1. Gradient method measurement of AC/A 2. Measure the pupil distance and select a trial frame that is consistent with the pupil distance; 3. The refractive error of the subject's eyes is completely corrected; 4. Insert +100-degree spherical lenses in front of both eyes at the same time; 5. Insert a Maddox rod lens in front of the right eye to keep the Maddox rod grid horizontal; 6. Place the proximal Torrington plate 40cm in front of the eyes, turn on the power, and keep the light source and of sight horizontal; 7. Ask the subject about the positional relationship between the light source and the vertical line he sees, and record it; 8. Remove the +100-degree spherical lens, and then insert a -100-degree spherical lens in front of both eyes at the same time; 9. Ask the subject about the positional relationship between the light source and the vertical line he sees, and record it; 10. Subtract the first recorded value from the second recorded value, and then divide it by 2, which is the AC/A value. Power Supply: 2 x AA batteries(Not included batteries) Colour:White Material:ABS Package Contents: 1 * Squint Vision Detection Tool 1 * Stand Only the above package content, other products are not included. Note: Light reflection and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.