Introduction of logarithmic visual acuity chart:
Since the introduction of 1958, logarithmic visual acuity chart has been paid attention to and affirmed by the majority of ophthalmologists in China. Its main point is that statistics requires homogeneity and comparability. In the five-point method, there are optotype, finger, palm and light used to measure vision, and accordingly their measured vision is different, that is, optotype and finger are used to check the cone function, that is, central vision.
Scope of application:
National standard of eye chart-standard logarithmic eye chart GB11533-1989 This standard specifies five main standards, printing specifications and usage methods of eye chart. This standard is applicable to the near and far vision measurement in children and adolescents' general physical examination, enrollment, flight recruitment and other physical examinations, and should also be used for reference in clinic.
Print specifications:
Paper: The far view is printed by offset printing paper, with a basis weight of more than 70g/m2 and a whiteness of not less than 85%. The close-up view is printed with white cardboard, the basis weight is more than 200g/m2, and the whiteness is not less than 90%.
Format: 5-seam (787mm× 1092mm) format is adopted in the far view, and the allowable deviation is 3 mm. The myopia visual acuity chart adopts 145mm×270mm format, with a three-fold style and an allowable deviation of 2 mm.
Terms, symbols and design standards of eye chart;
1, term symbol
Sight: refers to all kinds of words, numbers, graphics, etc. Used to measure vision. According to different measuring objects or occasions, one or more visual targets can be selected. Angle of view: refers to the angle (n) between two points on the external object at the eye node, which is expressed by α and expressed by minutes. Junction point: refers to the optical center of the refractive system of the eyeball, which is about 5mm behind the corneal apex on the optical axis of the eyeball, and the direction of light does not change when it passes through the junction point.
Design distance: refers to the distance from the target to the eye node when the angle between each stroke or gap width of the target at the eye node is exactly 1 min (1'), which is also called 1' viewing distance or 1.0 normal visual distance. Inspection distance: refers to the distance from the eye chart to the eye node n of the examinee during inspection. You can choose the design distance of all targets as the actual inspection distance.
2. Design criteria
Sight shape: use three equal-length square "E", each pen or gap is one-fifth of the side length of the square. Normal vision standard: the vision that can distinguish 1' is defined as the normal vision standard, and it is recorded as 5.0, which is equivalent to 1.0 vision recorded in decimals and 6/6 or 20/20 vision recorded in fractions.