Overview of LDS-3 software features
LDS-3 can be used to design both GP and SOFT rotationally-symmetrical contact lenses to be manufactured on the OPTOFORM series of lathes (models 30, 40, 50 and 80).
Background
LDS was first developed by Sterling in 1992, and was subsequently replaced by LDS-2 and LDS-3 during the mid-1990s. The program is extremely stable and has been used all over the world by a wide cross-section of ophthalmic labs for many years.
With the development of Fast-Tool-Servo (FTS) devices (also referred to as “oscillating tool” attachments) the ability to design Toric lenses was added to LDS-3.
Sterling does not recommend the use of LDS-3 for designing Toric lenses as there are far superior methods for doing so (see MiniFiles). The incorporation of toric design tools into LDS-3 was abandoned, in favour of a superior design methodology, prior to full debugging of the ÒtoricÓ module in LDS-3. LDS-3 has therefore never been sold as a design tool for toric lenses.
However LDS-3 is an excellent tool (with graphic capabilities) for the design of spherical, multi-spherical (up to 9 curves per side), and advanced aspherics (14th order polynomial).
Output Files from LDS-3
There are two outputs from LDS-3 – lens design (LDS) files and Optoform job (JFL) files. The LDS files hold data for a specific lens type (e.g. basic geometry, minimum / maximum power for this design, initial job file information). This may be then converted into a JFL file for a specific lens of a specific power and diameter. The JFL file describes the surface of the lens to be cut in a point-to-point fashion, for each of the three surfaces that are cut on the lens (back, edge and front).
Lens Definition within LDS-3
LDS-3 defines a lens in terms of three surfaces – the front, back and edge surfaces. Note that these three surfaces are different in definition to the front back and edge surfaces defined in the JFL file, as shown in the figures below. In LDS-3, the edge is defined as the smooth area joining the front and the back surface. This edge extends to a merge region with both the front and back surfaces. In the JFL file, each surface is defined as the part of the surface that will be cut using the front, back or edging tools. As such, the three surfaces are defined as the three quadrants that each of these tools is oriented to cut.
The front and the back surface can be divided into up to nine zones. Each zone is defined by a curve with a spherical radius and possibly a conic (eccentricity) term, causing the surface to be aspheric in nature. This asphericity can be also be modified by the use of the even aspheric terms A2 – A14 (taken from the universal optical equation), to provide specific features on the lens. The zones merge at a given diameter of the lens, and a merge area between the zones may also be defined, to provide a smooth transition between different zones.
The edge of the lens is calculated to provide a smooth surface that merges the front and back surfaces of the lens. This is calculated to fit each individual lens, based on three parameters – where it merges with the front surface, where it merges with the back surface, and the height at which the edge touches the outside diameter of the lens. These parameters are held in dimensionless form. The distance from the edge to the merge point is stored as a fraction of the edge thickness, and the height at which the edge touches the diameter is stored as the offset from the front surface as a proportion of the distance between where the front surface meets the diameter and where the back surface meets the diameter.
Overview of Software Operation
LDS-3 is used to create lens design (LDS) files, which define a family of lenses. This design is created using the five Data Entry / Selection Screens. Individual lens designs can be created by pressing the END key from one of the Data Entry screens, which will then calculate the shape of that lens, and display it on the DISPLAY screen. From here, the job file can be created by pressing END again to bring up the OUTPUT JOB FILE screen, which allows the user to choose the name of the job file to be produced, and to generate that file.
On pressing END, the shape of the lens is calculated. This shape is modified according to a number of rules that can be set by the designer (see Appendix 1). The radius of the front curve will be modified for a given lens power, along with one of the following:
1. Lens centre thickness,
2. Lens edge thickness, or
3. The optical zone’s diameter.
Alternatively, both the centre thickness and the edge thickness may be varied in order to obtain a minimum lens thickness. The shape of the lens may also be varied if a Lenticular zone is included in the front surface of the lens.
When designing a lens, the designer would normally define the type of lens to be produced (e.g. GP/SOFT, SPHERICAL/TORIC, containing conic / aspheric terms or not), and some basic information about it (e.g. number of front/back surface zones, the thickness of the lens). This is performed in the PARAMETER MENU. Specific zone information may then be entered. This would normally consist of specifying the zone’s radius and outside diameter, along with any aspheric terms that may be required. The radius and diameter can be specified with respect to the base curve radius of the back surface and the diameter of the lens respectively. Also, one of the front zones’ radius may be specified as a “Lenticular” radius, which joins two radii together. This is performed in the ZONE MENU.
Once the zones have been defined, individual lenses may be defined by specifying individual lens powers, base curve radii and lens diameters. This is performed in the CALCULATION MENU.
Other information may also be required, depending on the type of lens to be produced. If a Slab-Off front surface is to be produced, the information relating to it should be entered in the SLAB-OFF MENU. Also, toric lenses will require extra information to be input into the PARAMETER and CALCULATION menus.
When designing a soft lens, all parameters are entered in the wet state. The linear and radial shrinkages for the material being used should also be entered in the PARAMETER menu. When END is pressed, the lens is first displayed in its WET state. Pressing END a second time will display the lens in its DRY state. Pressing END for a third time will enable the user to generate a job file.
Lens designs may be loaded / saved from the FILE MENU. However, LDS-3 “remembers” the last design being programmed by storing this in the file “DEFAULT.LDS” in the current directory. This is automatically loaded every time LDS-3 is run.
If required, LDS-3 can be run in a mode that will only enable the user to enter detail into the PARAMETER menu (not the other data entry menus). This is known as running in PROTECTED mode, and may be used in a production environment where lens design families have already been created, and slight changes may be required for individual lenses.
