The double pass technique is a well-established method to evaluate the optical quality of a patient’s eye, being capable of determining its aberrations as well as scattering. It is based on imaging a point source onto the retina, where the light is reflected and recorded by a camera after its second pass through the ocular media.
For double pass imaging, mainly lasers are being used as illumination source due to their high beam directionality and power. However, the combination of high coherence of a laser and the relative unevenness of the human retina lead to a speckle pattern that superimposes the actual double pass image and hinders its interpretation.
In commercial double pass systems, speckle formation is avoided by the use of a vibrating mirror in the optical path in combination with a long enough exposure time of the camera. The final image is the average over many images superimposed by different speckle patterns. The disadvantage of this method is that the moving parts are prone to malfunction.
We are investigating semiconductor-based incoherent light sources in order to reduce speckle formation in a cost-effective non-mechanical way.