Surface - Modifiers

The Modifiers tab of a surface dialog allows a user to specify both surface sag deformations and a ray path node number specification for ray path reporting purposes.

Sag Deformation

Mathematically, deformations are implemented by shifting the zeros of the base surface's function. Let f_b(x,y,z) = 0 define the base surface and f_d(x,y,z) = 0 define the deforming surface. Further, assume that the deforming surface is defined as a height map along z (e.g, f_d(x,y,z) = z - g_d(x,y) ), which is the "sagability" assumption. FRED doesn't enforce this assumption, but will generally print a warning to the output window when it detects a deformer that is not "sagable". When the deformation is applied to the base surface, the resulting combined surface is defined by (ignoring coordinate system transformations):

0 = f(x,y,z) = f_b( x, y, z - A*g_d(x,y) ) = f_b( x, y, A*f_d(x,y,z) - (A-1)*z )

where A is the deformation scale factor (this scales the deforming surface's height).

Ray Path Entity

The Ray Path Entity specification allows an object to report a node ID other than itself for ray path purposes, simplifying and reducing the reported ray paths following an advanced raytrace. This feature can be particularly useful when working with models imported from CAD, which often define a single structure as a composite of smaller surfaces. In such cases the composite surfaces can be set to report a single node ID in the ray paths list, greatly simplifying path based analyses.

Surface Roughness

The Surface Roughness specification applies a surface roughness model to the selected surface. The surface roughness model must exist in the Surface Roughness folder prior to assignment from the Modifiers tab of the surface dialog.

Retro-reflection

When active, the Retro-reflection specification enforces the condition that specular reflected rays from the selected surface travel back along their incident direction. Retro-reflected rays will continue to accrue positive path length along their trace direction unless the Phase Conjugation box is also checked, in which case the sign of the distance traveled becomes negative and therefore the retro-reflected rays' path length will decrease as they travel. The retro-reflection setting will be ignored on surfaces featuring gratings or birefringent materials.

Navigation

This feature can be accessed by selecting the Modifiers tab in a surface dialog box.

Controls

Control	Inputs / Description	Defaults
Sag Deformation
Apply another surface's sag as a deformation	Select whether the base surface will have a deformation	Unchecked
Deforming Surface	Choose from a list of valid deforming surfaces	All valid surfaces
Scale deformation by	Scale factor multiplying the amplitude of the deforming surface.	1
Deforming surface is applied in coordinate system of base surface	This check box controls whether coordinates are transformed before the deformation surface is evaluated. When checked it has the effect of placing the deformer in the same coordinate system as the base surface. This could be useful, for example, if the same exact deformation is being applied to several similar base surfaces each at a different location. When not checked it has the effect of applying the deformer globally to the base surface. This would be used, for example, to apply different parts of one large deformation to the spatially correspond parts of a segmented mirror.	Unchecked
Ray Path Entity
Designated entity for a ray path event at this surface	Select a node from the object tree using the entity picker interface. The selected node will then be reported in place of the surface for ray path purposes.	Currently assigned ray path node.
Surface Roughness
Surface roughener	Select the surface roughness model from the drop down list to be applied to the surface. The drop-down list contains all surface roughness models which currently exist in the Surface Roughness folder on the object tree. Only one surface roughness model can be applied at a time.	-None-

OK	Accept settings and close dialog box.
Cancel	Discard settings and close dialog box.
Apply	Apply settings and keep dialog box open.
Help	Access this Help page.

Application Notes

Elements and bevels

Deformations can be applied to surfaces that are parts of Lens/Mirror elements. When a deformation is applied the Lens/Mirror will use the optical surfaces to bound its outer tube, ensuring the element won't leak rays. Currently, the bevel option is not handled correctly, so there can be leaks if a deformation is applied to a lens/mirror with bevels.

Prisms

Deformations cannot be applied to elements of a prism. The user interface tries to prevent this from happening. If by chance the user finds a way to do this (by scripting perhaps) bad things are to be expected.

Non-sagable surfaces

If a non-sagable deformer is selected a warning will generally be written to the output window after the base surface is tessellated. Because tessellation can occur more than once, when such warnings occur they are frequently seen repeated multiple times.

Height maps

To apply a sampled height map as a deformer, read the data into a bicubic mesh surface and apply it as the deforming surface.

Example 1 - Sinusoidal deformation

This short example demonstrates how a sinusoidal surface is used to deform a conic mirror. The two surfaces are shown below, with the sinusoidal surface on the left and the conic on the right. The sinusoidal surface is an Implicit Script Surface.

We now edit the conic surface and select the “Modifiers” tab:

Check the “Apply another surface sag as a deformation…” to activate the deformation feature. The deforming surface “Geometry.Elem 1.x sinusoid” is selected from the dropdown list under “Deforming Surface”. The “Deforming surface is applied in coordinate system of base surface” option is checked since both surfaces have the same aperture and lie on the z-axis. To finish, click OK. The deformed surface is shown in the view below after having set the sinusoidal surface to be NotTraceable.

Example 2 - Zernike deformations

This next example uses a Zernike base surface to deform a set of off-axis sections of a parabolic mirror. Once again, the base and deforming surfaces are created in FRED as shown below

In this case, the desire is to use the single Zernike surface to deform each mirror section according to its radial position. Thus, the “Deforming surface is applied in coordinate system of base surface” option is unchecked which uses the deforming surface in the global coordinate system. The end result is that the mirror sections are deformed by the single base surface.