Understanding And Applying Machine Vision Part 5 pptx

However, due to geometric distortion and field angle characteristics, short focal length lenses should be evaluated as to suitability.. The lens extender is used behind the lens to incre

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Figure 6.27Vignetting

Video Camera Objectives

These are primarily designed for TV closed circuit surveillance applications The primary requirements are to provide maximum light level (large numerical aperture or low f-stop number) and a recognizable image Aberrations are of secondary importance, and they are really suitable only for low-resolution inspection work such as presence or

absence They are available in a wide range of focal lengths, from 8.5 mm (wide-angle lens), through the midrange, 25–

50 mm, to 135 mm (telelens) In general, wide-angle objectives (8.5–12.5 mm) have high off-axis aberrations and should be used cautiously in applications involving gauging

For most of these objectives, a 5–10% external adjustment of the image distance is provided, allowing one to focus anywhere within the range of working distances for which the objective has been designed This range can be

increased on the short end by inserting so-called extension rings of various thicknesses between the objective and the camera body By further increasing the image length li, they automatically increase magnification, or the size of the imaged object The use of extension rings offers great flexibility It should be remembered, however, that commercial objectives have been corrected only for the range of working distances indicated on the barrel The abusive use of extension rings can have a disastrous effect on image distortion These remarks on the use of extension rings apply not only to video camera but also to any type of objective

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35-mm Camera Objectives

These are made specifically for 35-mm photographic film with a frame much larger than the standard video target size Their larger format provides a better image, particularly near the edge of the field of view Their design is usually optimized for large distances, and they should not be used at distances much closer than that indicated on the lens barrel, meaning that no extension ring other than the necessary C-mount adapter should be added between objective and camera body They are widely available from photographic supply houses with various focal lengths and at

reasonable cost

Reprographic Objectives

We classify under this heading a number of specialized high-resolution, flat-field objectives designed for copying, microfilming, and IC fabrication Correction for particular aberrations has been pushed to the extreme by using as many as 12 single lenses This is generally done at the price of relatively low numerical aperture and the need of a high level of light.

Copying objectives are generally of short focal length and have high magnification At the other end of the spectrum, reducing objectives used in exposing silicon wafers through IC masks have very small magnification, extremely high resolution, and linearity Their high cost (up to $12,000) may be justified in applications where such parameters are of paramount necessity

Microscope Objectives

These are designed to cover very small viewing fields (2–4 mm) at a short working distance They cause severe

distortion and field curvature when used at larger than the designated object distance They are available at different quality grades at correspondingly different price levels They mount to the standard CCTV camera through a special C-mount adapter

Zoom Objectives

Allowing instantaneous change of magnification, zoom objectives can be useful in setting up a particular application It

is to be noted that the implied ''zooming" function is only effective at the specified image distance, that is, at the

specified distance between the target and the principal plane of the objective In all other conditions, the focus needs to

be corrected after a zooming step The addition of any extension ring has a devastating effect on the zooming feature Also, the overall performance of a zoom objective is less than a standard camera lens, and its cost is much higher

Mounts:

1 C Mount The C mount lens has a flange focal distance of 17.526 mm, or 0.690 in (The flange focal distance is the

distance from the lens mounting flange to the convergence point of all parallel rays entering the lens when the lens is focused at infinity.) This lens has been the workhorse of the TV camera world, and its format is designed for

performance over the diagonal of a standard TV camera

Generally, this lens mount can be used with arrays that are 0.512 in or less in linear size However, due to geometric distortion and field angle characteristics, short focal length lenses should be evaluated as to suitability For instance, an 8.5-mm focal length lens should not be used with an array greater than 0.128 in in length if the application involves metrology Similarly, a 12.6-mm lens should not be used with an array greater than 0.256 in in length

If the lens-to-array dimension has been established by using the flange focal distance dimension, lens extenders are required for object magnification above 0.05 The lens extender is used behind the lens to increase the lens to image distance because the focusing range of most lenses is approximately 5–10% of the focal length The lens extension can

be calculated from the following formula:

Lens extension = focal length/object magnification

2 U Mount The U-mount lens is a focusable lens having a flange focal distance of 47.526 mm, or 1.7913 in This lens

mount was primarily designed for 35-mm photography applications and is usable with any array less than 1.25 in in length It is recommended that short focal lengths not be used for arrays exceeding 1 inch Again, a lens extender is required for magnification factors greater than 0.05

