Design for manufacturability - Optics€¢Spring ratio is a trade off with surface figure ... Property of Edmund Optics, Inc. ... Design for manufacturability

DESIGN FOR MANUFACTURABILITY

March 2016

Jeremy Govier

Principal Engineer

PROPRIETARY - Property of Edmund Optics, Inc. | 2016 Copyright© Edmund Optics, Inc.

WE MAKE OPTICS2


HOW TO REDUCE COST IN YOUR OPTICS3

Choose specifications that increase yields while

getting the performance you need

Add constraints to your designs to improve

shapes in ways that will make it easier to achieve

tolerances

Reduce sensitivity in the design to improve as

built performance with looser tolerances


SPECIFICATIONS: SPHERICAL MANUFACTURING4


YOU CANNOT HAVE IT ALL5

Tolerances in the high precision column can be achieved in a few

places but all parameters can be in the high precision

• Multiplying yields for each specification in the high tolerance

• Some tolerances are coupled or effect each other

– Power/Irregularity and Center Thickness

– Centering and Scratch Dig


CONSTRAINING SHAPES SPRING RATIO6

• Spring ratio is the ratio of Diameter/thickness

– Typically the largest thickness (center or edge) is used, but thick

edges and very thin centers can still be a problem

• High spring ratios can cause the lens to flex during polishing

• Spring ratio is a trade off with surface figure

• Spring ratio < 8 ideal

• Spring ratios >20 achievable but increased surface figure


CONSTRAINING SHAPES Z- FACTOR7

R1 (convex) R2 (convex)

D1 D2R1 (concave)

R2 (convex)

D1

D2

𝑧 =𝐷1

4 ∗ 𝑅1−

𝐷2

4 ∗ 𝑅2

• Z-factor is a measure of the angle between the tangent lines of where

two bell chucks will contact a lens

• z > 0.08 considered good for centering

• The larger the angle, the higher the Z-factor, and the more cupping

force is applied from the bells to center the lens

• The clear aperture is usually used for the cup diameter

• Radii in equation assume positive means curvature to the right of the

surface, negative to the left


CONSTRAINING SHAPES CONCENTRIC RADII8

If the centers of curvature of both surfaces are very close a large

amount of roll must occur to move a tilt the surface back into having

no wedge

|R1-R2-CT| < 2mm can be a problem

|R2-R1+CT| < 2 if Radii are negative


CONSTRAINING SHAPES CLEAR APERTURE9

Clear Aperture defines the area of the lens where the surface figure,

scratch-dig, and coatings are specified in.

There needs to be a margin between the clear aperture and the edge

of the lens.

This allows for many things:

• Room for bevels and the chip zone of the optic

• A place for coating fixture to hold the lens while the coating is being

applied

• This gives a place for the lens to be held without risking scratching

the surface during centering and other tests

• This helps account for the edge roll effect during polishing

• Typically a millimeter less than the diameter is needed as a maximum

but the more margin between the clear aperture and edge the better

for reducing costs


CONSTRAINING SHAPES EDGE ROLL10

Edge roll explained

During all forms of polishing part of the tool will pass off the edge of a

part. This causes the polishing media to compress as it moved back

onto the tool which increases removal right at the edge of the part.

• Typically about 1mm from the edge of the part will have a high form

error due to edge roll effects in polishing

• This means 2mm on the diameter will be lost to high irregularity

• Oversizing the part during polishing and then edging off the excess

diameter during centering helps eliminate this


CONSTRAINING SHAPES EDGE THICKNESS11

• To prevent chipping an edge thickness of at least 1mm is desired

• Consider edge thickness during all stages of manufacturing not just

the finished part when constraining edge thickness

• Typically a lens is oversized 1mm during polishing to help reduce the

effect of the edge roll

• For lenses with Clear Apertures very close to the edge assume

diameter +2mm instead when constraining edge thickness

Finished

diameter

Diameter +1mm


HOW TO REMOVE SENSITIVITY12

• Best nominal design does not necessarily equate to best as built

design

• You care about how the design will perform in hardware not in the

raytrace

• The best nominal performance often comes from design forms that

have significant losses in performance when fabrication tolerances

are added

• Insensitive designs can often be found that have small drops in

performance that are only slightly lower performing nominally


HOW TO REMOVE SENSITIVITY 13

The leading cause of design sensitivity, particularly relating to

positional tolerances of lenses, is the steepness of the incident angle of

rays on the surface of the optics.

This can be constrained in your optimization with controls on incident

ray angles or what is often easier to control and has a direct effect on

these ray angles is the surface spherical aberration on each surface.

In Zemax this is the SPHA operand, and there are similar constraints in

all the ray trace codes.


HOW TO REMOVE SENSITIVITY14

Another option for optimizing your design for insensitivity is to design

with perturbations in the design. Most codes now have automated

function like this that allow you do put a small amount of tilt, decenter,

and other tolerances into the design while it optimized to find the best

design after production tolerances. In Zemax this is the TOLR

operand. This often can take more time and requires more design effort

to achieve than the surface spherical method but many designers have

shown good success with this method as well.


QUESTIONS?

15

Documents

Design for manufacturability - Optics€¢Spring ratio is a trade off with surface figure ... Property of Edmund Optics, Inc. ... Design for manufacturability