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Construction parameters include surface types ( Spherical , Aspheric , Holographic , Diffractive , etc.), and different parameters for each surface type, such as Radius Of Curvature , thickness to the next surface, glass type, and tilt and/or decenter. Performance requirements can include: # Optical performance such as image sharpness, quantified by Encircled Energy , Modulation Transfer Function , Strehl Ratio , ghost reflection control, etc. and/or pupil performance (size, location, aberration control) #Physical requirements such as Weight , static Volume , Swept Volume , Center Of Gravity , and overall configuration requirements, and #Environmental requirements (ranges for Temperature , Pressure , Vibration , Electromagnetic Shielding , etc.) Design constraints can include realistic lens element center and edge thicknesses, minimum and/or maximum airspaces between lenses, physically realizable glass Index Of Refraction and Dispersion properties, etc. Optical design is both a science and an art. It is scientific because ray paths and Wavefront structure can be very accurately calculated anywhere along the Propagation path through the lens. Glass and coating optical properties can be measured and modeled with sufficient precision for use in lenses. If tolerancing is included during the design, parts can usually be manufactured accurately enough that the resulting lens assembly performs fairly closely to the paper design. But optical design is also an art, as the multi-dimensional design volume within which a constrained lens design is free to roam is literally beyond human imagination or comprehension if more than two to three construction parameters are free to vary. The number, type and placement of optical elements is partly driven by the requirements, but is also often based on previous similar designs obtained from published data, patents, books, etc. Skill and intuition in lens design is acquired over years of experience spanning hundreds to thousands of different lens design projects, preferably leading to additional experiences (and headaches) dealing with fabricating and aligning systems. As an example of the complexity of lens design space, a simple two-element Airspaced lens has nine variables (four radii of curvature, two thicknesses, one airspace thickness, and two glass types). Even for this simplest case, the design space is thus 9-dimensional, and local or global solutions within this space can at least be imagined as smaller or larger bubbles in a sponge-like 9-D foamscape. A complex multi-configuration lens corrected over a wide spectral band and field of view, at multiple zoomed focal lengths and over a realistic temperature range, can have a 100-dimensional design volume or greater. Lens optimization techniques have been studied since the 1940's, beginning with early work by James G. Baker , and later by D. Feder , Wynne , Glatzel , D. Grey and others. Prior to the advent of digital computers, lens design was an agonizingly slow hand-calculation process requiring high-precision Trigonometric and Logarithmic tables, reams of paper, and significant patience and understudying from previous masters. Tracing a single ray through a given lens surface could take more than an hour of painstaking calculations and checks, and a lens designer could not design more than a very few complex, high-performance Anastigmatic objectives in an entire lifetime. Modern desktop computers can now Raytrace tens to hundreds of millions of rays per second through a lens, and perform hundreds to thousands of optimization cycles per second, rapidly exploring the n-dimensional design volume and even hill-climbing in and out of local minima in the search for the best solution. The day is approaching when computers will be able to perform "real-time raytracing", calculating the ray/wave propagation of light through a lens as fast as the light actually travels. However, even with lightning-fast optimizers, seasoned experience is still needed to guide solution Trajectories through unacceptably shallow local Minima and achieve the desired performance requirements. Experience in glass, metals and coating properties is also needed, especially in systems required to give good performance over wide temperature ranges. SEE ALSO REFERENCES #Smith, Warren J., ''Modern Lens Design'', McGraw-Hill, Inc., 1992, ISBN 0-07-059178-4 #Kingslake, Rudolph, ''Lens Design Fundamentals'', Academic Press, 1978 #Shannon, Robert R., ''The Art and Science of Lens Design'', Cambridge University Press, 1997. |
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