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2021-02-10 03:40
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2021年2月10日发(作者:骑兵)


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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