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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.
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