Overview: Integration of Optical Systems Survey on current optical system design Case demo of optical system design

Transcription

1 Outline Chapter 1: Introduction Overview: Integration of Optical Systems Survey on current optical system design Case demo of optical system design 1

2 Overview: Integration of optical systems Key steps in integration application requirements specifying performance fundamental design form paraxial solution real lens solution mechanical integration testing/evaluation Optical testing Optical system design 2

3 Application requirement What is the applications? The first step to obtaining the best solution is to understand the problem posed in terms of the fundamental parameters of the applications examples: CCD imager/camera MTF, FOV, Fiber coupling N.A., coupling efficiency, PMT tube Throughput, FOV, But you also need to know/fit Optical testing 3

4 Basic terminology Conjugate distance The distance from the lens to the object/source or the detector/image plane In an infinite conjugate design, one of these distances approaches infinity (these are sometimes called afocal systems) conjugate size The size of the object/source and the size of the detector/image plane Size can be angle (angular field of view)/ beam waist In afocal systems, this also can be known as the beam waist on either side of the lens. numerical aperture (NA) F-number (F/#) A measure of the cone of light accepted or emitted by the lens NA increases with decreasing F/# resolution/ Spot size Smallest feature of an object distinguishable by the system (imaging system) Spot size (non-imaging system) Angular resolution (for afocal system) 4

5 Specifying performance The performance of an optical system is linked to many factors. Roughly, we need to consider two different types of system imaging systems The goal of an imaging system is to provide sufficient image quality to enable extraction of desired information about the object from the image. non-imaging systems (to be further considered) 5

6 Important factors Resolution contrast distortion perspective depth of field Specifying performance of imaging system 6

7 Specifying performance of nonimaging system Important factors field efficiency system s ability to accommodate a large detector area or source size throughput the amount of energy transmitted by the lens Why is throughput important? focusing system: spot size to evaluate a focusing lens performance -afocal system: angular resolution to specify the minimum angular separation between two objects that can be resolved by the lens 7

8 Throughput How large the lens diameter we should chose? (What kind of f/# we need to use?) A lens at discrete f/# (F1, F1.2, F1.4) with respect to its diameter for a 100 mm 2 detector relation TP versus lens diameter Best choice Example: F/1 at Larger f/#, smaller throughput 8

9 Specifying performance Non-imaging system In a focusing system Two fundamental reasons for a large spot size Magnification limited spot size All sources have a finite size and the resulting spot diameter is dependent on the magnification of the system By decreasing magnification, the source can be imaged to a smaller diameter Resolution limited spot size When the source size is small, a resolutionlimited spot size tends to only noticeable. The majority of problem for spot size are magnificationlimited. 9

10 Fundamental design form Most applications can be simplified and characterized to three types of design finite/finite conjugated design The light from a source placed at a near conjugate is focused down to a spot finite/infinite (infinite/finite) conjugated design Focusing a source placed at infinity to a spot infinite/infinite conjugated design Takes incoming collimated (parallel) light, changes the beam diameter and direction according to the magnification, and emits the light collimated 10

11 Paraxial solution A paraxial element is a theoretically perfect lens with no thickness, glass, or curvature. Because it does not introduce aberration into the system, it allows us to concentrate first order properties of the optical platform (conjugate distances and heights, magnification, etc.) The only specification of the paraxial lens is its position, diameter and focal length. Note: each design form has infinite paraxial solutions! Behind some of them, calculations and integrations are simple. 11

12 Description of variables Image and object height (Hi, Ho) Image and object distance measured from the lens closet to them (I,O) Focal length for the lens closet to the image and object (Fi, Fo) Focal length of the whole system (F) Magnification (M) Full angle of the cone of light accepted or emitted by a lens system (closely linked to numerical aperture) (Θ) Angular half field of view in infinite conjugate system (αi,αo) 12

13 Finite/finite conjugates Single element: Using a single element to create the relay Advantage: simplicity and cost-effective Two element: Yields increased image quality Equation F = Equation F = Fo Fi Fo + Fi d O I O I I F M = = = O F + O I Fi M = = = O Fo Hi Ho Hi Ho 13

14 Infinite/infinite conjugates This can be achieved Two positive elements A positive and negative elements Note: Fi is a negative value in the latter case and the resulting angular magnification is therefore negative d = Fo + Fi Equation M = Fo Fi = Ho Hi = αo αi 14

