| Optics can be found virtually everywhere, from fiber | | | | outer cylindrical edge of the lens or simply its |
| optic couplings to machine vision imaging devices to | | | | geometrical axis. The mechanical axis coincides with |
| cutting-edge biometric iris identification systems. Many | | | | the rotating axis of the centering machine that edges |
| people don't realize that designing an optic is | | | | the lens to its final diameter. This centering process |
| significantly different from designing the mechanical | | | | also, in turn, defines the diameter tolerance, which is |
| components of a system. | | | | typically +0, given mounting considerations. |
| Often an optic will be designed at either of two | | | | If a ray of light is coincident with the mechanical axis, |
| extremes: under-specified or over-specified. This article | | | | then a lens will deviate the ray so that it passes the |
| will discuss the definitions and use of common optical | | | | optical axis at the focal plane. The separation of the |
| specifications. | | | | two axes at the focal plane is then defined as the |
| SURFACE ACCURACY | | | | decentration, or axial displacement centering error. The |
| After a design is successfully completed, optical | | | | centering accuracy value used in optical fabrication is |
| manufacturers can determine the characteristics of | | | | actually twice this value and is often called the Total |
| each optical surface in the system and tolerance them | | | | Indicator Run-out or TIR. The deviation is then the angle |
| according to manufacturing capabilities. This is done | | | | equal to the decentration divided by the focal length of |
| with an emphasis on the value and uniformity of the | | | | the lens. The concentricity or centration of a lens is |
| shape, as well as on the cosmetics of each surface. | | | | typically specified by the deviation angle, however it is |
| The maximum allowable deviation of an optical | | | | typically tested at double the value while the lens is |
| surface from a perfect surface is described by | | | | rotated. An angular deviation of 1 to 3 arc minutes is |
| Surface Accuracy. There are several terms | | | | common for precision components. |
| associated with accuracy, as follows: | | | | EDGE TREATMENT |
| 1. Surface Flatness is the deviation for a plano surface | | | | There are several terms associated with the |
| such as a window or mirror. When a test plate | | | | treatment of edges. The most basic is a cut edge; this |
| (typically an optical flat) is held in contact with the work | | | | is literally what it means. A large sheet of glass is |
| piece (the part under inspection), a contour map is | | | | either "cut" using a scribe and break technique or |
| visible as light and dark bands. These dark bands are | | | | cored for circular pieces. The edges are left as is |
| called Newton's rings or fringes. Due to the air gap | | | | which can leave sharp edges. The next edge type is |
| between the surfaces, each ring corresponds to the | | | | swiped or seamed edges which means that all the |
| vertical distance between the test plate and the | | | | sharp edges are removed. The final type is a ground |
| surface under inspection. Since the test plate in this | | | | edge which provides an even mounting surface and |
| case is a clear, flat reference, the air gap is very small | | | | gives a uniform cosmetic appearance to the perimeter |
| so the surface flatness is defined in terms of | | | | of the optic. The better the treatment of the edge, the |
| wavelength (very small unit of measure); i.e. 1/4 wave | | | | less likely it may become chipped in handling. Edge |
| or 1/4λ. The spacing between rings is equal to | | | | chips are not permitted within the optics' stated clear |
| one-half the wavelength of the illumination source; i.e. 1 | | | | aperture. Edge chips are typically defined for optical |
| 4 wave = 1/2 ring. A monochromatic green light at the | | | | windows and first surface mirrors to have maximum |
| 546.1 nm mercury line or helium-neon red laser line at | | | | values of 0.25 to 0.5mm. |
| 632.8 nm is used for illumination. Typically, only values | | | | Bevels are clean ground edges used to prevent edge |
| less than 1/4 wave are considered to be precision and | | | | chips or simply as protective chamfers. Our bevels are |
| values less than 1/10λ to be high precision. | | | | defined as maximum face widths at 45°, with a |
| 2. Power is used when dealing with a curved surface | | | | standard tolerance of ±15°. For micro optics, optical |
| to define the deviation of the fabricated surface radius | | | | manufacturers may not bevel the edges (since the |
| from the radius of an inversely shaped test plate. For | | | | attempt will likely cause chips). Also, manufacturers |
| this example, let's assume the test plate is a highly | | | | may not bevel the edges for small radii meeting the |
| calibrated reference gauge. This deviation is also | | | | diameter edge at large angles. If the diameter = (0.85 x |
| referred to as surface fit; i.e. how well the work piece | | | | radius of curvature), then no bevel is used. The actual |
| "fits" the test plate. The number of rings visible is used | | | | clear aperture (CA) value used will typically be smaller |
| to identify the power of the surface. Again, each ring is | | | | than that defined by the bevels with a maximum |
| equivalent to 1/2 of the test wavelength. The surface | | | | possible CA calculated as follows: |
| is checked using this procedure at several different | | | | PRISM ANGLE ACCURACY |
| stages of production. Note that even if the optical | | | | Typically, the relative angle between the reflecting |
| prints use power and irregularity to specify maximum | | | | surfaces (as in a roof) needs to have a critical |
| allowable deviations, radii tolerances are used for the | | | | tolerance in order to maintain a maximum allowable |
| fabrication of actual test plates. | | | | angular deviation. However, depending on placement in |
| 3. Irregularity is used to define how the surface | | | | a system, the other angle(s) could be toleranced to limit |
| deviates from the perfect shape of the test plate, as | | | | aberration effects. Angle tolerances for prisms are |
| demonstrated by a spherical or cylindrical surface. | | | | inspected using an autocollimator with the prism |
| Thus, the uniformity of the rings' shape indicates the | | | | oriented as a retro-reflector. This is only suitable for |
| limit of the surface's regularity. This deviation is also | | | | testing 90° and 45° angles; i.e. as in a right angle |
