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Understanding Optical Specifications

Optical

specifications

are

utilized

throughout

the

design

and

manufacturing

of

a

component

or system to characterize how well it meets certain performance requirements. They are

useful for two reasons: first, they specify the acceptable limits of key parameters

that

govern system performance; second, they specify the amount of resources (i.e. time and

cost) that should be spent on manufacturing.

An

optical

system

can

suffer

from

either

under-specification

or

over-specification,

both

of which can result in unnecessary expenditure of resources. Under- specification occurs

when not all of the necessary parameters are properly defined, resulting in inadequate

performance. Over-specification occurs when a system is defined too tightly without any

consideration

for

changes

in

optical

or

mechanical

requirements,

resulting

in

higher

cost

and increased manufacturing difficulty.

In order to understand optical specifications, it is important to first review what they

mean.

To

simplify

the

ever-growing

number,

consider

the

most

common

manufacturing,

surface,

and

material

specifications

for

lenses

,

mirrors

,

and

windows

.

Filters

,

polarizers

,

prisms

,

beamsplitters

,

gratings

,

and

fiber

optics

also

share

many

of

these

optical

specifications,

so understanding the most common provides a great baseline for understanding those for

nearly all optical products.

MANUFACTURING SPECIFICATIONS

Diameter Tolerance

The diameter tolerance of a circular optical component provides the acceptable range of

values

for

the

diameter.

This

manufacturing

specification

can

vary

based

on

the

skill

and

capabilities of the particular optical shop that is fabricating the optic. Although

diameter

tolerance

does

not

have

any

effect

on

the

optical

performance

of

the

optic

itself,

it

is

a

very

important

mechanical

tolerance

that

must

be

considered

if

the

optic

is

going

to be mounted in any type of holder. For instance, if the diameter of an

optical lens

deviates from its nominal value it is possible that the mechanical axis can be displaced

from the optical axis in a mounted assembly, thus causing decenter (Figure 1). Typical

manufacturing tolerances for diameter are: +0.00/-0.10 mm for typical quality,

+0.00/-0.050 mm for precision quality, and +0.000/-0.010 mm for high quality.

Figure 1:

Decentering of Collimated Light

Center Thickness Tolerance

The

center

thickness

of

an

optical

component,

most

notably

a

lens

,

is

the

material

thickness

of

the

component

measured

at

the

center.

Center

thickness

is

measured

across

the

mechanical

axis of the lens, defined as the axis exactly between its outer edges. Variation of the

center thickness of a lens can affect the optical performance because center thickness,

along

with

radius

of

curvature,

determines

the

optical

path

length

of

rays

passing

through

the

lens.

Typical

manufacturing

tolerances

for

center

thickness

are:

+/-0.20

mm

for

typical

quality, +/-0.050 mm for precision quality, and +/-0.010 mm for high quality.

Radius of Curvature

The radius of curvature is defined as the distance between an optical component's vertex

and the center of curvature. It can be positive, zero, or negative depending on whether

the surface is convex, plano, or concave, respectfully. Knowing the value of the radius

of curvature allows one to determine the optical path length of rays passing through the

lens

or

mirror

, but it also plays a large role in determining the power of the surface.

Manufacturing tolerances for radius of curvature are typically +/-0.5, but can be

as low

as +/-0.1% in precision applications or +/-0.01% for extremely high quality needs.

Centering

Centering, also known by centration or decenter, of a

lens

is specified in terms of beam

deviation

δ

(Equation 1). Once beam deviation is known, wedge angle W can be calculated

by a simple relation (Equation 2). The amount of decenter in a lens is the physical

displacement of the mechanical axis from the optical axis. The mechanical axis of a lens

is

simply

the

geometric

axis

of

the

lens

and

is

defined

by

its

outer

cylinder.

The

optical

axis

of

a

lens

is

defined

by

the

optical

surfaces

and

is

the

line

that

connects

the

centers

of curvature of the surfaces. To test for centration, a lens is placed into a cup upon

which pressure is applied. The pressure applied to the lens automatically situates the

center of curvature of the first surface in the center of the cup, which is also aligned

with

the

axis

of

rotation

(Figure

2).

Collimated

light

directed

along

this

axis

of

rotation

is

sent

through

the

lens

and

comes

to

a

focus

at

the

rear

focal

plane.

As

the

lens

is

rotated

by rotating

the cup,

any decenter

in the

lens

will

cause the

focusing beam

to diverge

and

trace out a circle of radius

Δ

at the rear focal plane (Figure 1).

Figure 2:

Test for Centration

(2)

(1)

where

W

is the wedge angle, often reported as arcminutes, and n is the index of refraction.

Parallelism

Parallelism

describes

how

parallel

two

surfaces

are

with

respect

to

each

other.

It

is

useful

in

specifying

components

such

as

windows

and

polarizers

where

parallel

surfaces

are

ideal

for

system

performance

because

they

minimize

distortion

that

can

otherwise

degrade

image

or light quality. Typical tolerances range from 5 arcminutes down to a few arcseconds.

Angle Tolerance

In components

such as

prisms

and

beamsplitters

, the angles

between surfaces

are critical

to the performance of the optic. This angle tolerance is typically measured using an

autocollimator assembly

, whose light source system emits collimated light. The

autocollimator is rotated about the surface of the optic until the resultant Fresnel

reflection back into it produces a spot on top of the surface under inspection. This

verifies

that

the

collimated

beam

is

hitting

the

surface

at

exactly

normal

incidence.

The

entire

autocollimator

assembly

is

then

rotated

around

the

optic

to

the

next

optical

surface

and

the

same

procedure

is

repeated.

Figure

3

shows

a

typical

autocollimator

setup

measuring

angle tolerance. The difference in angle between the two measured positions is used to

calculate the tolerance between the two optical surfaces. Angle tolerance can be held to

tolerances of a few arcminutes all the way down to a few arcseconds.

Figure 3:

Autocollimator Setup Measuring Angle Tolerance

Bevel

Glass

corners

can

be

very

fragile,

therefore,

it

is

important

to

protect

them

when

handling

or mounting a component. The most common way of protecting these corners is to bevel the

edges. Bevels serve as protective chamfers and prevent edge chips. They are defined by

their face width and angle (Figure 4).

Figure 4:

Bevel on an Optical Lens

Bevels are most commonly cut at 45°

and the face width is determined by the diameter of

the

optic.

Optics with

diameters less than 3.00mm, such

as micro-lenses or micro-prisms,

are typically not beveled due to the likelihood of creating edge chips in the process.

It is important to note that for small radii of curvature, for example, lenses where the

diameter

is

≥

0.85

x

radius

of

curvature,

no

bevel

is

needed

due

to

the

large

angle

between

the surface and edge of the lens. For all other diameters, the maximum face widths are

provided in Table 1.

Clear Aperture

Clear aperture is defined as the diameter or size of an optical component that must meet

specifications. Outside of it, manufacturers do not guarantee the optic will adhere to

the stated specifications. Due to manufacturing constraints, it is virtually impossible

to produce a clear aperture exactly equal to the diameter, or the length by width, of an

optic. Typical clear apertures for lenses are show in Table 2.