3 L Mount The L-mount lens is a fixed-focus flat-field lens designed for committed industrial applications This lens

mount was originally designed photographic enlargers and has good characteristics for a field of up to 2 1/4 in The

Page 126flange focal distance is a function of the specific lens selected The L-mount series lenses have shown no limitation using arrays up to 1.25 in in length

Special Lenses

There are standard microscope magnification systems available These are to be used in applications where a

magnification of greater than 1 is required Two common standard systems for use with a U system are available, one with 10 magnification and one with 4 magnification

Cleanliness

A final note about optics involves cleanliness (Figure 6.29) In most machine vision applications some dust is

tolerable However, dirt and other contaminants (oily films possibly left by fingerprints) on the surface of lenses, mirrors, filters, and other optical surfaces contribute to light scattering and ultimately combine to reduce the amount of light transmitted to image area

Figure 6.29Transmission, reflection and scattering of light at optical surface

Practical Selection of Objective Parameters

6.3.4.1—

Conventional Optics

The best guide in selecting an objective for a particular application is undoubtedly experience and feeling An

experienced engineer will generally borrow a rough estimate of the focal length needed in a particular geometry from previous experience either in CCTV techniques or in photography Examination of the thus obtained image will

immediately suggest a slight correction in the estimate, if necessary The purpose of this tutorial would not fully be achieved, however, if the case of a novice engineer were not considered, having little or no such experience

It is best to start the design from the concept of field angle (the angle whose tangent is approximately equal to the object height divided by the working distance), the only single parameter embodying the other quantities specifying an imaging geometry: field of view, working distance and magnification The maximum field angle ϕmax, or the angle of view when the camera is focused at infinity,

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is listed in the manufacturer's specifications Table 6.1 lists ϕmax values for COSMICAR CC objectives The minimum field angle ϕmin depends on the distance of the principal plane of the lens to the image plane It has been calculated and listed in the table for the case of the maximum reasonable amount of extension rings set at 4 times the focal length.The following steps should be followed:

1 From Table 6.1 determine the range of objectives that will cover the field at the specified working distance

2 Adjust the image distance by adding extension rings to obtain the desired magnification

3 If the total length of extension rings arrived at by performing step 2, dangerously approaches the value of 4 times the focal length, and/or particularly if some image aberration begins to show up, switch to the next shorter focal length objective and repeat step 2

6.3.4.2—

Aspherical Image Formation

The concept of an image as being a faithful reproduction of an object is, of course, immaterial in those machine vision applications where only presence, consistent shape, color, or relative position are involved An elongated object will not fill the field of the 4/3 aspect ratio of a conventional CCTV In order to make more efficient use of the full

capability of vision hardware and vision algorithms; it is sometimes desirable to use a different magnification for the two-field axis (Figure 6.28) A simple such arrangement is to use a conventional spherical objective, conferring the same magnification in the two directions A cylindrical beam expander is then added to change the magnification, as desired, in one image axis only

Figure 6.30Beam splitter.

If the telecentric aperture stop is at the front focus (toward the object), the system is considered to be telecentric in image space; if the telecentric stop is at the back focus (toward the image), then the system is telecentric in object space Doubly telecentric systems are also possible

Since the telecentric stop is assumed to be small, all the rays passing through it will be nearly collimated on the other side of the lens Therefore, the effect of such a stop is to limit the ray bundles passing through the system to those with their major axis parallel to the optical axis in the space in which the system is telecentric Thus in the case of a system that is telecentric in image space, slight defocusing of the image will cause a blur, but the centroid of the blur spot remains at the correct location on the image plane The magnification of the image is not a function (to first order, for small displacements) of either the front or back working distance, as it is in non-telecentric optical systems, and the effective depth of focus and field can be greatly extended

Such systems are used for accurate quantitative measurement of physical dimensions Most telecentric lenses used in measurement systems are telecentric in object space only Its great advantages are that it has no z-dependence and its depth of field is very large Hence, telecentric lenses provide: a constant system magnification; a constant imaging perspective and solutions to radiometric applications associated with delivering and collecting light evenly across a field of view

Large telecentric beams cannot be formed because of the limiting numerical aperture (D/f), and the large loss of light

at the aperture In practice, telecentric lenses rarely have perfectly parallel rays Descriptions such as "telecentric to within 2 degrees" mean that a ray at the edge is parallel to within 2 degrees of a ray in the center of the field

