Source:Optical System DesignCHAPTERBasic Optics andOptical SystemSpecificationsDownloaded from Digital Engineering Library McGraw-Hill (www.digitalengineeringlibrary.com)Any use is subject to the Terms of Use as given at the website.Basic Optics and Optical System Specifications2Chapter 1This chapter will discuss what a lens or mirror system does and howwe specify an optical system.You will find that properly and completelyspecifying a lens system early in the design cycle is an imperative ingre-dient required to design a good system.The Purpose of an Imaging OpticalSystemThe purpose of virtually all image-forming optical systems is to resolvea specified minimum-sized object over a desired field of view.The fieldof view is expressed as the spatial or angular extent in object space,andthe minimum-sized object is the smallest resolution element which isrequired to identify or otherwise understand the image.The word "spa-tial"as used here simply refers to the linear extent of the field of view inthe plane of the object.The field of view can be expressed as an angleor alternatively as a lateral size at a specified distance.For example,thefield of view might be expressed as10×10，°or alternatively as350×350m at a distance of 2 km,both of which mean the same thingA good example of a resolution element is the dot pattern in a dotmatrix printer The capital letter E has three horizontal bars,and hencefive vertical resolution clements are required to resolve the letter.Hori-zontally,we would require three resolution elements.Thus,the mini-mum number of resolution elements required to resolve capital letters isin the vicinity of five vertical by three horizontal.Figure 1.1 is an exam-ple of this.Note that the capital letter B and the number 8 cannot bedistinguished in a 3 x 5 matrix,and the 5 x 7 matrix of dots will dojust fine.This applies to telescopes,microscopes,infrared systems,cameralenses,and any other form of image-forming optics.The generallyaccepted guideline is that approximately three resolution elements or 15line pairs over the object's spatial extent are required to acquire anobject.Approximately eight resolution elements or four line pairs arerequired to recognize the object and 14 resolution elements or seven linepairs are required to identify the object.There is an important rule of thumb which says that this smallestdesired resolution element should be matched in size to the minimumdetector element or pixel in a pixelated charged-coupled device (CCD)orcomplementary metal-oxide semiconductor (CMOS)-type sensor WhileDownloaded from Digital Engineering Library McGraw-Hill (www.digitalengineeringlibrary.com)Any use is subject to the Terms of Use as given at the website.Basic Optics and Optical System SpecificationsBasic Optics and Optical System Specifications3Figure 1.1lllustration of Num-ber of Resolution Elements Required toResolve or Distin-guish Alphanumericsresolution elementsresolution elementsnot rigorous,this is an excellent guideline to follow for an optimummatch between the optics and the sensor.This will become especiallyclear when we learn about the Nyquist Frequency in Chap.21,where weshow a digital camera design example.In addition,the aperture of thesystem and transmittance of the optics must be sufficient for the desiredsensitivity of the sensor or detector.The detector can be the human eyea CCD chip or film in your 35-mm camera.If we do not have cnoughphotons to record the imagery,then what good is the imagery?The preceding parameters relate to the optical system performance.Inaddition,the design form or configuration of the optical system must becapable of meeting this required level of performance.For example,most of us will agree that we simply cannot use a single magnifyingglass element to perform optical microlithography where submicronline-width imagery is required,or even lenses designed for 35-mm pho-tography for that matter.The form or configuration of the systemincludes the number of lens or mirror elements along with their relativeposition and shape within the system.We discuss design configurationsin Chap.8 in detail.Furthermore,we often encounter special requirements,such as coldstop efficiency,in infrared systems,scanning systems and others.Thesewill be addressed later in this book.Finally,the system design must be producible,meet defined packag-ing and environmental requirements,weight and cost guidelines,and sat-isfy other system specifications.How to Specify Your OpticalSystem:Basic ParametersConsider the lens shown in Fig 1.2 where light from infinity enters theDownloaded from Digital Engineering Library McGraw-Hill (www.digitalengineeringlibrary.com)Any use is subject to the Terms of Use as given at the website.