The Spectral Response of the Fundus

The reflectance of the fundus strongly depends on the wavelength of the incident light. It is very high for the red light [3]; this is why fundus images seem to be "reddish." However, this does not mean that the response to red light reveals most information about the retina. In order to analyze the information content of the color, we have to analyze, more precisely, the process of reflection at and absorption in the different layers of the inner eye (see [27] for a detailed discussion).

In [4] and [3], a radiation transport model is proposed that helps understanding the color content of fundus images. The light enters the eye by the pupil and transverses the layers of the inner eye. In all layers, there is a part of this incoming light which is absorbed, a part which passes the layer, and another part which is reflected. It is this last part of the light that characterizes the perceived color. The transmission, absorption, and reflection of light with a given wavelength depend mainly on the tissue properties, mainly on the concentration of the two pigments melanin and hemoglobin.

The light in the blue spectrum is strongly absorbed by both melanin and hemoglobin. This is one of the main reasons why the reflected light does not contain a lot of blue: It is nearly entirely absorbed in the pigment epithelium layer. Furthermore, the dispersion of the light depends on its wavelength and it

Hemoglobin Melanin

\ s 1 s 1 s I s I s I s \ s \ N I s

Wavelength

Figure 7.3: The extinction coefficient of hemoglobin and melanin depending on the wavelength of the incoming light.

is stronger for smaller wavelength. As a consequence, opacities disturb the blue light more than light of other wavelengths.

Green light is also absorbed by the two pigments, but less than blue light. We can also observe in the Fig. 7.3 that the absorption coefficient has a peak for the green light. As a consequence, features containing hemoglobin absorb more green light than the surrounding tissue; they appear dark in images taken with green light. The green light is reflected on the pigment epithelium and it does not enter into the choroidal layer.

Red light, in contrary, whose absorption by hemoglobin and melanin is quite weak, penetrates deeper into the layers of the inner eye; it is mainly reflected at the sclera. Hence, the red part of the reflected light comes from the choroidal layer or the sclera; it does not contain much information about the retina itself.

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