The prism can change the propagation direction of the incident beam. Therefore, in the actual optometry work, it is mainly used to correct the glasses to alleviate the symptoms of diplopia and visual fatigue. The main purpose is to deflect the incident light to the corresponding area of the retina of both eyes with the help of the prism. For example, the Bo (base out) prism is used to alleviate esotropia or esophoria(Fig. 1), The exocline or exocline is relieved by Bi (base in) prism (Fig. 2).

Different from the relief prism, the bottom direction of the prism used for training is the same as the hidden oblique direction. After the hidden oblique eyes are placed with the bottom outward prism in front of the training, the target light shifts to the non corresponding area of the temporal retina synchronously to induce binocular diplopia. In order to avoid diplopia, the binocular position must be focused appropriately to resist the refraction of the prism, so as to increase the binocular collection ability; After the bottom inward prism is placed in front of the implicit oblique eyes, the target light shifts to the non corresponding area of the nasal retina synchronously to induce binocular diplopia. In order to avoid diplopia, the binocular position must be abducted appropriately to resist the refraction of the prism, so as to increase the ability of binocular divergence.

When training, you can use the rotating prism on the comprehensive refractometer, the stepped prism and the block prism.

Method: 

① The prism of esophoria training is base in, and the added prism value is 2-3 times that of implicit strabismus. The target light is refracted by the prism and biased towards the nasal side of macular fovea. In order to overcome diplopia, the eye position is abducted, so as to increase the impact tension of external rectus muscle and strengthen the load exercise;

② The prism of exophoria training is base out, and the magnitude is 2-3 times higher than that of the exotropia. The target light is refracted by the prism and biased towards the temporal side of the macular fovea. In order to overcome diplopia, the eye position is retracted to increase the impact tension of the medial rectus muscle and strengthen the load exercise  

Indications: Simple esotropia and simple exotropia.