With the aging of population and high prevalence of diabetes and hypertension in recent years, the prevalence of retinopathy has increased significantly. Fundus photography is an important method currently used to evaluate and quantify the health status of the human eye. In this technique, a non-mydriatic fundus camera has been widely used to diagnose ocular fundus diseases. Cornea alignment is a classic problem in fundus photography. If the cornea is not at the correct axial position when the fundus camera acquires images, the annular spot of illumination at the pupil will introduce strong stray light, resulting in blurred images and reduced signal-to-noise ratio, which affects the imaging quality of the fundus camera. Therefore, it is essential to achieve axial alignment of the cornea to obtain high-quality results using ophthalmic devices. While a considerable amount of literature has been published on alignment in the x and y directions, there is still very little scientific research on alignment methods in the z direction of the cornea. In this study, we report an alignment method based on the two-light-spot vision marker that identifies the axial position of the cornea with the separation distance of two light spots. Such an alignment method has high precision and a large alignment range. We hope that our alignment method and study results will be useful for designing advanced ophthalmic examination devices such as the fundus camera and confocal laser scanning ophthalmoscope.
In this study, we propose an alignment method based on two-light-spot vision marker. The beam passes through the two-aperture diaphragm and forms two light spots called vision markers. The alignment of the pupil center is achieved based on the x-y position coordinates of the two light spots that coincide on the detector after reflection from the anterior surface of the cornea. The axial position of the fundus camera relative to the cornea is then determined based on the separation distance between the two light spots on the detector after reflection from the anterior surface of the cornea. When designing the two-aperture diaphragm in the alignment optical path, the suitable aperture of the two-aperture diaphragm is determined by considering the effects of edge diffraction phenomenon, light intensity, and aberration. According to the formula and simulation, we then calculate the relationship between the aperture distance with the alignment range and alignment precision and select the aperture distance of the double-aperture diaphragm. We conduct a model eye alignment experiment on a 45° non-mydriatic fundus camera experimental system and measured the alignment precision and alignment range of this alignment method. This is finally validated in human eye experiments.
Model eye alignment experiments demonstrate that the alignment method proposed in this study achieves an alignment precision of 0.25 mm in both positive and negative directions, alignment ranges of 3.6 mm and 4.6 mm, and a total alignment range of 8.2 mm in the fundus camera system (Fig. 7). The experimental precision is worse than the simulated theoretical value, and the alignment range is slightly less than the theoretical value (Fig. 5). The axial distance between the cornea and the objective lens has an approximate linear relationship with the distance between the two light spots on the detector, and the separation speed of the two light spots is almost the same (Fig. 8). In addition, when the error of the distance between two apertures of two-aperture diaphragm is the maximum value of ±0.01 mm, the measurement error (ε) is ±0.0018 mm. The results of the human eye alignment experiments suggest that this alignment method can be applied in practice, thus avoiding stray light in fundus images (Fig.9).
In this study, we demonstrate a method to realize the cornea alignment in z direction. We propose a cornea alignment method based on two-light-spot vision marker, which is used to achieve precision alignment of the human eye in the axial position with respect to the instrument during ophthalmic imaging. It assists in eliminating the effect of stray light from the anterior surface of the cornea on the fundus image. According to the linear relationship between the distance of two separate light spots on the detector and the axial distance from the cornea vertex to the objective lens, the fundus camera can be guided to adjust the axial distance between the instrument with the examined eye. Considering the mutual constraints of alignment precision, alignment range, and vision maker standard intensity, we select a suitable set of two-aperture diaphragm parameters that are verified by model eye experiments, and the 0.25 mm axial alignment precision and 8.2 mm alignment range are achieved. It has been used on human eyes preliminarily. This precision cornea alignment method can meet the demand of fundus photography for eliminating stray light reflected from the cornea.
