了解光学指标.docx

1、了解光学指标Understanding Optical SpecificationsOptical 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 paramet

2、ers 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-spe

3、cification 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 man

4、ufacturing 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, beamsp

5、litters, 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 SPECIFICATIONSDiameter ToleranceThe diameter tolerance of a circular optical component

6、 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,

7、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

8、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 LightCenter Thickness ToleranceThe center thickness of a

9、n 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 pe

10、rformance 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.Radi

11、us of CurvatureThe radius of curvature is defined as the distance between an optical components 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 o

12、ne 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 ex

13、tremely high quality needs.CenteringCentering, also known by centration or decenter, of a lens is specified in terms of beam deviation (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 displ

14、acement 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

15、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

16、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 at the rear focal plane (Figure 1). Figure 2: Test for Centration

17、 (1) (2)where W is the wedge angle, often reported as arcminutes, and n is the index of refraction.ParallelismParallelism 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 sys

18、tem 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 ToleranceIn components such as prisms and beamsplitters, the angles between surfaces are critical to the performance of the op

19、tic. 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.

20、 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

21、 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 ToleranceBevelGlass corners c

22、an 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:

23、 Bevel on an Optical LensBevels 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 import

24、ant 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 ApertureClear apert

25、ure 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 th

26、e diameter, or the length by width, of an optic. Typical clear apertures for lenses are show in Table 2. Figure 5: Graphic Indicating Clear Aperture and Diameter of a FilterTable 1: Bevel TolerancesDiameterMaximum Face Width of Bevel3.00mm 5.00mm0.1mm5.01mm 25.4mm0.25mm25.41mm 50.00mm0.3mm50.01mm 75

27、.00mm0.4mmTable 2: Clear Aperture TolerancesDiameterClear Aperture3.00mm 10.00mm90% of Diameter10.01mm - 50.00mmDiameter 1mm 50.01mmDiameter 1.5mmSURFACE SPECIFICATIONSSurface QualityThe surface quality of an optical surface describes its cosmetic appearance and includes such defects as scratches an

28、d pits, or digs. In most cases, these surface defects are purely cosmetic and do not significantly affect system performance, though, they can cause a small loss in system throughput and a small increase in scattered light. Certain surfaces, however, are more sensitive to these effects such as: (1)

29、surfaces at image planes because these defects are in focus and (2) surfaces that see high power levels because these defects can cause increased absorption of energy and damage the optic. The most common specification used for surface quality is the scratch-dig specification described by MIL-PRF-13

30、830B. The scratch designation is determined by comparing the scratches on a surface to a set of standard scratches under controlled lighting conditions. Therefore the scratch designation does not describe the actual scratch itself, but rather compares it to a standardized scratch according to the MI

31、L-Spec. The dig designation, however, does directly relate to the dig, or small pit in the surface. The dig designation is calculated at the diameter of the dig in microns divided by 10. Scratch-dig specifications of 80-50 are typically considered standard quality, 60-40 precision quality, and 20-10

32、 high precision quality. Learn more about surface quality here.Surface FlatnessSurface flatness is a type of surface accuracy specification that measures the deviation of a flat surface such as that of a mirror, window, prism, or plano-lens. This deviation can be measured using an optical flat, which is a high quality, highly precise flat reference surface used to compare the flatness of a test piece. When the flat surface of the test optic is placed against the optical flat, fringes appear whose shape dictates the surface flatness of th