Defining TracePro Solids

FIGURE 2.12 - Aspheric Tab in Insert Lens dialog

Fresnel Lens

The Insert|Fresnel Lens menu item opens a modeless dialog box that allows you to specify a Fresnel lens based on the material and the object and image distances. The dialog box allows you to enter:

1.Ring width or lines/unit length

2.Thickness of the substrate

3.Radius of the lens substrate

4.Material catalog, name, and design wavelength

5.Object distance

6.Image distance

7.Origin or location of the center of the part

8.Rotation angles specifying the orientation

TracePro interprets an object or image distance equal to zero as an infinite distance. Negative distances can also be entered for the case of a virtual object and/or image.

TracePro 5.0 User’s Manual

2.13

Creating a Solid Model

The Fresnel grooves are on the back surface of the element as drawn.

FIGURE 2.13 - Insert Fresnel Lens dialog box and resulting Fresnel Lens

The ring width specifies the width of the individual rings in the Fresnel lens, while the lines/unit length specifies the inverse of the ring width: the number of rings per unit length (e.g. 50 lines/inch). You can specify either one of these quantities, and the other will be updated.

The object and image distances specify point-to-point imaging. After entering all the data for the Fresnel lens, TracePro will build a Fresnel lens with conical facets that provide the requested imaging. The facet angles will be chosen so that light from a point on the left side of the lens at a distance equal to the object distance is imaged optimally into a point at a distance equal to the image distance on the right side of the lens.

The formula for calculating the facet angles on the Fresnel lens is

tanβ	=	sinθ1 + sinθ2		,	(2.2)
		(n2 – sin2θ1	-------------------
			) – cosθ2

where θ1 is the incident angle of light on the Fresnel lens, θ2 is the angle of light

leaving the Fresnel lens, n is the index of refraction of the material, and β is the facet angle. This formula produces an optimized or “aspheric” Fresnel lens for the case where the object distance is infinite, and is not equivalent to collapsing segments of a sphere onto a flat substrate.

2.14	TracePro 5.0 User’s Manual