ISO 11979-2:1999 pdf download – Ophthalmic implants-Intraocular lenses -Part 2: ptical properties and test methods.
NOTE 2 With respect to the wicidence of light, a convex raus is positive and a concave radius is negative.
NOTE 3 Thes equations assume that there is exact alignment of front and beck surtacss along 11* optical axis
NOTE 4 509914 161 descries a nwihod that may be used to dtormeie i,, whicti shou be known to the third decimal
Use n = 1,336, and the dimensions and refractive index of the IOL under in situ conditions to obtam the dioptflc power in situ, D. tram equation (A1).
It the measured dimensions and the refractive ridex of the 101 were not obtained under N, situ condItions, proper corrections therefore should be made.
A.3 Determination of dioptric power from measured back focal length
A.3.1 Principle
The back focal Length (BFL) Is the distance from the back vertex of the IOL to the local point with parallel light incident onaxis upon the 101
NOTE I The position of the focsl poaIt Is dependent on the spatial frequency focused at. K is not cowicident with the paraxial focal posit of the lens under measiaernenl if there ö sphencal erration. The focus found is often referred lose besl focuV.
In order to obtain the paraxial focal length from the measured BFL corrections have to be made for the distance from the back vertex to the back principal plane of the 101, and for the distance from the paraxial focal point to the best focal point
NOTE 2 Bft and the two corrections are a vector quantities The positive direction is that of the optical axw towarde the
A.3.2 Apparatus
A.32.1 Optical bench, such as that illustrated r Figure Al, used to determine BFL.
NOTE his a matter of cocrveiience whether to use a straiit bench or emptoya mirror as illustrated in Figure A.l.
The target is at the focus of the collimator, so that parallel bgtit is wicident upon the IOL The focal length ci the coltimator should be more than ten times that of the 101. The collimator is an achrornat that Is virtually free of aberrations for the wavelength band transmitted by the filter. The fiter should transmIt green light with the transmittance peak close to 546 nm.
The microscope is connected to a position-measurmg device so that its position along the optical axis can be determined with an accuracy of 0,01 mm.
&3.3 Procedure
Mount the IOL on the optical bench just behind the aperiure
Focus the microscope at the back surface of the lOt. and note the position of the microscope. Focus the microscope at the image of the target and note the position of the microscope. NOTE I Focusing should be done at a spatial trequency dose to 0.3 Ct the cut-off frequency of the IOL
The distance from the back vertex of the 101 to the focal point is the back focal Length. BFL, of the lOL
NOTE 2 The procedure given here ass,nes that measurement is done ii air at normal ambient condelons of a laboratory. The catculationa assume that the dimensions of the lOt. are not appreciatity different under ur situ conditions. Should that not be the case, BFt. should be measured with the IOL under simulated Ni situ conditions, with eppropnale changes xi the calculations
B.1 Principle
The resoitition limit ol an 101, expressed as a percentage of the diflraction-Iim.ted cuto4l spatial frecency of an ideal lens having the same focaP length, is determined under identical conditions of aperture, wavelength and surrounding medium
B.2 Apparatus
8.2.1 OptIcal bench, e.g. as ilbistraled wi Figure A.1 having the following features:
a) a collimator achromat which is virtualfy free from aberrations in combination with the light source used, having a focal length preferably at least ten times that of the 101.. being measured;
b) a target known as the U.S. Air Force 1951 Resolution Target (U.S. Mu Std 150-A-1961: Photographic lenses,; see Figure 0.1), diffusely ilurninated by a monochromatic light source of 546 nm ± 10 rim, arid being in the focal plane of the coLmator
C) an aperture stop of 3,0 mm ± 0,1 mm, placed at most 3 mm in front of the 101 being measured;
d) a surrounding medium of air.
e) a microscope objective with a numerical aperture greater than 0,3 and capable of magnifying
B.3 Procedure
Ptace the lOt. on the optical bend,, taking care to centre it as wel as possdle on the optical axle of the bench.
By moving the microscope obi.ctive, focus the image of tne target to obtain the best pos&ble overal balance between coarse and fine patterns (see Figure B. 1).
Then detrmine the fin*st pattern (group, element) for which both horizontal and vertical bars are resotved, with the additional requirement that all coarser patterns are also resolved. Refer to of ISO 6328:— regarding how to determine If a pattern Is resolved, Further examine the image for aberrations other than sphencal aberration.