IODA has made this octagonal lens with one spherical surface and one toroidal surface that is used to check the non-symmetric error given by the automated focimeter after calibration.
The specially mounted spherocylindrical-power reference lens with a power of –2,00 D sph/–1,50 D cyl, of ophthalmic form and quality conforming to ISO 8980-1, shall have a diameter of not less than 25 mm.
The spherocylindrical-power reference lens is centred and firmly affixed onto an octagonal disk so thatthe optical centre of the lens is coincident with the geometric centre of the disk.
The length of the sidesof the disk shall be between 25 mm and 30 mm.
The axis of the cylinder should be aligned as close aspracticable to one of the faces of the disk.
The disk shall have a clear central aperture of at least 10 mm diameter centred on the geometrical centre of the disk.
As an alternative , the lens may itself be octagonal, in this case no need for a frame. (lens created by IODA)
The length of the sides of the edged lens shall be at least between 20 mm and 25 mm in order to have stable and accurate measurement results. The optical centre of the edged lens shall be coincident with its geometrical centre.
1) 10 mm clear central aperture within octagonal disk or edged lens
2) spherocylindrical lens, at least 25 mm in diameter, mounted on an octagonal disk
3) octagonal disk, with length of sides between 25 mm and 30 mm
4) octagonally edged lens, with length of sides between 20 mm and 25 mm
For more information check the ISO 8598-1:2014(E)