Image acquisition

Industrial Vision: Image acquisition
CCD camera
Digitalisation
Data acquisition cards
Vision software
Cameras and sensors; lenses
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Image acquisition: CCD camera
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Image acquisition: CCD camera
CCD camera
CCD: Licht wordt in lading omgezet. CCD chip bevat
aan elkaar geschakelde capaciteiten. Ladingen
worden doorgeklokt naar buiten. ADC kan niet op de
CCD chip, dus steeds een analoge uitgang.
CMOS: elk beeldelement bevat 1 fotodiode en 3
transistoren: een voor lading-spanningsconversie,
twee voor adressering.
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Image acquisition: CCD sensor
Photon
Gates
Silicon
Charges
Potential
Well
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Image acquisition: digitalisation
0...255
A
Volts
D
Greyvalue
8 Bit
Volts
0,7
0,348
brighter
127 255 Greyvalue
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Digitalisation
Pixel = Picture Element
camera image
Pixel mask
8 bit grayscale
digital image
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Digitalisation
Grayscale image
& numeric representation
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Video sources
The video source can be:
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Video camera
Camcorder
Video recorder (VCR)
Television broadcasts
X-ray equipment
Scanning Electron Microscope (SEM)
CT scanner
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Signal types for Image acquisition boards
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Composite video = signal containing both video data (luminance + colour)
and the timing (synchronisation) information. It is the standard which
interconnects almost all video equipment (TVs, laserdisc, videorecorders,
camcorders) at home.
It can be transported over RCA connector or SCART (=Peritel=Euro) connector.
Examples of composite video standards:
• RS-170:
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used in North America and Japan
• Monochrome signal
• Spatial resolution: 640 pixels x 480 lines
• Frequency: 60 fields/second (equivalent to 30 frames/second)
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NTSC/RS-330
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used in North America and Japan
• Equivalent to RS-170 but colour information is superimposed on the
monochrome signal.
• NTSC = National Television System Committee
Signal types for Image acquisition boards
More composite video standards:
• CCIR
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used in Northern Europe
Monochrome signal
Spatial resolution: 768 pixels x 576 lines
Frequency: 50 fields/second (equivalent to 25 frames/second)
CCIR = Comité Consultatif International Radio
PAL
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used in Northern Europe
• Equivalent to CCIR but colour information is superimposed on the
monochrome signal.
• PAL = Phase Alteration Line
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SECAM
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used in France, Russia and the Sovjet Republic States
• Equivalent to CCIR but colour information is superimposed on the
monochrome signal.
• SECAM = Séquencial Couleur Avec Memoire
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Signal types for Image acquisition boards
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S-Video (also called Y/C video): luminance (Y) and chrominance (C) are separate
signals. The Y signal contains timing (synchronisation) information. S-video can be
transported over 4 pin mini DIN connector, or over SCART connector.
Some image sources produce “nonstandard” video signals:
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Video and timing information can vary in format as well as in single or multiple
signals. They do not adhere to particular spatial resolutions, signal timing
schemes, signal characteristics … Consult the documentation provided with
your video source.
Progressive scan (25-30 frames/sec) cameras produce non interlaced signals.
All previous camera signals are analogue.
DIGITAL CAMERAS: No frame grabber required!
• Cameras with FireWire (IEEE 1394) interface.
Supported by Apple, Windows XP
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Cameras with USB interface
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Fuga
Allegro
Image acquisition boards
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The video capture device is often called frame grabber card.
Frame grabber puts a pixel mask over the image: the card converts the
analogue image (or images) supplied by a video source into a digital array
(or arrays) of data points.
It is a plug in card (PCI) with AD convertor. The ADC must have video
speed: 20 MHz or higher (30 or 25 video frames per second, 300 kB [640 x
480 x 8 bit] per frame.
