of the density of the fine structures that are visible in the anterior part of the stroma (cf. Figure 3b). The width increase can be explained by an enlargement of structures that cause the correlation signal, as can be observed in the corresponding en-face images (cf. Figure 5b-f). For further quantification, the autocorrelation function was integrated over the central part (the extension can be seen in Figure 6a, 1 pixel corresponds to 11 \(\upmu\)m) and the result was plotted (integrated auto correlation factor (IACF) over the en-face image) as a function of depth for two healthy subjects (cf. Figure 6b). The slight decrease that is followed by an increase of the IACF with depth is clearly visible in both subjects. This observation might be caused by the varying ultrastructure of the stroma as the fibrils in deeper layers are more strictly organized than in the superficial ones [1]. The strong peak at the corneal surface is an artefact caused by specular reflection that can be observed in a large region of the central part of the cornea. As in this area a large connected and highly correlated structure is present, the IACF is large. It should be noted that the presence of a strong peak at the corneal surface is not a significant effect. Figure 5: Volume data of the cornea recorded in the cross-polarized detection channel. (**a**) Representative B-scan (lateral average of 10 frames) The depth location of the corresponding en-face images is marked with red horizontal lines; (**b**)–(**f**) En-face frames retrieved at a depth of 100, 200, 300, 400, 500 pixels (101, 202, 303, 404, 505 \(\upmu\)m) from the corneal surface (cf. red lines 1–5 in (a); The field of view of (a) is 11 mm (x) and 860 \(\upmu\)m (z), of Figure 5b–f is 11 mm (x) and 11 mm (y). that the small peak that can be observed in the plots of both subjects at pixel 316 (and pixel 725) is caused by an intensity artefact at this imaging depth that arises from the strong reflection at the corneal surface. For a comparison, the integrated intensity (of each en-face image of the central part of the cornea) was also plotted as function of depth (Figure 6c). In this graph, a slight decrease of the overall intensity with depth can be observed, clearly indicating that the signal in Figure 6b is not dominated by varying intensity.