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The design of the optical system of the Phoropter

Oct. 28, 2020

With the decline in vision of contemporary people, the use of optometry tools has become more and more common. The refractor examines the convergence of light after entering the eyeball and measures the degree of difference in vergence and dispersion between the examined eye and the emmetropic eye. The autorefractor used in modern times is usually an infrared phoropter. The optical components of the infrared phoropter include infrared collimating lasers, optical lenses with coating, optical filters, optical mirrors and optical prisms.

The optical system of the phoropter is composed of a light source, a folding light path, a focusing light path, and an image acquisition and analysis light path. The light source generates a standard image, that is, an infrared aperture. It throws the image into the pupil of the human eyes. The folded light path puts the light emitted by the light source into the pupil of the human eyes, and transmits the reflected image of the fundus to the image acquisition unit. The focusing optical path is mainly used to accurately measure the diopter and other parameters of the human eyes. Image acquisition sequentially records the images of each position during the focusing process, collects and compares these images. Finally, through calculation and judgment, the best imaging point is found and the corresponding diopter is calculated.

The structure of the retinoscope

Through the retinoscope, the degree of reflection of the retina can be observed. Part of the light reflected by the retina enters the retinoscope, passes through the aperture of the mirror, and exits the retinoscope. Among them, the retinoscope includes a focusing lens and a reflecting mirror.

The process of subjective optometry

The principle of this part  is to use two objectives or focusing lenses and a beam splitter. The cursor is clearly imaged on the reflective surface of the retina by changing the degree of convergence and dispersion of light entering the eye, and the refractive power of the human eyes is automatically calculated.

When using colored light as the detection light, some refractometry will place a filter in the illuminating light path to reduce the brightness of the light entering the pupil of the subject, thereby reducing the glare of the human eyes. Since the light reflected by the retina is orange-red, it is sufficient for the examiner to distinguish the light. Change the distance between the optotype and the trial lens by moving optical prism in the optical path back and forth.

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