Other features:
• input multiplexer (to select one of the 4 inputs)
• Colour notch filter = chrominance filter (to acquire monochrome signals
from colour sources)
• Programmable gain stage (to match the signal into the ADC input range)
• Timing and acquisition control (to synchronise grabbing with sync pulses
of incoming signal: PLL or Digital Clock Synchronisation)
• Camera control stage (to send to the camera or to receive from the
camera setup and control signals, e.g. horizontal and vertical sync
signals, pixel clock and reset signals)
• Most cards provide digital I/O for input or output operations, to
communicate with external digital devices (e.g. industrial process). This
saves a separate I/O board.
Block diagram of analog frame grabber
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© Data
Translation
Image acquisition boards (continued)
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Plug-in cards (image grabber, frame grabber card) for analogue cameras
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Are plugged in at a VME or PCI bus
Are delivered with Windows 98 or NT drivers
Accept cameras according to the EIA (30 frames/sec) or CCIR (25)
standards
Good cards have their own processor (DMA data transfer to PC) and
large RAM
Others (cheaper ones) use the PC processor
They accept the signals: S video, composite video TV or VCR signals
(NTSC/PAL/Secam)
Some cards have camera control output
Image acquisition - Cameras
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Sensor types:
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Line
• Array
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Interface standards:
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CCIR / RS-170 (B&W, 50-60 fields/sec.)
PAL / SECAM / NTSC (Colour)
Progressive scan (25-30 frames/sec.)
FireWire (IEEE 1394)
USB
Sensor technology:
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CCD (Charge Coupled Device)
• CMOS (Complementary Metal Oxide
Semiconductor). A CMOS camera produces a
1000*1000 pixel image
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Spatial resolution
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• The number of rows (N) from a video source generally corresponds
one-to-one with lines in the video image. The number of columns,
however, depends on the nature of the electronics that is used to
digitize the image. Different frame grabbers for the same video camera
might produce M = 384, 512, or 768 columns (pixels) per line.
• a CCIR / PAL image source can result in max 768 x 576 pixel image
• a RS-170 / NTSC source can result in max 640 x 480 pixel image
• Depending on video source or camera used, the spatial resolution can
range from 256 x 256 up to 4096 x 4096.
• Most applications use only the spatial resolution required. For fast
image transfer and manipulation, often 512 x 512 is used. For more
accurate image processing, 1024 x 1024 is common.
• The pixel aspect ratio (pixel width : pixel height) can be different from
1:1, typical 4:3. Some frame grabbers don’t convert video data into
square pixels but into rectangle ones. This creates the effect of a circle
appearing ovular, and squares appearing as rectangles.
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Spatial resolution
• Example 768 x 512 (aspect ratio 3 : 2)
768 pixels
512 rows
520 rows max CCIR
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Brightness resolution
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Brightness resolution = bit depth resolution: number of gray levels
(monochrome) or number of colours
• RS-170 / NTSC image: 8 bits = 256 gray levels
• A standard RS-170 image is 307 kB large: 640 x 480 x 8bit.
Interlaced / non interlaced formats
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A video signal consists of a series of lines. Horizontal sync pulses
separe the lines from each other.
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All composite video sources (RS-170/NTSC, CCIR/PAL) and some
nonstandard video sources transmit the lines in interlaced format: first
the odd (first field), afterwards the even lines (second field).
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Vertical sync pulses separate the fields from each other.
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Some nonstandard video sources transmit the lines in non-interlaced
format = progressive scan. Only one field, containing all the lines, is
transmitted.
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Progressive scan is recommended for fast moving images.
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If one is planning to use images that have been scanned from an
interlaced video source, it is important to know if the two half-images
have been appropriately "shuffled" by the digitization hardware or if that
should be implemented in software. Further, the analysis of moving
objects requires special care with interlaced video to avoid "zigzag"
edges.
