Lens drawings

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 use the lens drawing routines most effectively, you should understand some concepts that are unique to OSLO. The most important of these is that many of the data items needed to set up a lens drawing are operating conditions, i.e. they are attached to the lens data. For example, once you enter the data describing the default rays to be shown on lens drawings, those rays will be shown on all drawings that have default rays. If you don’t want those rays, however, you can make the drawing with no rays, and then add the rays that you do want later.

Click Show >> Lens Drawing. The lens drawing dialog box will appear.

The dialog box indicates the three types of drawing that OSLO can produce. The Plan view is a two dimensional drawing, made from a view point along the x, y, or z-axis. It is not a sectional view of a three-dimensional drawing. The Wire frame and Solid model drawings are both three-dimensional drawings, one with only apertures and meridional and sagittal lines shown, and the other with full hidden-line removal. The Plan view and Solid model drawings can also be prepared using the SHIFT+F1 and SHIFT+F2 toolbar icons.

Click the Operating conditions button in the lower left corner of the box. The Update lens drawing operating conditions spreadsheet will appear (this spreadsheet can also be obtained through the Update menu).

Most of the items in this spreadsheet provide advanced control over the way that OSLO makes lens drawings, and are described in the reference manual. However, the Number of field points for ray fans, together with the accompanying table of values for the fans, is routinely used to set up default ray patterns appropriate for the particular lens under study. The initial and final distances are routinely used to show the trajectories of rays in object and image space.

The salient features of lens drawings can be easily illustrated using a version of demotrip.len that is modified to work at a magnification of -1.

3. Close the Lens Drawing spreadsheet, open the demotrip.len file, then open the lens surface data spreadsheet, and modify the lens as follows.

Change the lens id to "Triplet with -1 magnification".
Click Draw On to turn on Autodraw.
Open the Paraxial data spreadsheet and enter -1 in the magnification field under Conjugates. Close the Spreadsheet to return to the Surface data spreadsheet. Note that th[0] and th[6] have been changed.
In the Autodraw window, note that the upper and lower rim rays do not pass through the last element. Change the apertures on surfaces 5 and 6 to 7.0. Now the upper and lower rim rays can pass. The Autodraw window and spreadsheet should look as follows.

This example shows the effects of aperture checking. Note that the upper and lower rim rays pass through the negative element, even though they are outside the aperture of the surfaces. This is because the apertures were not set up as checked. On the other hand, the rays could not pass through surface 7 until the aperture was increased to 7.0, because the aperture was marked as checked.

Next, suppose that you want lens drawings to show the rays propagating all the way from the object to the image surface.

4. Click Update >> Operating Conditions >> Lens Drawings. In the Intial distance field, enter th[0], and in the Final distance field, enter th[6]. (Note that you can enter lens data symbolically) Click OK to close the spreadsheet and return to the surface data spreadsheet. Now the Autodraw window looks as follows.

Next, suppose you want to show not just the chief ray off axis, but the entire vignetted beam.

5. Execute the command User >> Check Vignetting. Accept the proposed field point 1.0 by pressing ENTER in response to the prompt. The following will appear in the text window.

*VIGCHK FOB FYMIN FYMAX APPROX_FXMAX
1.00 -0.508 0.619 0.917
This command shows the fractional apertures that must be used for the upper and lower rays from the edge of the field

6. Click Update >> Operating Conditions >> Lens Drawings, and change the number of rays from the second field point to 3. Change the Min Pupil for this field point to -0.508, and the Max pupil to 0.619. Close the spreadsheet. The final Autodraw window looks as follows.

7. As a final step, save the lens to disc under a new file name, delete the lens from memory by clicking File >> New (note that the surface data spreadsheet must be closed), accept the defaults, then click OK). Then re-open the lens, and note that the Autodraw window uses the new default rays that you set up. Click on the Draw lens toolbar icon (SHIFT+F1) and you will see the same drawing as appears in the Autodraw window. Click on the Solid model toolbar icon. A solid model picture of the mirror will be drawn, including the default rays.

Changing the Initial distance and the Final distance from their default values can sometimes help to clarify a drawing. A parabolic mirror can serve as an example. Enter a parabolic mirror in OSLO as follows.

Close the surface data spreadsheet, then click File >> New. Enter paratest for the file name, and 1 surface. Click OK, and the surface data spreadsheet will open.
Click Draw On to turn on Autodraw.
Enter 20 for the entrance beam radius.
Click on the Glass button for surface 1. Select Reflect from the options list.
Click on the Special button for surface 1. From the pop-up menu, select Polynomial Asphere >> Conic/Toric, and enter -1 for the conic constant. Close the spreadsheet.
Enter -10 for the Radius, -5 for the Thickness, and 20 for the Aperture. Since the focal length of a conical mirror is half the radius of curvature, surface 2 will be the image surface.

Now click Update >> Operating Conditions >> Lens Drawings, and change the number of field points to 1, and the number of rays from that field point to 11, with a Min Pupil of 0 and a Max Pupil of 1. Close the spreadsheet. The Autodraw window will appear as follows. What is happening?

The problem is that the Initial and Final distances are not set correctly. The default value for these quantities, which is used when they have a value of 0.0, is about 10% of the intial and final apertures, respectively. This is good for most lens sytems, but not for the present reflector.

Click Update >> Operating Conditions >> Lens Drawings. Change the initial distance to 30.0 and the final distance to 5.0. Close the spreadsheet, and the Autodraw window will appear as follows.

Much better. But wait! There seems to be a missing ray that should be converging to the image. It looks like this ray would follow an almost vertical trajectory. To investigate further, you should trace a fan of rays through the system numerically and look at the data.

Execute Calculate >> Setup Object Point, with 0.0 for the object coordinates.

Click Calculate >> Ray Analysis. In the dialog box, select Print y ray fan. Set the Minimum fractional y height to 0, and the Maximum to 1. Set the number of rays in the fan to 11. This will give the same data as used for the drawing. Then close the dialog box by clicking OK. The following data will appear in the text window.

*TRACE FAN
RAY      FY          DYA         DXA         DY          DX          DZ
  1    1.000000   -1.333333      --          --          --          --
  2    0.900000   -1.607143      --      3.5527e-15      --          --
  3    0.800000   -2.051282      --      3.5527e-15      --     -8.8818e-16
  4    0.700000   -2.916667      --      1.7764e-15      --          --
  5    0.600000   -5.454545      --     -8.8818e-15      --          --
  6    0.500000  1.0000e+20  1.0000e+20  1.0000e+20  1.0000e+20  1.0000e+20
  7    0.400000    4.444444      --     -1.7764e-15      --          --
  8    0.300000    1.875000      --     -8.8818e-16      --          --
  9    0.200000    0.952381      --          --          --          --
10    0.100000    0.416667      --     -4.4409e-16      --          --
11      --      5.5511e-16      --          --          --          --

The "1.0000e+20" values for ray 6 indicate that the ray trace failed for that ray. Evidently, the ray emerged from the mirror at exactly 90 degrees and failed to intersect the image plane. To fix the drawing, you could select 10 rays instead of 11.

This example illustrates some important points about using OSLO to make drawings. First, it is often necessary to use some experimentation to obtain satisfactory results when you are drawing systems that are out of the ordinary. Second, you should always remember that OSLO is primarily a numerical program. The numbers are used to make the drawing, not the other way around. Whenever anything unusual occurs in a drawing, it is well to check the numbers. In fact, this is why drawings and graphical output are included in OSLO. They provide an effective visual way to check the correctness of the numerical results.

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