The ray-tracing program OSLO LT is available as a free download from Sinclair Optics (http://www.sinopt.com). To make effective use of OSLO LT, you should be familiar with several basic concepts concerning the user interface and the basic algorithms that OSLO uses.
OSLO data can be divided into four major groups: surface data, operating conditions, preferences, and the Spreadsheet Buffer.
 | Surface data describe the refracting and reflecting surfaces and the material in the space between the surfaces and include items such as the radius of curvature, the thickness, the type of glass, the aspheric constant, etc. Surfaces are numbered, starting with 0 for the object surface. In a normal, sequential optical system the surface numbers increase monotonically in the order that rays strike them. The most common way to enter and modify surface data is the "Surface Data" spreadsheet. |
| Operating conditions include items that pertain to the whole optical system. They specify not only fundamental optical items such as the entrance beam radius or the object height, but also data that relate to system analysis, such as the size of grid cells in a spot diagram, or the number of rays to show on a lens drawing. |
| Preferences are to be distinguished from operating conditions. Preferences are attached to the program, and remain the same for all lenses. Examples include the appearance of the graphics windows, number formatting, the selection of fonts, etc. |
| The Spreadsheet Buffer is an important part of the OSLO data structure. As far as OSLO is concerned, the spreadsheet buffer is an output-only data structure, but it serves as the principal source of communication between the program and user macro commands. |
OSLO is fundamentally an interactive program. Each command is executed after it is entered and the display is updated. This means, for example, that the surface data spreadsheet is always up to date. There are a few exceptions, mostly relating to entering optimization data, but for the most part OSLO is based on an interactive model.
User Interface
The OSLO LT user interface is similar to that of other windows programs. The on-line help system explains how to make use of the command line, menu, toolbars, dialog boxes, sliders, graphic and text windows etc.
Data entry
OSLO is a surface-based program, which means that the objects it deals with are surfaces. If you want to work with a solid model, you must enter data that describe all the surfaces that bound the solid. An optical system is specified in OSLO as a series of surfaces that a ray intersects consecutively in passing through the system. Light enters the system traveling from left to right. The object surface is given the number 0. In a sequential system, surfaces are numbered in the order in which a ray or its extension (in the case of a virtual surface) would intercept them. The highest numbered surface is called the image surface, whether or not there is an image formed on it. The correct ordering of the surfaces is essential for lens input specification to OSLO.
Quantities that describe a surface, such as the radius of curvature and the aspheric coefficient, carry the number of the surface. Quantities that pertain to the space between two surfaces, such as the refractive index and the thickness, are assigned to the lower-numbered surface.
Nominally refracting surfaces having no refractive index change have no effect on ray trajectories, and are called dummy surfaces. A dummy surface is often used to keep track of ray data, or to establish a base coordinate system for other surfaces.
Sign conventions
The proper signs for the radius of curvature, the thickness and index of refraction are determined easily for systems that do not contain tilted surfaces. Then the following rules apply:
Sign conventions for centered systems
|
RADIUS OF CURVATURE |
The radius of curvature, or curvature of a surface is positive if the center of curvature lies to the right of the surface. |
|
THICKNESS |
The thickness separating two surfaces is positive if the next surface lies to the right of the current surface; otherwise it is negative. |
|
REFRACTIVE INDEX |
OSLO expects all refractive indices to be provided with positive signs. Reflecting surfaces are specified explicitly by the designation, rfl. |
A simple example illustrates the sign conventions used for setting up optical systems. Consider a glass bead in which light enters from the left, reflects from the back edge, and then emerges from the same surface that it entered, as shown in the figure. The correct surface data for this system is shown below:
| SRF | RADIUS | THICKNESS | APERTURE RADIUS | GLASS |
| 0 | -- | 1.0000e+20 | 1.0000e+14 | Air |
| 1 | 5.000000 | 10.000000 | 4.999999 A | BK7 |
| 2 | -5.000000 | -10.000000 | 4.999999 | Reflect |
| 3 | 5.000000 | -- | 4.999999 | Air |
| 4 | -- | -- | 4.999999 S |
Surface types
OSLO provides many ways of identifying surfaces beyond direct specification. These methods fall into three groups: pickups, solves, and variables.
| Pickups pick up parameters from another surface and apply them to the current surface. This is useful, for example, in designing a system with a mirror that reflects the light back through some part of the system. The surfaces in the path back from the mirror are the same physical surfaces as in the path toward the mirror. By specifying the radii of the surfaces in the path back as pickups from those in the forward path, you guarantee they will always be identical. |
| Solves tell the program to calculate the value of a parameter that satisfies a given condition. For example, specifying the thickness of the next-to-last surface of a lens by an axial ray height solve with height zero tells the program to calculate the thickness of this surface so that the axial ray will pass through the vertex of the image surface. This is the same as saying the image surface remains at the paraxial focus. As other surfaces change, OSLO changes the thickness of the next-to-last surface to satisfy the condition. |
| The Variable specification (which is only applicable for directly specified items) tells OSLO it may vary the item during optimization. |
Whenever lens data is changed, OSLO retraces the axial and chief ray through the system, resolving all pickup and solve requests. The routine that does this is called lens setup.
Lens drawings
Although the main purpose of OSLO is to carry out optical design, the program contains extensive routines for making drawings of lenses that can be viewed on screen, printed as hard copy, or exported to other software in various graphical formats.
To become familiar with OSLO LT, please complete this exercise.