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Image sensors - Interlacing
Interlaced image (TV)
Field 1
Field 2
Industrial Vision software
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Labview
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…
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Image enhancement
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Neighbourhood operations
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Sharpening / blurring
Edge detection
Image
-1 -1 -1
-1 9 -1
-1 -1 -1
1 1 1
1 1 1
1 1 1
1 0 -1
1 0 -1
1 0 -1
1 1 1
0 0 0
-1 -1 -1
Result
Image enhancement
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Contract enhancement
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Thresholding
Histogram equalisation
See course Digital Image Processing, R. Catthoor
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Image acquisition: moving objects
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Resolution calculation
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Cd resolution = cross detection
• Md resolution = moving detection
• Example cd resolution:
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Image width = 1900 mm
Required resolution = 0.25 mm
Number of pixels: 1900/0.25 = about 8000
This requires minimum 2 line scan CCD cameras
Example md resolution:
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Required resolution = 0.25 mm (same as cd)
Object velocity = 1600 mm/sec
This requires 1600/0.25 = 6400 lines per second, or a camera with
6.4 kHz line scan frequency.
Image acquisition: cameras and sensors
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Camera
• Contains CCD or CMOS chip. It produces an analogue signal
• CCD chip contains opt. sensors with bucket memory and shift register
• The shift register is controlled by a pixel clock
• Synchronous reset camera: the pixel clock runs independently
• Asynchronous reset camera: the shift register is set externally: a picture
is captures immediately after an external trigger.
• A Matrix camera captures interlaced or progressive scan (=noninterlaced) images
• Interlaced camera: sensor can be slower; reduced flickering
• Standards: an image has a 2x3 ratio
• A Linescan camera captures one single line only
• Sensors producing an interlaced image are not good for moving images
(comb effect)
• Now there are digital cameras: The CCD is coupled to a built in ADC
• Digital camera’s allow digital remote camera settings; no frame grabber
card is required.
Image acquisition: cameras and sensors
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CCD or CMOS
• CCD chip bestaat uit lichtsensoren in de vorm van een emmertjes
geheugen, en een shift register. Dit laatste wordt gecontroleerd door
een pixel clock
• Het licht wordt in lading omgezet. CCD chip bevat aan elkaar
geschakelde capaciteiten. Ladingen worden door deze capaciteiten
naar buiten geklokt. Is er 1 pixel stuk, dan is er geen informatie over
de volledige lijn die erop volgt.
• Een ADC kan niet op de CCD chip, dus steeds een analoge uitgang.
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CMOS: elk beeldelement bevat 1 fotodiode en 3 transistoren: één
voor lading-spanningsconversie, twee voor adressering. Bij CCD is
de ganse cel beschikbaar voor belichting, dus 10x meer gevoelig
dan CMOS. De donkerstroom is 10x hoger bij CMOS.
Elke lichtgevoelige cel is individueel uitleesbaar. Is er 1 pixel stuk,
dan bijft de rest van de lijn intact.
Dynamisch bereik is hoger. Bij CCD is er bij overbelichting een
“overlopen” van ladingen.
CMOS is sneller uit te lezen.
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Image sensors - CCD versus CMOS
Property
Dynamic range
Pixel read out
Manufacturing costs
Integration complexity
On-chip-processing
Power usage
Distortions
Technology
Image technology
CCD
Up to 45 dB
Frame/field wise
High
High
Very difficult
High
Blooming/smearing
Well known
Well known
CCD
CMOS
Up to 140 dB
Random/Area wise
Low
Low
Relatively easy
Low
Fixed pattern noise
Well known
Experimental
CMOS
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Optics
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Choices:
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Magnification
Depth of field
Distance to object
Distance to image sensor
Light intensity
Optical resolution
Telecentric
Artefacts:
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Added size
• Added weight
• Image distortion
Image acquisition: cameras and sensors
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Lens
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Choose a lens with right focus distance
A “telecentric objective” sees only the surface plane of a prismatic
object
Price
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B/W Camera: 500 EUR
Frame Grabber card with software: 300 EUR