1 Scope
This Standard specifies design standards, printing dimensions, application method and visual acuity statistic method for visual acuity chart.
This Standard is applicable to distant and near optometry used for common physical examination, recruitment and employment of children at 3 or above, adolescent and adults; it may apply visual acuity disorder screening, ophthalmology and visual optics clinic by reference.
2 Terms and Definitions
For the purposes of this Standard, the following terms and definitions apply.
2.1
Optotype
Letter, digit and figure for visual acuity measurement
2.2
Visual angle
The angle of two points of an external object projecting on eye nodal point (N), expressed by (a) in (') (see Figure 1)
Figure 1 Diagram for Visual Angle (a) and Nodal Point N
2.3
Nodal point, N
Optical center of the eyeball dioptric system, about 7mm to the corneal vertex on the eyeball optical axis; the lights have no direction change when they pass through the nodal point (see Figure 1)
2.4
Design distance, D
The distance from the optotype to the eye nodal point when the angle formed each stroke or gap width of an optotype on the eye nodal point is just 1', also called as the distance of 1' perspectival distance (visual angle distance) or the distance of normal visual acuity 1.0. It is expressed from equation:
D=da……………………………………………(1)
Where,
d——The standard distance;
a——The visual angle.
2.5
Examine distance, d'
The actual examine distance
The distance from the visual acuity chart to the eye nodal point (N) of the examined person in the examination. The design distance of all optotype may be selected as the examine distance for actual applicaiton.
2.6
Standard distance, d
A examine distance specified by the visual acuity chart (see Figure 1), also known as standard examine distance
2.7
Increasing rate of optotype size
The patio of the sizes of two adjacent lines of optotypes
2.8
Geometrical progression
It refers to a number sequence in which the ratio of two adjacent numbers is a constant (common ratio), i.e. geometric progression (GP)
Example: the number of visual angles in this Standard is a geometrical progression.
2.9
Arithmetical progression
It refers to a number sequence in which the difference of two adjacent numbers is a constant (common difference value), i.e. arithmetical progression (AP)
Example: the number of 5-mark record in this Standard is a arithmetical progression.
2.10
(visual acuity) record
The mode expressing visual acuity level, by fraction, decimal fraction or logarithm record.
2.11
Fraction record
The expression of visual acuity by examine distance (numerator) and design distance (denominator), i.e. d/ D or d' / D, its substance is the visual acuity expression by the reciprocal visual angle.
2.12
Decimal record, V
The expression of visual acuity by reciprocal visual angle; the visual angle is expressed in ('). It is expressed from equation:
…………………………………………(2)
2.13
Logarithm record
The expression of visual acuity by visual angle logarithm, lga.
2.14
5-mark record, L
It is a logarithm record, a visual acuity expression by 5 marks deducted by the visual angle logarithm; it is expressed by equation below:
L=5-1ga……………………………………(3)
This method is a visual acuity recording mode (Miao's Recording) exclusive in China; in this record system, normal visual acuity is set 5 mark, no light perception is set 0, and the visual acuity levels between (including) them form a complete number system (see Table 1).
Table 1 5-mark Record and Corresponding Decimal Record
5-mark record 0 1 2 3.0 4.0 5.0 6.0
Decimal record 0
(no light perception) 1/∞
(light perception) 0.001
(manual) 0.01 0.1 1.0 10.0
2.15
Illumination
Physical quantity to express the luminance flux in unit object area, expressed in 1m/m2 i.e. Lux (lx).
2.16
Luminance
The physical quantity to express the illumination intensity from light-emitting surface, expressed in cd/m2.
2.17
Emendation, e
In the variable distance application of visual acuity chart, the visual acuity record value shall be added with corresponding emendation, and the equation is as:
Contents
1 Scope
2 Terms and Definitions
3 Design Standards for Visual Acuity Chart
4 Printing Dimensions of Visual Acuity Charts
5 Application Method of Visual Acuity Chart
6 Visual Acuity Statistics
7 Others
Appendix A (Informative) Standard Logarithm Distant Visual Acuity Chart
Appendix B (Informative) Standard Logarithm Near Visual Acuity Chart