15 Infinite/finite conjugates Single element: A single element is usually sufficient for this design form Helps to reduce Cost Integration complexity The infinite/finite conjugates setup can be used to increase throughput to a detector Equation θ = 2sin 1 ( NA) 1 f /# = ( ) = 2NA 1 Hi α = tan ( ) F F Dia 15

16 Real lens solution The paraxial solution is great for simplicity. In real world, however, lenses have thickness, glass, and curvature. Substituting glass and curvature into the system requires understanding how each type of lens was optimized in order to maintain good performance. In general, we still can have some criteria to evaluate the performance. Low F/# Polychromatic Field efficiency cost But, you still need to be careful about Your applications. You don t have to limit to these criteria. 16

17 Low F/# Criteria Lens ability to work at low f/# (large incoming light diameter or high NA) low F/# applications should use lenses with ( ) rating polychromatic lens ability to hold performance with white light illumination (as opposed to monochromatic sources such as lasers or some LEDs) application with froadband illumination and no filtering should use lenses with ( ) rating field efficiency lens ability to accommodate a large sensor (image), source (object), or angular field of view (in afocal system). Applications with large image/objects should use lenses with ( ) rating cost this is a comparision of the cost of each type of configuration (i.e., takes into consideration the number of elements in that configuration). Low cost -$, high cost- $$$$$. 17

18 Real lens solution (1): infinite/finite This configuration is often used to focus a source in detector applications. Singlets are sometime used in low cost, long working distance imaging systems. 18

19 Real lens solution (2): finite/finite Achromats are often used in this configuration for relaying in imaging applications. Singlets are usually reserved for illumination based relays. 19

20 Real lens solution (3): infinite/infinite+finite-2 element Adding negative lenses enables compact afocal systems. They can be used to extend a focal plane. Positive/negative achromats create long working distance, high magnification imaging lenses. 20

21 Mechanical integration How to hold the elements appropriately to maintain the spacing of the lenses keeping in mind the effect of sag. Once the sag of each surface is determined, the spacers and seats can be designed to yield appropriate center-to-center lens spacing. Sag is defined as the distance between the edge of a mount and the vertex of a surface. The concept of sag only exists when the surfaces have curvature, because the inner diameter of the spacer or seat determines the location of the vertex. Since spacing in optical layouts are often defined from vertex to vertex (along the optical axis), it is important to be able to calculate sag. Equation SAG = r r 2 ( D 2 ) 2 21

22 Mechanical integration There are three common techniques used to mount lenses: Retainer rings Generally offer tighter control over the position, tilt and centering of the elements since each of lens sits on an individual counterboard seat. Costly Not always practical for small assemblies Spacers Ideal for small assemblies Cost effective (both in manufacturing and assembly) Lens sapcing tolerance is looser A combination of retainer and spacer 22

23 Mechanical integration Focusing and adjustment All optical designs should incorporate some amount of focusing or adjustment For best optimization of performance Helps to reduce the optical and mechanical tolerances which in turn reduces cost Focusing can be accomplished by moving the image (sensor), object, or some of the elements. Tolerance Tolerance affect the production yield of a compliant lens. Probability description is necessary to have more accurate estimate of yield Understanding the balance between increased cost due to overtolerancing and a decrease in yield due to non-confirming lenses can reduce cost significantly. 23

24 Sag calculation Seat inner diameter: 4 mm SAG: 0.29 mm Center spacing: 9.71 mm Seat inner diameter: 8.5 mm SAG: 1.42 mm Center spacing: 8.58 mm Sag effects increase with increasing seat inner diameter. This is also true for decreasing radius 24

25 Testing and integration Once the system is integrated, it is important to quantify its parameters and performance in order to understand whether the application requirements have been met. When prototyping, slight alteration to original designs are typical in order to accommodate component limitations. Fundamental parameters: First order requirements: conjugate distances, size, It is important to understand whether or not these have been met Adjustments of the element positions may be necessary to meet the design specifications Note: metal housing should be kept in mind. Not just only glass performance of a design resolution and contrast throughput 25

