Importance Sampling

Apply the Importance Sampling Property

Once the number of importance rays, the direction of the importance rays, the shape of the importance target and the location, orientation, and dimensions of the target have been defined, verify that the proper surface or object is selected in the System Tree, and click Apply to apply the Importance Sampling Property.

Automatic Setup of Importance Sampling

For complex optical systems of many lenses that require scatter and stray light analyses, manually defining the importance sampling targets for each surface is complicated and time-consuming. TracePro lets you define Importance Sampling targets for all the optical surfaces automatically.

The following discussion assumes you are familiar with the terminology of optical imaging.

Define the Prescription

Prior to the automatic set up of importance sampling targets, you must first define the prescription, which is a sequence of surfaces that light rays pass through. Use the prescription tab of the Define|Apply Properties dialog box to define the prescription. For more information about “Prescription”, see page 4.20.

Select the Target Shape

After the prescription is correctly set up, select the Define|Auto Importance Sampling dialog box, labeled Automatic Setup of Importance Sampling (see Figure 4.21). Specify the importance sampling target shape as annular or rectangular for the surfaces in the prescription.

Choose the target shape (annular or rectangular) that most closely matches the shape of the field of view of the optical system. You can choose a circular field of view by selecting the Annular radio button.

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Applying Properties

FIGURE 4.21 - Automatic Setup of Importance Sampling Dialog Box

Specify a gut ray, marginal ray, and one or two chief rays.

Gut Ray	A ray passing through the center of the aperture stop and the center of the
	field of view.
Marginal Ray	A ray passing through the edge of the aperture stop and the center of the field
	of view.
Chief Ray	A ray passing through the center of the aperture stop and the edge of the field
	of view. For an Annular target specify an Outer Chief Ray and an Inner Chief
	Ray. For a Rectangular target specify a Height Chief Ray and the Width Chief.

In some cases, it is helpful to use a lens design program such as OSLO to determine the starting positions and directions of the rays. Two chief rays are needed to define rectangular or annular targets. To define circular importance sampling targets, select Annular and leave the data for the Inner Ray set to all zero.

Note: If the gut ray is blocked by the optical system (e.g. by the secondary mirror in a Cassegrain Telescope) move the gut ray to the inner edge of the obscured aperture, keeping its direction the same.

Annular Targets

To define importance sampling for annular targets, enter the required data for the rays. The Inner Ray is one that passes through the center of the aperture stop and the inner edge of the annulus, as shown in Figure 4.22. The Outer Ray passes through the outer edge of the annulus, on a line passing through the Gut Ray and Inner Ray points.

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Importance Sampling

FIGURE 4.22 - Ray locations for automatic setup of importance sampling targets with an annular field of view

Rectangular Targets

To define importance sampling for rectangular targets, enter the data for the rays. The Height Ray passes through the center of the aperture stop and the center of one edge of the image rectangle, as shown in Figure 4.23, and the Width Ray passes through the center of an adjacent edge. The Marginal Ray passes by the edge of the aperture and meets the image field at the same location as the Gut Ray. TracePro calculates the distance and location of the image field from the meeting point of Marginal and Gut Rays.

FIGURE 4.23 - Ray locations for automatic setup of importance sampling targets with a rectangular field of view

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