Spectral Transmission of the Human Corneal Layers

Abstract

We have assessed the spectral transmittance of the different layers of the human cornea in the ultraviolet (UV), visible, and near-infrared (IR) spectral ranges. Seventy-four corneal sample donors were included in the study. Firstly, the corneal transmittance was measured using a spectrophotometer. Then, all samples were fixed for histopathological analysis, which allowed us to measure the thickness of each corneal layer. Finally, the absorption coefficients of the corneal layers were computed by a linear model reproducing total transmittance. The results show that corneal transmission was almost in unity at the visible and IR ranges but not at the UV range, in which the layer with higher transmission is Descemet's membrane, whereas the stroma showed the lowest transmittance. Regarding the absorption coefficient, the most absorptive tissue was Bowman's layer, followed by the endothelium. Variations on transmittance due to changes in the stroma, Bowman's layer, or Descemet layer were simulated, and important transmission increases were found due to stroma and Bowman changes. To conclude, we have developed a method to measure the transmittance and thickness for each corneal layer. All corneal layers absorb UV light to a greater or lesser extent. The absorption coefficient is higher for Bowman's layer, while the stroma is the layer with the lowest transmittance due to its thickness. Variations in stroma thickness or changes in the corneal tissue of Bowman's layer or the endothelium layer due to some pathologies or surgeries could affect, to a greater or lesser degree, the total transmission of the cornea. Thus, obtaining accurate absorption coefficients for different layers would help us to predict and compensate these changes.

Keywords: absorption coefficient; cornea; layer; transmittance; ultraviolet.

Figure 1

Process of transmittance measurement: corneal sample preparation using a 7 mm diameter trephine, placement between sapphire sheets and then inside the sample caliper and inside the spectrophotometer.

Figure 2

Absorption coefficient for the five corneal layers for all the measured ranges.

Figure 3

Changes in transmittance in the ultraviolet A+B region for: (a) total cornea with two stroma thicknesses (400 μm and 500 μm) and (b) total cornea with and without Bowman’s layer. Data of average thickness of each layer in a normal cornea have been used for this simulation: 50 μm for the epithelium, 10 μm for Bowman’s layer, 500 μm for stroma, 9 μm for Descemet’s layer, 5 μm for the endothelium [7,8,14,16].

Figure 4

Comparison of the Boettner and Wolter [1] experimental result of total transmittance cornea with our calculated results from the absorption coefficients and mean thickness of corneal layers.

Figure 5

Comparison of our absorption coefficient results for epithelium, Bowman’s, and stroma with those obtained from Kolosvári et al. [17] epithelium_k, Bowman_k and stroma_k.