Technical Reference

Housing/Mount

L1	L2	Detector

FIGURE 7.3 - Scatter

To compute the probability of a ray that hits the lens, then scatters in a direction such that it strikes an on-axis detector which is 1 milliradian on a side, as shown in Figure 7.3. As before, the probability is approximately:

p =	fS(θ)cosθΔΩ ,	(7.6)
where:ΔΩ = (10-3 radian)2, and suppose θ = 30 degrees.
Evaluating the probability, we get:
p2	= 1.04 × 10–12 .	(7.7)

The probability of a ray scattering from the sunshade directly to the lens and then directly to the detector is the product of those probabilities:

ptotal = p1p2 = 1.30 × 10–13 .

(7.8)

This means that we will have to start about 1013 rays to have a good chance of getting one ray to go along a path like the one described. The most obvious way to improve our chances is to set up importance sampling from the lens to the detector. That increases p2 to 1, resulting in a much more tolerable situation:

1

ptotal = p1 = -- . (7.9)

8

The detector surface cannot be defined as an importance sampling target for the lens surface, as can be seen in Scatter, Figure 7.3. We need to set up importance sampling toward the image formed by the lens surface, rather than the image formed by the system. For surface L2, however, we set up importance sampling directly to the detector.

Importance sampling targets must be located at the image formed by the optical surface. In this way, any ray that strikes any optical surface always produces a ray that hits the detector.

Material Properties

In TracePro, a material property specifies an index of refraction and a coefficient of bulk absorption. In the Apply Properties dialog box, the Material tab lets you choose a material property from the TracePro Property Database and apply it to an object in a model.

7.6	TracePro 5.0 User’s Manual