| known as surface figure. As a specification, it is | | | | prism. Note that although this specification relates to |
| important to note that in order to properly inspect | | | | the physical edge of two reflecting surfaces, it is |
| irregularity, it cannot be much smaller than the power | | | | typically tested as beam deviation. |
| or else you will not be able to ensure the irregularity | | | | THICKNESS |
| value. A typical rule of thumb is to use a maximum | | | | The importance of an element's axial thickness |
| power of 4 or 5 times the irregularity. Most optic shops | | | | depends greatly on its role in a system and can vary |
| work the power out from a stated irregularity. As a | | | | dramatically. Thickness refers specifically to the center |
| common practice, irregularity is easier and more | | | | thickness of a lens or spacing between elements. For |
| accurately inspected using a laser-based | | | | curved surfaces, a reasonable operating tolerance |
| interferometer, such as a Zygo GPI-XP Interferometer. | | | | runs ±0.1mm. For flat surfaces, however, the |
| A power/irregularity ratio of 4/1 is an acceptable | | | | production of large sheets of non-polished glass yields |
| tolerance to meet in volume production. | | | | larger variances in thickness. Thickness will vary |
| SURFACE QUALITY | | | | greatly depending on sheet size and where on the |
| This refers specifically to the cosmetic condition of an | | | | sheet the measurement is made. In order to |
| optical element's surface. During the grinding and | | | | accommodate this fact a nominal tolerance value is |
| polishing stages of fabrication, small defects can occur, | | | | used meaning that no specific thickness tolerance is |
| such as scratches and digs. A scratch is any mark or | | | | defined. Over time, nominal thickness tolerance has |
| tear and a dig is any pit or divot in the element's | | | | generally been accepted to be ±0.015" to 0.020". |
| surface. The specification used for the maximum | | | | Again, this refers to glass that is not polished after |
| allowable flaws is denoted by a combination of | | | | fabrication. |
| numbers, the scratch number followed by the dig | | | | If a specific thickness or precision surface accuracy is |
| number; for example 60-40. The lower the number, the | | | | needed then polishing is clearly required and higher |
| higher the level of quality. For example, a 60-40 value | | | | orders of tolerancing can be maintained. Typically, a 6:1 |
| is common for research and industrial applications, | | | | diameter to thickness ratio is used as a rule of thumb |
| whereas a 10-5 value represents a high quality | | | | for high accuracy plano surfaces in order to prevent |
| standard for laser applications. | | | | warping in fabrication or in the final mounting. Higher |
| It is important to note that neither the scratch nor the | | | | ratios may be used for lenses depending on radii and |
| dig numbers actually correspond to a specific number | | | | diameter values. |
| of defects. Instead, they reflect the quality of an optical | | | | Edge thickness is used as a "reference" for lenses |
| surface as determined by a visual comparison to a | | | | meaning that it is not a manufacturing limit. Edge |
| precisely manufactured set of standards. This process | | | | thickness is actually a calculated value which depends |
| is in accordance with the MIL Spec. Scratch and dig | | | | on radii, diameter, and center thickness. It is thus used |
| evaluation is as defined by the US Military Specification | | | | as a reference to indicate physical limitations for |
| for the Inspection of Optical Components, | | | | mounting considerations. |
| MIL-O-13830A. | | | | MATERIAL |
| There is no direct correlation between scratch number | | | | Glass Index and Abbé Number values are the most |
| and the actual size of a scratch on an optical | | | | important criteria in comparing one material to the next. |
| element's surface. As a common reference, the | | | | The index of refraction is actually a ratio of the speed |
| scratch number relates to the "apparent" width size of | | | | of light in a vacuum to that of light in a medium (i.e., a |
| an acceptable scratch. However, there is some | | | | specific type of glass). Since the speed of light in any |
| ambiguity since it also includes the total length and | | | | glass varies with the wavelength of light, the index of |
| number of allowable scratches. Dig numbers do relate | | | | refraction also changes with wavelength. Typically, a |
| to a specific size of dig. For example, a 40 dig number | | | | glass is defined at nd , which is the index at yellow |
| relates to a 400 µm (or 0.4mm) diameter pit. Coating | | | | helium or 587.6 nm. |
| quality is also held to the same Scratch-Dig | | | | Dispersion, or spectral variations in index of refraction, |
| specification as the surface of an optic. | | | | results in differences of focal distances for light of |
| Surface Quality inspection typically includes additional | | | | different wavelengths. This means that even though a |
| criteria, such as staining and edge chips. Overall | | | | lens will transmit a particular wavelength, if it was not |
| cosmetic inspection also includes defects within the | | | | designed at that wavelength then the performance will |
| material, such as bubbles and inclusions, including striae. | | | | not be the same as that stated for the design |
| Imperfections of this nature can contribute to | | | | wavelength. The Abbé number (vd) quantifies the |
| scattering in systems involving lasers and image | | | | amount of dispersion for a particular frequency range. |
| defects (if at or near the image plane). Inspection of | | | | This defines how much index changes with |
| surface accuracy and quality is limited to the | | | | wavelength and the smaller the value means the |
| component's clear aperture. | | | | quicker the change: v d=(nd-1)/(nF-nC) where |
| CENTRATION | | | | nF=486.1nm and nC=656.3nm |
| Centration is defined as the maximum allowable | | | | Glasses are typically defined as either crowns or flints. |
| deviation between the optical and mechanical axes for | | | | Crown glasses have the following combination of |
| a spherical lens. The optical axis is defined as the line | | | | values: n d55 or nd >1.6 and vd >50. Flints define the |
| connecting the centers of curvatures of both lens | | | | rest and are typically referred to as high index glass. |
| surfaces. The mechanical axis is the centerline of the | | | | |