mounting lens

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Tutorial: Mounting Optical Lens


Mir Salek

Optomechanics, Fall 2008

1- Introuction

An assembly of lens is hold usually in a lens barrel. Each lens is fixed in this barrel by a

mounting structure. An example of a lens barrel is shown in Figure .The followingcriteria should be met by a lens mount:

! "t should define the position of the lenses to the re#uired accuracy.

$! %oes not deform or change the optical properties of the optics by stress

& ! 'ro(ides the re#uired ad)ustments for the optics* ! 'rotects the optics from the maximum predicted shoc+ to the de(ice

, ! 'rotect the optics in presence of en(ironmental extremes such as extreme cold and

heat or moisture. The de(ice might not be operational in these extreme situations.

- ! Maintain its function upon operational en(ironmental (ariations. The most commonen(ironment effects are temperature and (ibrations.

Figure : a lens barrel assembly

The position of the optics is defined by a fixed mechanical structure and a retainer pushes

the optics to this reference point. Figure $: Lens hold in the cellillustrates a lens hold in a

cell. "n this article different methods of mounting the lens are presented. "n each casethermal properties and resistance to shoc+ and (ibration of the mount is discussed.

Figure - of reference


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Figure 2: Lens hol in the cell2

2- Mounting the Lens

2-1- Mounting !ith "e#erence to the Mechanical $%is

"n loose tolerance or less expensi(e systems the position of the lens may be defined by

its mechanical axis. Mechanical axis is defined by the mechanical edges of the lens. Thema)or issue here is of course misalignment of optical axis of the lenses in the system. Theother problem occurs when the lens has appreciable edge thic+ness. "n this case lenses

might tip and their edges might crac+. To a(oid the latter problem either the edge of the

lens is made in a circular format or a relief should be made in the cell. The former case isshown in Figure &and the latter case inFigure *.

Figure &: 'etails (ie! o# a lens !ith spherical rim&

$Figure , of reference &Figure / of reference

$


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Figure ): *utting a relie# in the cell to a(oi +amming)

2-2- Mounting !ith "e#erence to the Optical $%is

The optical surfaces of the lens determine its optics axis. There are three well +nown

methods to seat the lens on its optical surfaces: obtuse angle tangent and sphericalcontact. All three types are shown in Figure ,."n terms of ease of production and

precision ran+ing order of the three designs is 0 obtuse angle $0 tangent and &0

spherical. "n terms of tolerance to stresses the order is the re(erse. The obtuse angle edgesharpness can (ary. 1e will discuss this in more detail in the stress section.

All the structures in Figure ,are shown for con(ex surfaces. The mount shapes for fourconfigurations of mechanical interface to optical surfaces are shown inFigure -. The

tangent structure can not be used with conca(e surfaces.

Figure : Mounting lens on its optical sur#aces

*Figure from reference $ page 2/,Figure 3 of reference

&


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Figure : Four con#iguration o# mechanical inter#aces on spherical lens sur#aces.

&- "etainer

"n this article retainer is any structure which pro(ides the spring force to press the optics

to the reference mount. "n this section different retainer types are presented. The next

sections would explain the stress and thermal issues of the most common types ofretainers.

&-1 Snap "ing

4nap ring is a flexible ring which is bent and slid in a groo(e by the lens. This structure is

shown in Figure /. 4ome snap rings ha(e two contact holes. These contact holes alle(iate

gripping and tightening them. 5owe(er most snap rings used to hold optical de(ices do

not ha(e these contacts. Therefore ta+ing them out of the groo(e is (ery difficult.

Figure /: Snap"ing/

&-2 Shim

Mount

4himming is

used to reducethe radial

distance

between the lens and the mount. 4hims help to isolate the lens from (ibration and thermalexpansion. Effect of shim is somewhat similar to flexures. 6rass shims are easier to insert

but are not (ery compliant. Mylar and Teflon shims are (ery compliant. A shim mount isshown in Figure 3.

-Figure 2 of reference /Figure from 7eference $ page 23

*


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Figure 8: Shim mount8

As shown in Figure 2the shims can be inserted in three $89 segments. The radial gap

between the optics and the cell ranges from 8.83 to 8.&mm. This techni#ue is usually

combined with 7T potting ;discussed later


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Figure 10: $ #le%ure mount: & tangent ars holing a lens10

&-)- Threae "etainer "ing

This is the most popular type of retainer. Threaded retainers can ha(e any of thestructures of Figure ,. Figure $shows a lens held by a threaded retainer. The height of the

retainer and the cell mount should be the same. Otherwise a tor#ue would be applied to

the lens which would cause additional stress. The situation is shown in Figure .

Figure 11: Stress as a result o# misaligne #orces11

The ma)or problem with this type of retainer is stress at the contact. A nylon retainer

could reduce contact stress. An O0ring could be inserted in the retainer to a(oid metal0glass contact. Another way to reduce stress is to combine a threaded retainer with a

flexure one. Figure $illustrates these retainers.

8Figure of reference &Figure from reference $ page $& with modifications

-


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Figure 12: 4pecial types ofThreaded 7etainers12

7etainers also tend to tilt the optics. Threaded pieces are generally not precise. Thereforethey should only apply the spring force to the seat and not change the position. One +ey

factor is that threaded retainers should ha(e loose fit. 4ometimes an intermediate non0

rotating ring is first inserted to hold the lens in position.