The use of telecentric lenses is most appropriate when:

The field of view is smaller than 250 mm

The system makes dimensional measurements and the object has 3D features or there are 3D variations in the working distance (object-to-lens distance)

The system measures reflected or transmitted light and the field of view (with a conventional lens) is greater than a few degrees

6.3.4.4—

Beam Splitter

A beam splitter arrangement (Figure 6.30) can be a useful way to project light into areas that are otherwise difficult because of their surroundings In this arrangement a splitter only allows the reflected light to reach the camera

Two or more camera fields can also be synthesized into a single split field using a commercial TV field splitter The compromises here are a loss of field resolution and more complex and expensive hardware Both these methods have drawbacks and are not always practical An alternative method consists in imaging the two (or more) parts of interest

on the front end of two (or more) coherent (image quality) fiber-optic cables and, in turn, reimage two (or more) other ends of the cables into a single vision camera The method, while providing only moderate quality imaging, provides extreme flexibility

Scanned Sensors: Television Cameras

In one method an object is illuminated, and all of its points are simultaneously imaged on the imaging plane of the sensor The imaging plane is then read out, that is, sensed, point by point, in a programmed sequence It outputs a sequential electrical signal, corresponding to the intensity of the light at the individual image points The most common scanned sensor is the CCTV camera This is discussed in greater detail in the next chapter

6.4.2—

Flying Spot Scanner

The same correlation can be established if we were to reverse the functions of light source and sensor In this method, the sensor is a point source detector, and it is made to look at the whole object, all object points together (Figure 6.31) These points are now illuminated one by one by a narrow pencil of light (a scanned CRT or, better, a focused laser) moving from point to point according to a programmed (scanned) sequence Here, again, the sensor will output a sequential signal similar to that of the first method In a flying spot scanner, extremely high resolutions of up to 1 in

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Figure 6.31Flying spot scanner for web surface inspection

for the transverse axis A popular system, for example, capable of achieving high resolution uses a line scanner, a dimensional array of individual photosensors (256–12,000), in one axis and a numerical control translation table in the other

one-References

Optics/Lighting

Abramawitz, M J., "Darkfield Illumination," American Laboratory, November, 1991

Brown Lawrence, "Machine Vision Systems Exploit Uniform Illumination," Vision Systems Design, July, 1997.Forsyth, Keith, private correspondence dated July 9, 1991

Gennert, Michael and Leatherman, Gary, "Uniform Frontal Illumination of Planar Surfaces: Where to Place Lamps," Optical Engineering, June, 1993

Goedertier, P., private correspondence, January 1986

Harding, K G., "Advanced Optical Considerations for Machine Vision Applications," Vision, Third Quarter, 1993, Society of Manufacturing Engineers

Higgins, T V., "Wave Nature of Light Shapes Its Many Properties," Laser Focus World, March, 1994

Page 131Hunter Labs, "The Science and Technology of Appearance Measurement," manual from Hunter Labs Reston, VA.Kane, Jonathan S., "Optical Design is Key to Machine-Vision Systems," Laser Focus World, September, 1998.

Kaplan, Herbert, "Structured Light Finds a Home in Machine Vision," Photonics Spectra, January, 1994

Kopp, G and Pagana, L A., "Polarization Put in Perspective," Photonics Spectra, February, 1995

Lake, D., "How Lenses Go Wrong - and What To Do About It," Advanced Imaging, June, 1993

Lake, D., "How Lenses Go Wrong - and What To Do About It - Part 2 of 2," Advanced Imaging, July, 1993

Lapidus, S N., "Illuminating Parts for Vision Inspection," Assembly Engineering March 1985.

Larish, John and Ware, Michael, "Clearing Up Your Image Resolution Talk: Not So Simple," Advanced Imaging, April, 1992

Mersch, S H., "Polarized Lighting for Machine Vision Applications," Conference Proceedings, the Third Annual Applied Machine Vision Conference, Society of Manufacturing Engineers, February 27–March 1, 1984

Morey, Jennifer L., "Choosing Lighting for Industrial Imaging: A Refined Art," Photonics Spectra, February, 1998

Novini, A., "Before You Buy a Vision System" Manufacturing Engineering, March 1985.

