1. The composition of the refractive system of the eye
It is composed of four refractive media: cornea, aqueous humor, lens, and vitreous body.
1. Cornea: Horizontal diameter: 11~11.5mm Vertical diameter: 10~10.5mm
Peripheral thickness of cornea: 1.1mm, slightly thinner in the center (CCT): 0.5~0.7 mm
Front radius of curvature: 7.7mm Rear radius of curvature: 6.8mm
Refractive index: 1.376 Transmittance: above 97
Dioptric power is approximately: 43.05 D accounts for about 73% of the total refractive power of the eye.
2. Aqueous humor: The depth of the middle part of the anterior chamber is about 2mm. The water content is 0.1ml, and the content of posterior aqueous humor is 0.06 ml. The refractive index of aqueous humor is: 1.336.
3. Lens: diameter is about 10mm; thickness is 4mm. The refractive index is 1.406
The radius of curvature: the front is 10 and the back is -6. The refractive power is 16.00D~~ 20.00D
4. Vitreous body: The vitreous body is not glass, it is similar to the human eye. A glass-like substance, it is colorless, transparent, semi-solid, and colloidal. Its main component is water, which accounts for about 99% of the volume of the vitreous body. There is a concave surface in the front of the vitreous body that can accommodate the lens, which is called the vitreous concavity. When you are young, the lens and the vitreous body are better able to adhere tightly. As you age, the adhesion between the lens and the vitreous body gradually becomes worse, so it is easy to separate them during senile cataract surgery. There is a layer of very dense material around the vitreous body called the vitreous membrane, which is divided into two parts: the anterior limiting membrane and the posterior limiting membrane. There are no blood vessels in the vitreous body. The nutrients it needs come from the aqueous humor and choroid. Therefore, the metabolism is slow and cannot regenerate. If there is a defect, the space will be filled by the aqueous humor. When the vitreous body becomes cloudy due to various reasons, when you look at something, you will feel like there are mosquitoes flying in front of your eyes. In addition, as age increases, or due to high myopia and other reasons, the semi-solid gel-like vitreous body will gradually become liquid, which is called vitreous body liquefaction.
The refractive index of the vitreous body: 1.336
II. Optical constants of the eye's refractive system
The axial length of the eye is 24.387mm; the refractive power of the eye (at rest) ) 58.64D
1. Optical axis (eye axis): the vertical line passing through the anterior pole of the central part of the cornea surface. The nodes and pivot points of the eye are all on the optical axis. The point where this axis intersects behind the sclera is the posterior pole of the eyeball. The distance between the anterior and posterior poles is the length of the eye axis.
2. Visual axis: the line connecting the fixation point outside the eye through the node and the macula.
3. Fixed axis: the line connecting the fixation point outside the eye and the pivot point. (Rotation point: assuming that the eyeball rotates around a center point in the orbit, the center is approximately 13.5mm behind the simplified cornea.)
4. Simplified eye: also called simple eye or model eye, it is A model eye formed by simplifying the refractive system of the eye.
Figure 1 Simplified eye
Top: FF′: front and rear focus, two principal points of EE′, two nodes of NN′
Bottom: base point of simplified eye , including two main focuses FF′, a node N and the refractive surface of the cornea representing the average number of EE′.
5. Viewing angle (angle a): the angle formed by the two ends of the external object at the node inside the eye.
6. Kappa angle: the angle formed by the line connecting the external gaze point and the anterior pole of the cornea and the optical axis. The visual angle and Kappa angle can be roughly regarded as the same angle clinically. Since the Kappa angle is difficult to measure, it is often calculated based on the angle between the optical axis and the visual axis between the reflective points of the cornea.
The simplest method is to ask the patient to look at the light at 33cm and observe the corneal reflection point. If it coincides with the optical axis in the center of the cornea, the Kappa angle is 0; if it is on the nasal (temporal) side of the cornea center, it is positive (negative) Kappa The angle is usually within 5°, and if it is greater than 7°, it seems to be exoskew.
The light from point A of object AB is focused at a through node n, and the light from point B is focused at b through node n. Therefore, the image on the retina is real and inverted, and its size can be determined by the following formula: As shown in the figure
According to the above formula, assuming the object is 120 mm high and 5280 mm away from the node, the height of the retinal image is AB·nb/Bn=120×15/5280=0.34 mm. This value approximates the diameter of the fovea.