GB/T 40177-2021 English PDF (GBT40177-2021)

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GB/T 40177-2021: Optics and optical instruments - Ophthalmology - Graduated dial scale
GB/T 40177-2021
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 11.040.70
C 40
GB/T 40177-2021 / ISO 8429:1986
Optics and optical instruments - Ophthalmology -
Graduated dial scale
(ISO 8429:1986, IDT)
ISSUED ON: MAY 21, 2021
IMPLEMENTED ON: DECEMBER 01, 2021
Issued by: State Administration for Market Regulation;
Standardization Administration of the PRC.
Table of Contents
Foreword ... 3 
1 Scope ... 4 
2 Description of the system ... 4 
3 Example ... 5 
Optics and optical instruments - Ophthalmology -
Graduated dial scale
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.