26 Testing and integration performance of a design Paraxial calculations give no indication of the real world performance of a design. For this reason, it is important to test the optical platform before integrating it into a setup. Evaluation Image quality can be determined using a variety of targets. Evaluation of non-imaging systems is done using a combination of detectors and possibly more involved interferometric instruments. Basic Notation Resolution, Over-all image quality, Contrast, Depth of Field, and Distortion Can be measured by targets (see below) Throughput Can be measured as power with photodiode. Spot size, beam profiling Can be analyzed with a CCD by evaluating the grayscale of the resulting image Angular resolution Can be measured using a collimator to project a target 26

27 Performance of a design Imaging quality Resolution USAF/Ronchi Ruling, EIA, Star Target Modulation transfer function sinusoidal patterns Depth of field DOF target Contrast EIA grayscale/ Color checker Distortion distortion targets 27

28 Performance of a design USAF Ronchi Ruling EIA Star Target Imaging quality: Resolution USAF/Ronchi Ruling, EIA, Star Target 28

29 Performance of a design Imaging quality: Modulation transfer function sinusoidal patterns Sinusoidal Patterns 29

30 Performance of a design Imaging quality: Depth of field DOF target DOF target 30

31 Performance of a design EIA grayscale Color Checker Imaging quality: Contrast EIA grayscale/ Color checker 31

32 Performance of a design Imaging quality: Distortion distortion targets Distortion Target 32

33 Performance of a design Non-imaging quality Field efficiency coherence detector based system (CCD/visual) Throughput photodetector based system spot size coherence detector based system (CCD/visual) Angular resolution target projected with a collimator 33

34 Rules of thumb for testing Test with application in mind It is important to test parameters that are critical to the design, but testing every aspect of the optical system is costly and usually unnecessary. cost associated with testing Testing can represent a significant production cost. High volume production usually warrants allocation of extra resources to design an adequate testing fixture/procedure to maximize yield and minimize labor visual tests Beware of visual testing Visual tests need to be desensitized to human error. 34

35 A case study (10-1) Statement of application An imaging and laser based system is needed to help identify the presence of pills during a manufacturing process Idea Using laser light to illuminate the sample area and detect the reflected light The presence of the pill can be tested by analyzing the histograms of the sample area (about the laser line). With pill, average pixel count is high due to reflection. Pixel count 35

36 You need to specify the components Helicoid barrel C-mount lens mounts Achromats Red dichroic filter C-mount iris CCD camera Line-generator lasercapture board and software A case study: (10-2) Specifying Component 36

37 A case study (10-3) Application requirement The system must yield a 10 mm field of view at 250 mm working distance onto a 1/3 format CCD (width: 2.54/3~8.4mm; half range~4.2 mm) The imaging lens needs to be less than 75 mm long Specifying performance The depth of field requirements are unknown, for this reason, the lens should incorporate a manual iris. The object resolution needed is also uncertain, so we need a focusing mechanism that allows us to vary the magnification Fundamental design form/paraxial solution 37

38 A case study (10-4) Fundamental design form/paraxial solution This is a finite/finite imaging system A magnification of 0.48X (sensor/fov~4.2/10~0.42x; take a lager value 0.48X) Length limit on imaging lens (75 mm) forces us to use positive and negative element combination Object (FOV~10 mm) Sensor (~4.2 mm) 75 mm 38

39 A case Study (10-5) Real lens solution Used Achormats to correct spherical and chromatic aberrations (using optical software for testing) Object (FOV~10 mm) Quartz/glass to shorten optical path Sensor (~4.2 mm) 75 mm 39

40 A case Study (10-6) Mechanical integration C-mount is quite often to be used Evaluation: Flexibility was designed into the prototype in order to determine the different effects that each components has on the overall system performance. We may check (1) F-number, (2) magnification, (3) filter, (4) illumination 40

41 A Case Study (10-7) F number (f/#) Decrease light (increased F#) So, F/# is not so critical here. Why? 41

42 A Case Study (10-8) Magnification (PMAG) Reducing working distance (and subsequently refocusing) can be used to obtain high magnification images 42

43 A Case Study (10-9) Filter: Filtering can be used to make the laser line stand out. Red filter already was shown previously 43

44 A Case Study (10-10) Illumination Different illumination can be used Quartz halogen source with a cylinder lens to generate a line Laser is used because of its high power and monochromatic red output can be easily filtered to increase the image contrast 44

45 Choosing off-the-shelf optics Why off-the-shelf optics (using catalog lens) Ready for use Factors Continual supply Organized scientific stepped matrix Systematic progression Complete technical data Volume discount 45