The red-green test mainly uses the principle of chromatic aberration. White light is a composite light composed of seven colors of red, orange, yellow, green, cyan, blue, and purple. The wavelength and refractive index of each color light are different, so white light will occur after passing through a denser medium. Dispersion. In visible light, red light has the longest wavelength, the smallest refractive index, and the fastest speed, while the purple light has the shortest wavelength, largest refractive index, and slowest speed.

Emmetropia can make the focus of yellow light fall on the retina, then the focus of red light falls behind the retina, and the focus of green light falls in front of the retina. The distance between red light and green light is basically equal to the retina, so red light and green light The diameter of the circle of confusion formed on the retina is basically the same, so the clearness of the optotype in the red and green background is basically the same when looking at the eye. Therefore, after correcting the refractive error, if the spherical degree is appropriate, the patient will feel that the clarity of the red and green visual targets is the same. If the subject is near-sighted (that is, under-correction of myopia or over-correction of hyperopia), the focus of yellow light will fall in front of the retina, the focus of red light will be behind the yellow light, so it is closer to the retina, while the focus of green light The yellow light is more forward and therefore farther away from the retina. Therefore, the diameter of the circle of red light formed on the retina is smaller than that of the green light, so the subject will feel that the visual target in the red background is clearer. Therefore, during the red-green test, if the patient complains that the red vision mark is clearer, it is necessary to increase the nearsightedness or reduce the farsightedness to make the red and green visions equally clear.

If the subject is in a hyperopic state (that is, when myopia is overcorrected or hyperopia is undercorrected), the focus of yellow light will fall behind the retina, and the focus of red light will be behind the yellow light, so it is farther away from the retina, while the focus of green light is greater than The yellow light is more forward, and therefore closer to the retina. Therefore, the diameter of the circle of dispersion formed by the green light on the retina is smaller than that of the red light, so the subject will feel that the visual target in the green background is clearer. Therefore, during the red-green test, if the patient complains that the green vision mark is clearer, it is necessary to reduce myopia or increase the distance vision so that the red and green vision marks are equally clear.