GB/T 40177-2021Optics and optical instruments - Ophthalmology - Graduated dial scale (English PDF)

1 Scope

This Standard gives specifications for the angular coordinate system to be used in the design of scales, reticles or other display means incorporated in instruments for determining optical data on human eyes or corrective lenses for human eyes.

2 Description of the system

The coordinate system, specified in this Standard and sometimes called the TABO system, describes a) the angular orientation of the cylinder axis, when used to describe the refractive error of the eye or the refractive effect of a contact or spectacle lens;

b) the angular orientation of the prism base, when used to describe the prismatic effect of a contact or spectacle lens in polar coordinates;

c) the angular orientation of the principal curvature meridians, when used to describe the curvature of a surface such as the cornea or a lens.

The coordinate system is the same whether it refers to the right eye or to the left eye. When used, the coordinate system is described as it appears to a person viewing the eyes and the spectacle lenses - correctly placed in front of the eyes - from outside.

The zero axis of the coordinate system is horizontal and, in the case where the numbering is from 0° to 360°, it is the right-hand portion of the horizontal axis.

The angular value increases in the counter-clockwise direction with 90° corresponding to the vertical axis. When used to define cylinder axis or orientation of principal curvature meridians, the angular value shall be expressed as a number between 0° and 180°. When used to define prism base orientation, the angular value shall be expressed as a number between 0° and 360°, marked by the point where the base of the prism touches the TABO circle.