Adaptive Optics (AO) is a technology that directly improves the image quality obtained by measuring wavefront aberrations involved in the observation target and inside the optical instrument with a wavefront sensor and correcting it dynamically. This correction usually adapts a deformable mirror that directly changes the optical path length. However, in order to correct the aberrations with higher precision, we are researching technology using a phase modulator (LCOS-SLM) which able to shift the phase of the light wave locally and finely by changing the refractive index of liquid crystal materials. The current major application of adaptive optics technology is in the field of fundus imaging, where high-resolution images of the human retina can be obtained and thus the early diagnosis of eye diseases is highly expected.
By applying adaptive optics technology using LCOS-SLM to retina observation, it is possible to measure the retina with a spatial resolution of several microns (so that the photoreceptor cells can be seen). Figure 1 shows an overview of an adaptive optics retina imaging system. The human eye is irradiated with very weak light that sufficiently satisfies the laser safety standards, and the light scattered from the retina returns to the wavefront sensor via LCOS-SLM. The wavefront sensor consists of a microlens array and a highly sensitive vision camera. This wavefront sensor measures wavefront distortion caused by eye optics and imaging equipment. The negative feedback controller controls the input signal to the LCOS-SLM based on the wavefront distortion measured by the wavefront sensor so that the output wavefront becomes a plane wave (or a spherical wave with the desired curvature).
Figure 1: Principle of wavefront correction with adaptive optics
Figure 2 shows examples of human retina images demonstrating the effect of aberration correction. Without aberration correction, only a blurred and noise image was obtained (Left: Without AO Correction). This will not provide useful information for observation. However, when aberration correction is activated, the photoreceptor cells (bright spots) become resolvable (Right: With AO Correction). The smallest photoreceptor cells in the human eye are only 2 µm to 3 µm; the result indicates that adaptive optics is essential to observe photoreceptor cells.
Figure 2: Results
The retina is known to have a multilayer structure. By using LCOS-SLM to control the focus position, it is possible to visualize the tissue of each layer of the retina. Figure 3 shows examples of retinal images obtained by controlling the focal depth position to the photoreceptor cell, blood vessel and nerve fiber layers. The left image shows the distribution of photoreceptors, wherein the bright spots are photoreceptor cells. From the photoreceptors image, it is possible to detect retinal abnormalities by analyzing the distribution density, cells arrangement, the presence or absence of defects, etc. The image of the blood vessel and blood cell in the middle shows a bifurcated blood vessel, and the blood vessel wall (the black line) and the blood cell in the blood vessel (the bright area between the two black lines). Blood flow can also be measured by video recording. In the image of the nerve fiber on the right side, the bundles of nerve fibers extending from the upper left to the lower right can be seen. The development of advanced medical technology is expected by analyzing the observed high-resolution fundus images.
Figure 3:Imaging results when focus on different depth position.
Observation by focus adjustment with controlling the LCOS-SLM
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