A tutorial for designing fundamental imaging systems

1. Abstract

This tutorial shows what to do when we design opto-mechanical system for imaging system. Basic principles about designing paraxial systems are introduced. In particular, you can find how to decide specifications, how to select optical elements, how to calculate image shift and how to estimate wavefront errors.

2. Introduction

Optical elements have been developed due to development of manufacturing tools, and we can use various kinds of mirrors, lenses, prisms or filters in various systems, and we can get high-quality elements. When we develop optical systems, we need to decide specifications according to cost, size or image quality. Figure.2.1 is an example of imaging microscope optical system. This includes features which we have to decide when we are designing basic optical systems.

3. Specifications of optical system with imaging devices

When we design optical systems, we need to decide specifications of optical systems. We need to decide following basic specifications :

(a) Magnification and focal length,

(b)Resolution and depth of focus.

(1) Magnification and focal length

We should decide magnification of designed system according to the resolution of the image. We should decide it at first. Magnification for fundamental optical system is shown in Figure.3.1. There are 2 lenses and object with height of ho. Object is placed at front focal plane of an objective lens, and an image is given at back focal plane of an imaging lens. Magnification of this optical system m is given by the following equation[1]. This shows that we can calculate back focal length fi after we decide front focal length fi and magnification m. The distance between the objective lens and imaging lens need not to be fo+fi because the ray is collimated between the 2 lenses. The image height is independent of this distance

2) Resolution and depth of focus (DoF)

Resolution and depth of focus (DoF) depend on system F#. F# is important factor for image brightness and resolution of optical system, which is defined by following equation. We can get higher resolution with larger F#, but the depth of focus (DoF) becomes shorter. We have to balance both the DoF and resolution. There are 2 pictures taken with different F# in Figure.3.2. The background is focused in The picture because the camera with large F# has long DoF. On the other hand, the background is defocused in the picture because the camera with small F# has short DoF. We use aperture stop in order to control F#.

Where f is the focal length of optical element, and D is the diameter. Resolution is also decided with F#. We can get high resolution with small F#. The resolution is described with Airy disc, explained in Figure.3.3. Airy disc is the size of image of a small point. The size of Airy disc is given by following equation. The resolution of optical system is often described by the Airy disc.

Where d is diameter of Airy disc. Resolution is a minimum distance in which two points can be distinguished, which is explained in figure.3.4. Resolution is almost the same size as the size of the Airy disc.

Figure.3.2 Picture with large F#(left, f5.6) and with large F#(right, f/32)
(Image by Wikipedia)

4. Optical elements

After we decide optical specifications, we need to think of what optical elements to use.

(1) Lens, mirrors and prisms

Lens, mirrors and prisms are most basic optical elements. These deviate rays and we can condense rays or rotate images. At first we decide what lens to use and calculate focal length or other basic features.

(2) Image sensor ( Photo detector )

Image sensor ( Photo detector ) Image sensor transfers light energy to electrical energy. There are various kinds of sensors such as CMOS and CCD sensors, and it provides with each merit.[3][8] CCD or CMOS sensor is used for imaging devices such as digital camera or digital photo copier. PMT is a very high sensitivity photo detector which is used for detection few photon.[8] Table.4.1 shows merit and demerit of each sensor. We usually use CCD area sensors for imaging systems. CCD, CMOS image sensor and PMT is shown in Figure.4.1. CCD and CMOS is almost the same size, but PMT is much larger.

(3) Filters

Filters are used to attenuate specific wavelength of rays. We should use filters when we want to control brightness or colors of images, or need to detect specific wavelength. There are various kinds of filters such as band-pass filter that attenuate excluding specific wavelength rays[3]. ND filter attenuates thoroughly over a wide wavelength band, so we can control amount of rays without changing system F#. Sharp cutting filters that cut outside inferred ray or outside. Because of these features, these filters look like colored glass window. Light transmission spectrum of each filter is shown in Figure.4.2. PL (polarized light) filter can pass a specific polarized light, which is used to suppress reflected light and you can see them used for sunglasses or camera filter which can suppress reflected light from surface of water or other reflective surfaces. We can find many other filters from optical components manufacturer website and need to choose appropriate filters according to our purpose [4]-[6].

(4) Aperture stop

Aperture stop is used in optical systems in order to change system F# to limit light amount to control the brightness of the images, DoF and resolution. Aperture stop is shown in Figure.4.3. Aperture stop is placed to limit amount of light most effectively.

(5) Adjustments and mounts

Adjustments and mounts We need to consider adjusting optical components with precision. It is necessary to balance precision and cost. There are various kinds of kinematic mounts. It is preferable to decide it in consideration of the method of the mount and the processing cost. Figure.4.4 shows kinematic mirror holders. Kinematic mounts are often used for adjustment of element tilt. These are used for adjustment of mirror itself, but it is difficult for small lens system like a camera. Therefore it is desirable to loosen tilt error. Figure.5.2.(a) shows the structure of kinematic mount for 2-axis tilt. The ball constrains the motion of the optical element. Figure.5.2. (b) shows the ball constraints. These provide translation constraint, XYZ motion, but rotational motion is free. Kinematic mounts realize tilt or other necessary motion with these constraint structures.

5. Line of sight (LOS)

In design of optical system, lateral shift or angular deviation of LOS correspond to the image motion. It is necessary to minimize the image motion and optimize element position error. We need to think of 6 factor of element motion, which is shown in Table.5.1.

6. Wavefront aberration

At first, we design optical system ignoring elements surface error. In practice, there are defects or distortion on the surfaces of optical elements, which effect to wavefront error. Wavefront error cause degradation of optical resolution [1].

Wavefront error is important in high-resolution system, but it is not so important if we treat low resolution optics. We need to decide quality of surface error according to resolution and cost. We need to do following three tolerance analysis. The total wavefront error is given by RSS (root sum square) of each element effect.

(a)Assembly torrelance

(b)Lenses quality tolerance

(c)Thermal changes

Table.6.1. is example of tolerance analysis for lens qualities. Analyzed system is shown in Figure.6.1. This system has 3 lenses. (so this has 6 surfaces)( see. Report of OPTI521, Fall2009, Homework4) [1].

Figure.6.1. Analyzed optical system

We can calculate the sensitivity with optical coding program like ZEMAX or CODE-V. We have to consider the sensitivities and contributions as well as costs.The element whose sensitivity is low can be adjusted loosely, and the factor whose sensitivity is high has to be adjusted strictly. We have to think of difficulty of machining, fabricating, or adjustment.Tilt adjustment is generally more difficult than lateral adjustment, so contribution of tilt error is often decided larger than that of lateral error. But we always have to think of each system, so that is not always true.

7. Other effects

We need to consider thermal effects, deformation by fixing force, vibration of system or environment, adhesives used for fixing optical components. We also have to think of surface treatment of mechanical parts.