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Ophthalmoscopes are used for eye fundus observation in medical diagnostics. The fundus is illuminated through the eye pupil and imaged to infinity by the patient's cornea and lens which act as a magnifier and allow direct observation by the physician's eye. In currently available ophthalmoscopes the requirement of simultaneously illuminating and observing the fundus is met by tilting the optical axis of the illumination ray path with respect to the observation ray path to separate them. This also reduces the blinding effect of reflections from the patient's cornea. However by this tilt the illuminated and observed fields of the fundus no longer coincide which strongly reduces the usable field of view especially when the patient's eye pupil has a small diameter. Hence in most cases the pupil is dilated for such diagnostic check-up, which on the other hand is time consuming and very discomforting for the patient. We avoid this drawback by using coaxial ray paths for illumination and observation. To separate them the illumination beam is expanded to a hollow- cone shape by a double axicon. The illumination beam is guided into the patient's eye by means of a 90 degree deflecting mirror. For fundus observation a hole is drilled in the mirror within the dark center of the expanded illumination beam. This illumination system allows application of Kohler's illumination principle, i.e. the light source can be imaged onto the patient's eye pupil which reduces influence of the latter on the extent of the illuminated area. This illumination principle cannot be applied in conventional ophthalmoscopes. For practical application an ophthalmoscope with this novel illumination system offers the following advantages: (1) field of view is completely illuminated, (2) pupil dilation is not necessary, (3) corneal reflections do not affect fundus observation, (4) illumination light losses are reduced to minimum, (5) handling of such an ophthalmoscope is very convenient and easy to learn, (6) ophthalmoscopy can be performed in emergency situations under severe time restrictions. These advantages may enable ophthalmoscopic investigations in much wider fields compared to currently used illumination principles.
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