Schroeder, H., "Practical Illumination Concept and Techniques for Machine Vision Applications," Machine Vision Association/Society of Manufacturing Engineers Vision 85, Conference Proceedings

Smith, David, "Telecentric Lenses: Gauge the Difference," Photonics Spectra, July, 1997

Smith, Joseph, "Shine a Light," Image Processing, April, 1997

Stafford, R G., "Induced Metrology Distortions Using Machine Vision Systems," Machine Vision Association/Society

of Manufacturing Engineers Vision 85, Conference Proceedings

Visual Information Institute "Structure of the Television Roster," Publication Number 012-0384, Visual Information Institute, Xenia, OH

Wilson, Andrew, "Selecting the Right Lighting Method for Machine Vision Applications," Vision Systems Design, March, 1998

Wilson, Dave, "How to Put Machine Vision in the Best Light," Vision Systems Design, January, 1997

Generally, machine vision systems employ conventional CCTV cameras These cameras are based on a set of roles governed by Electronic Industries Association (EIA) RS-170 standards Essentially, an electronic image is created by scanning across the scene with a dot in a back-and-forth motion until eventually a picture, or "frame," is completed This is called raster scanning.

The frame is created by scanning from top to bottom twice (Figure 7.1)

This might be analogous to integrating two scans of a typewritten page With one scan all the lines of printed

characters are captured as organized; with a second scan all the spaces between the lines are captured Each of these scans corresponds to a field scan When the two are interleaved, a double-spaced letter page results, corresponding to a frame

A field is one-half of a frame The page is scanned twice to create the frame, and each scan from top to bottom is called

a field Alternating the line of each of the two fields to make a frame is called interlace In a traditional RS-170 camera, the advantage of the interlaced mode is that the vertical resolution is twice that of the noninterlaced mode In the

noninterlaced mode, there are typically 262 horizontal lines within one frame, and the frame repeats at a rate of 60 Hz Today

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Figure 7.1Interlaced-scanning structure(courtesy of Visual Information Institute)

non-RS-170-based cameras are available that operate in a progressive scan or non-interlaced mode providing full

"frame" resolution at field rates — 60 Hz Depending on the camera and the imaging sensor used, the vertical

resolution could be that of a full field or a full frame

In all cases the camera sweeps from left to right as it captures the electronic signal and then retraces when it reaches the end of the sweep A portion of the video signal called sync triggers the retrace Horizontal sync is for the retrace of the horizontal scan line to the left of beginning of the next line Vertical sync is for the retrace of the scan from bottom

to top

In the United States where the EIA RS-170 standard rules, a new frame is scanned every 30th of a second; a field is scanned every 60th of a second (Table 7.1) In conventional broadcast TV there are 525 lines in each frame In machine vision systems, however, the sensor often used may have more or less resolution capability although the cameras may still operate at rates of 30 frames per second In some parts of the world the phase-alternating line (PAL) system is used, and each frame has 625 lines and is scanned 25 times each second

In machine vision applications where objects are in motion, the effect of motion during the 30th-of-a-second

"exposure" must be understood Smear will be experienced that is proportional to the speed Strobe lighting or shutters may be used to minimize the effect of motion when necessary However, in RS-170 format synchronization with the beginning of a frame sweep is a challenge with conventional RS 170 cameras Since the camera is continually

sweeping at 30 Hz, the shutter/strobe may fire so as to capture the image partway through a field This can be handled

in several ways The most convenient is to ignore the image signal on the remainder of the field sweep and just analyze the image data on the next full field However, one sacrifices vertical resolution since a field represents half of a frame

Page 135TABLE 7.1 Television Frames

Scan lines in Two Fields

Scan lines per Field

Field Rate (Hz) Frame Rate (Hz)

In response to this challenge, camera suppliers have developed products with features more consistent with the

requirements of high-speed image data acquisition found in machine vision applications Cameras are now available with asynchronous reset, which allows full frame shuttering by a random trigger input The readout inhibit feature in these cameras assures that the image data is held until the frame grabber/vision engine is ready to accept the data This eliminates lost images due to timing problems and makes multiplexing cameras into one frame grabber/vision engine possible

In some camera implementations, in asynchronous reset operating mode, upon receipt of the trigger pulse, the vertical interval is initiated, and the previously accumulated charge on the active array transferred to the storage register

typically within 200 microseconds This field of information, therefore, contains an image that was integrated on the sensor immediately before receiving the trigger The duration of integration on this "past" field can be random since the trigger pulse can occur anytime during the vertical period This would result in an unpredictable output Hence, this field is not used