)- Stress Issues in Mounting Optics

4tress in glass is caused by se(eral factors. The mount needs to hold the optics in its

place. This force causes an initial stress. 4hoc+s can cause much greater stress.

Temperature change is also a ma)or cause of stress which is discussed in the next section.'ressure (ariation might be of concern in special systems as well.

The stress is problematic for two reasons. First it can ma+e the glass to brea+ or chip.This usually happens at relati(ely high stresses. >lass is resistant to tensile stress of +si

for permanent load and about * +si for short term loads. 5owe(er glass in

optomechanical de(ices is usually under compressi(e stress rather than tensile. Mostglasses ha(e about ,8 +si compressi(e strength.

4ometimes compressi(e stress in one part of a glass might cause tensile stress in other

parts and produce small crac+s. To ma+e sure that these crac+s do not occur themaximum compressi(e stress should not be more than a sixth of the maximum

compressi(e strength of the glass. 5owe(er usually these crac+s are ignorable.

A much more delicate effect of stress on glass is birefringence. This is usually caused by

smaller stress of about ,88 psi ?@. 5owe(er birefringence is mostly a concern in

polariation sensiti(e applications.

)-1 $%ial Stress

A contact between lens and mount is shown inFigure &. The sharper the edge the less

would be its radius. The stress of the mount on the lens is gi(en by e#uation 1&.

$From reference $ page $* and $,&From reference $ page $-

/


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Figure 1&: 3ontact point o# a mount an a lens1)

45uation 1

For tangentional contact dBB d$and for sharp edge mounts d$BB d. 5owe(er the

radius of the edge can be assigned to tailor the stress of the mount. For most materialused in mounts C B 8.& and therefore 0C$ D . Also if E> D Emas it is for Aluminum and

glass then e#uation reduces to

- Temperature 4##ect

Materials expansioncontraction as a result of temperature change depends on their

coefficient of thermal expansion ;TE


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Tale 2: Mechanical *roperties o# 3ommon Metals1

4ince the oefficient of thermal expansion of optical material and mounts are usually

different the

-1- "aial Stress

7adial stress is an issue when the lens flat edge is in contact with the mount. "f the de(ice

is tight to the barrel the stress on lens as a result of temperature drop is

45uation 2 1/

47is the radial stressaM is t he metal TEa> is t he glass TE

dT is the temperature change

EMis the metal modulus of elasticityE>is the metal modulus of elasticity

-Table * of reference /E#uation of reference

2


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%>is the lens diameter

tcis the metal wall thic+ness

For typical (alues of aMH&.ppm9Fa>H&.2 ppm9FdTH 0,8 9F EMH 8e0- lbin$

E>H./e0- lbin$ %>H$ inch and tcH 8.8-$, inch stress would be $/$ which is far less

than the compressi(e stress of the glass. "t is also about a factor of $ less thanbirefringence criteria of the glass.

The relati(e diameter change of glass to the mount can be calculated from

I7 H IJ7IT

For the same parameters of abo(e except dT H K,89F I7 would be */inch. This (alueis less than $.,m which is about the radial clearance of a typical small lens.

-2- $%ial Stress

For a sharp edge retainer the maximum axial stress as a result of temperature change canbe obtained from the following e#uation:

45uation &18

For the same typical (alues used in section *0 the maximum axial stress would be$.*3+si more than the birefringence limit but still much less than compressi(e strength of

the glass.

- 4lastomeric Mounting o# Lenses

Elastomers are widely used to attach optics to cells. The elastomer should be inert. 7oomtemperature (ulcaniing ;7T< silicon robber polymer is a common material used as

elastomer. haracteristics of some common 7Ts is summaried in Table &.

1hen elastomer is used the radial spacing between glass and metal would be larger.

Typical (alues are between 8.8 to 8.8$ inches ;8.$, to 8.,mm


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4ince 7Ts are not (ery stiff they might allow small motion of the optics. Therefore

some motion of the optics might occur during shoc+ or (ibration.

Figure 1): 4lastomeric mounting o# lens1

Tale &: 3haracteristics o# common "T6s20

/- Stacke 3ell Lens Mounting

"n this design indi(idual lenses are mounted on separate cells. Then all the cells are

mounted in housing. Each lens should be concentric and coaxial with its cell. This design

2Figure 8 of reference $8Table , of reference


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can be (ery precise and is used precision applications.Figure ,shows an stac+ed0cell

lens mount.

Figure 1: Stacke-cell lens mount

8- 3onclusions

4e(eral techni#ues for mounting lenses were re(iewed. %epending on the re#uired

alignment and en(ironmental situations a particular design is preferred to the others.

"e#erences

?@ oder '. 7. 'roc. of 4'"E ol 82,2 ' $ Nan 233

?$@ u+obrato(ich %. u+obrato(ich 4. "ntroduction to Opto0mechanical %esign

7atheon 4ystems o. %efence 4ystems 4egment.?&@ u+obrato(ich % 7ichard 7 M Flexure Mounts For 5igh 7esolution Optical

Elements 'roc of 4'"E ol. 82,2 Nan 233

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