Download ppt - Robot Vision SS 2005 Matthias Rüther 1 ROBOT VISION Lesson 5: Camera Hardware and Technology Matthias Rüther

Robot Vision SS 2005 Matthias Rüther 1

ROBOT VISION Lesson 5: Camera Hardware and Technology

Matthias Rüther


Content

Camera Hardware– Sensors

– Video Data Transfer

– Mechanics

Optics– Lenses

– Macroscopic

– Telecentric

– Microscopic

Illumination– Illumination systems

– Mechanical Arrays


Sensors

Goal: convert light intensity to electrical signal– Mostly visible light spectrum (~700nm to ~400nm)

provides color information, light intensity, like human eye

– Near infrared (~700nm to 5m)Similar properties as visible light, NO heat information; black sky, plants are

white, used for vegetation inspection, remote sensing, to detect reflective markers


Sensors

– Ultraviolet (~400nm to ~240nm)Used with special illumination,

UV microscopy (resolution up to 100nm)

surface inspection (detecting cracks, fluid leaks etc.)

flame inspection (alcohol flames are barely visible to human eye)

Forensics (finger print, blood, etc.)


Sensors

2 Basic Technologies– Charge Coupled Device– CMOS Sensor

Both are pixelated metal oxide semiconducters Accumulate in each pixel signal charge proportional to local

illumination intensity => spatial sampling function Properties

– Responsivity: amount of output signal per unit of input optical energy– Dynamic range: ratio of saturation level to signal threshold– Uniformity: consistency of response– Shuttering: start and stop of exposure– Speed: frame rate / readout time– Windowing: can subwindows of the chip be sampled?– Antiblooming– Biasing / Clocking– Reliability– Cost


Charge Coupled Device


CMOS Sensor


CCD vs CMOS


Line Sensor


Line Sensor


Video Data Transfer

Transfer of image data from Camera to System Memory

Properties:– Transfer distance

– Bandwidth / Framerate

– Analog / Digital

– Environment

– Cost


Analog Video Signal

Video Standards– Composite Video: de facto standard for consumer products, combines

color, brightness and synchronisation data to one „composite“ signal.

– S-Video: Y/C or Component Video; splits video data in two channels: luminance (Y) and chrominance (C). Provides less granularity and sharper image. C = U/V for PAL and C = I/Q for NTSC

– RGB: standard for computer monitors. Four signals (red, green, blue, sync)

– Large distances possible (~20m). Higher frequencies degrade with length (low pass) and noise adds to the signal.


Analog Video Signal

Broadcast Standards (combine technical and legal definitions)

– NTSC: National Television Standards Comitee; North/Central America, Mexico, Canada, Japan.

– PAL: Phase Alteration Line; UK, Western Europe, Middle East, Parts of Africa and South America

– SECAM: Systeme Electronic Pour Coleur Avec Memoire, similar to PAL, chrominance is FM modulated; France, Russia, parts of Africa, Eastern Europe

Format Country ModeSignal Name

Frame Rate [fps]

Vert. Line Resolution

Line Rate [lps]

Img. Size

NTSC US, Japan Mono RS-170 30 525 15.750 640x480

Color NTSC Color 29.97 525 15.734

PAL Europe except France

Mono CCIR 25 405 10.125 768x576

Color PAL Color 25 625 15.625

SECAM France, East.

Europe

Mono 25 819 20.475 N/A

Color 25 625 15.625


Analog Video Signal

Scanning Process


CameraLink

Serial Interface for digital image transfer. Standardized Fast (up to 2.38 Gbps) Not a High Volume Product -> expensive Max 10m cable, no power provided

Physical Layer: Low Voltage Differential Signaling (LVDS); high-speed, low-power general purpose interface standard; known as ANSI/TIA/EIA-644, approved in March 1996.– 350 mV nominal signal swing– Theoretical 1.923 Gbps

Connection Channellink: developed by Natioan Semiconducturs for flat panel displays, – 28bit I/O, serialized 7:1 and transferred– Up to 2.38 Gbps

Cameralink specializes Channellink for video data transfer.