After the 200 microsecond period, the active array is now ready to integrate the desired image, typically by strobe lighting the scene Integration on the array continues until the next vertical pulse (16.6 msec) or the next reset pulse, whichever occurs first Then the camera's internal transfer pulse will move the information to the storage register and begin readout With this arrangement only half of the vertical resolution is applied to the scene

7.1.2—

Video Signal

The video signal of a camera can be either of the following:

Contains only picture information (video and blanking) and no synchronization information Sync is a complex

combination of horizontal and vertical pulse information intended to control the decoding of a TV signal by a display device

Sync circuits are designed so a camera and monitor used for display will operate in synchronization, the monitor

simultaneously displaying what the camera is scanning Cameras are available that are either sync locked or genlocked

A camera with sync lock requires separate driving signals for vertical drive, horizontal drive, subcarrier, blanking, and

so on Genlock cameras can lock their scanning in step with an incoming video signal; a feature more compatible with the multicamera arrangements often required in machine vision applications Genlock optimizes camera switching speeds when multiplexing cameras but is not required to multiplex cameras

7.1.3—

Cameras and Computers

When interfacing a camera to a computer, the camera can be either a slave or a master of the computer When the camera is the master, the camera clock dictates the clocking of the computer, and the computer separates the sync pulses from the video composite signal generated in the camera When the camera acts as the slave, the computer clock dictates the clocking of the camera via a genlock unit

When the camera is the master, cameras of different designs can be interfaced to the same system as long as they all conform to EIA RS-170 standards However, only one camera at a time can be linked to the computer When the camera is the slave, using genlocking, multiple cameras can be interfaced to the same computer

7.1.4—

Timing Considerations

Timing considerations should be understood Typically, a horizontal line scan in a conventional 525-line, 30-Hz

system is approximately 63.5 microseconds (Figure 7.2) However, 17.5%, or 11 microseconds, of this time represents horizontal blanking The pixel time is therefore (63.5 – 11)/512, or approximately 100 nsec In other words, a machine vision system with these properties generates a pixel of data (2, 4, 6, or 8 bits) every 100 nsec

The actual pixel-generating rate dictates the requirement of the master clock of the camera In this instance the master clock has to be 1/100 nsec, or 10 MHz The selection of the number of pixels per line is limited by the ability of the TV system to produce data at the rate demanded While 10 MHz is typical and special systems can generate data at a rate

of 100 MHz, the practical limit is 40 MHz

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Figure 7.2Camera timing considerations(courtesy of Visual Information Institute).

The vertical rate is a function of the number of lines in the image If 260 lines are developed at 60 Hz, each line must

be scanned in 1/15,600 sec, or 64 microseconds Conventional TV operates at horizontal rates of 15.75 KHz, which is determined by

Vertical blanking is typically 7.5% of the vertical field period

What all this means is that in an RS-170 camera, the two halves of the picture which would be stitched together by the computer are not really taken at the same moment in time There are a few milliseconds between the shifting of all the

"odd" lines and all the "even" lines Any movement in the field of view of the camera will cause the scene to change slightly between reading out these fields

Progressive scanning cameras are available These do not operate to the RS-170 standard of two interlaced fields combined to form a single frame Instead the lines of video on the imager are read sequentially, one line at a time from top to bottom Hence, the full vertical resolution of the imager can be obtained, typically at field reading rates - 60 Hz

7.1.5—

Camera Features

RS-170 cameras come with a variety of features that may or may not have a positive impact on their use in machine vision applications An automatic black level circuit that maintains picture contrast throughout the light range can be im-

Page 138portant in machine vision applications Many include circuits (automatic light range, automatic light control, automatic gain control, etc.) to assure the maximum sensitivity at the lowest possible light level These provide automatic

adjustment as a function of scene brightness

In applications where the video output is a measure of light intensity, the automatic gain control should be disabled Under certain light conditions the camera may present images with better contrast

Gamma correction, or correction to compensate for the nonlinearity in the response of the phosphor of a cathode ray tube (CRT-based display), is designed to give a linear output signal versus input brightness Both automatic light control and gamma correction circuits are designed to optimize the property of the display monitor for human viewing

In machine vision applications these circuits may distort the linearity of the sensor, defeating a linear gray scale

relationship that may be the basis of a decision Disabling gamma correction can increase the contrast between the dark image and the bright image