CameraLink


IEEE 1394 (Firewire)

De-facto industrial standard

– Moderate volume product (Industrial cameras, Video Cameras, Webcams)

– Consists of both hardware and software specification– Completely digital--no conversion to analog – Data rates of 100, 200, or 400 Mb per second (800Mbps by 1394b)– Flexible--supports daisy-chain and branching cable configurations– Inexpensive – Max 4.5m cable length– Power provided by bus

– Invented by Apple in mid 90‘s as LAN bus (100Mbps)– Development hampered by license fees in 1998 ($1 per port)– Since 1999 owned by 1394LA ($0.25 per unit)– Firewire remains trademark of apple.


USB 2.0

Upcoming rival for IEEE1394– Fast (480Mbps)

– High volume (available on every PC)

– Plug and Play

– Emerged from USB 1.1 (1995)

– Provides Power

– 5m cable length

– Master-Slave Architecture (IEEE1394: Peer to Peer)

– IEEE1394 is faster (10-70%), due to protocol architecture!


Mechanics

Industrial cameras need to be ruggedized

– Up to 90% humidity– -5 to +50 degrees Celsius– Harder requirements for

outdoor/surveillance cameras

Common Sensor dimensions:– ¼“– 1/3“– ½“– 2/3“– 1“

Mounting usually by ¼“ screws Lens mount standards: C-mount and

CS-mount; 1“ thread; differing by flange focal distance


Optics

… or how to calculate the focal length.

Lenses (or lens systems, a „compound“ lens) are used to project light rays on an image sensor.

If all rays originating from a distinct point of light intersect in one point on the image plane, a sharp image of this point is acquired.


Lens Parameters

Magnification = size of image / size of object– E.g. size of object = 5cm; size of image =

5mm -> magnification = 0.1

– Depends on working distance (lens – object distance) -> impractical for standard lenses

Focal length = working distance * size of image / (size of object + size of image)– E.g. to capture a 1000m wide object from

500m on a CCD chip measuring 4.8x6.4mm, you need 3.2mm of focal length


Lens Iris

The Iris limits the amount of light getting through the lens.

-> the image appears darker (avoids overexposure in bright scenes)

-> less lens area is used -> fewer lens errors are incorporated

-> sharpness is increased

Sharpness: theoretically impossible to focus 3D object, but:

– Blurred points of some size appear sharp to human eye (e.g. on 35mm film, 1/30mm spots appear sharp)

– -> „Depth of field“– In practice: max. blurred spot is 1

pixel


Lens Iris

Depth of field limits:– Wd = working distance– Bs = size of blur spot– I = amount of iris aperture– F = focal length

2**1ffwd

Ibs

wdDOF

e.g.: a 10mm wide object is imaged on a 1/3“ Megapixel CCD from a distance of 100mm, the blurred spot size is max. 5μm

-> best f is 26.5mm, choose 25mm standard lens

-> DOF=0.08mm at full aperture

-> DOF= 0.24mm at aperture = 4


Lens types

Standard lenses: focal length from 5mm to 75mm– Adjustable/fixed focus

– Adjustable/fixed Iris

– Adjustable/fixed zoom (focal length)

Macro lenses– Near field imaging (wd ~75mm-90mm, dof ±0.06mm… ±5mm,

magnification 0.14…8)

Telecentric lenses– Parallel projection, moving object towards lens does not change the

image


Lighting

Illumination is the most critical part in a machine vision system.

Small illumination changes may severely affect performance of vision algorithms.

If possible, adjust lighting conditions and keep them fixed!

Properties:– Intensity

– Spectrum

– Frequency (amplitude change: flicker, strobe)

– Direction

Hazards:– Object: reflection, specularity, color, stray light, transparency, motion

– Lamp: heat, flicker, stability, lifetime, size, power, speed


Regulated Halogen Lamp Systems

Illumination by Quartz-Halogen lamps

High power output

Power control by Voltage regulation and adjustable shutter

Fiber optic light guidance to avoid heating

High power consumption (150W lamp)

Heavy DC power source necessary to avoid flicker

Lamp life: 200-10000hrs


Light Emitting Diodes

Possible to generate all primary colors

Bright White LED‘s possible (up to 5W per piece) -> Cooling

Life time: 100000+ hrs

Low power consumption -> Small DC current source

Small/light housing

Fast strobe (time limited by driver circuit, down to 1μs pulses)

Packed in LED arrays


Types of Illumination

Directional

Glancing

Diffuse



Ring Light

Diffuse Axial

Brightfield/Backlight



Darkfield

Structured Light (Line Generators)