1 Imaging Techniques for Flow and Motion Measurement Lecture 5 Lichuan Gui University of Mississippi 2011 Imaging & Recording Techniques

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Imaging Techniques for Flow and Motion Measurement

Lecture 5

Lichuan Gui

University of Mississippi

2011

Imaging & Recording TechniquesImaging & Recording Techniques

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Particle ImagingParticle Imaging Geometric imaging

2D standard PIV imaging system- Light sheet coordinate (X,Y,Z)- Image coordinate (x,y,z) - Constant magnification factor M=z0/Z0

- Relation between particle and image position: X=x/M, Y=y/M

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Particle ImagingParticle Imaging Geometric imaging

Thin lens formula

- Focal length f

- Image distance z0

- Object distance Z0

- Focus criterion

fZz

111

00

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Particle ImagingParticle Imaging Imaging of small particles

Circular Aperture Diffraction - Airy pattern of a point light source - Image of sub-micron particle- Airy disk diameter

)/(

144.2

44.2

#

#

dff

Mfd

Dddiff

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Particle ImagingParticle Imaging

I0

dp

0r

I

Intensity distribution of particle image: Ip(x,y) = I0exp(-2r2/dp

2) r2=(x-x0)2+(y-y0)2

x0, y0 — particle position I0 — brightness at particle center dp — particle image diameter

3D-function view image view

Imaging of small particlesGaussian image profile

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Particle image size

- Particle image diameter d

- Example

- Measurement area: 100100 mm2

- Image area: 99 mm2

- F-number (f#): 8- Particle diameter: 0~100 m- Laser wavelength: 532 nm

22diffp dMdd

Particle diameter (m)

Imag

edi

amet

er[

m]

0 100 200 300 400 5000

10

20

30

40

50

d

Mdp

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Depth of field Z

- Unsharp imaging due to misalignment

- Acceptable diameter bias =ddiff

- Estimation of Z

2# /12 MMdf diffZ

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Ip(i,j)

( a )

Isp(i,j)

( b )

Ilf(i,j)

( c )

Itot(i,j)

( d )

- Particle image with Gaussian profile (Ip)

- Single pixel random noise, e.g. thermal noise (Isp)

- Low frequency background noise (Ilf): non-uniform illumination, flow boundary etc.

- Total intensity distribution (Itot): root-sum-square (RSS) of Ip, Isp and Ilf

Particle Image RecordingParticle Image Recording Imaging of small particles

Gray value (intensity) distribution

222, lfspptot IIIjiI

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Particle Image RecordingParticle Image Recording PIV recording type

Single frame recordings- Single exposure

- Long exposure time- Velocity determined by trajectory- Direction ambiguity - Low particle number density required

- Double exposure- Short exposure time- Velocity determined by displacement - Direction ambiguity- Methods to avoid direction ambiguity: a. color/intensity tagging b. Image shifting techniques

- Multi-exposure- Short exposure time- Velocity determined by displacement- Direction ambiguity- Used to increase particle image number- Limited in steady flow

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PIV recording typeMulti frame recordings- Velocity determined with particle image displacement between frames - Double/Multi exposure used to increase image number in steady flow

Particle Image RecordingParticle Image Recording

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PIV recording modes

Low image density (LID) mode- single particle can be identified

- particle tracking possible

- low information density

Laser speckle (LS) mode - single particle can not be identified

- high information density

High image density (HID) mode- single particle can be identified

- particle tracking impossible

- high information density


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PIV camerasPhotographic camera- High resolution black & white film - Single frame recording- Direct evaluation with Young’s fringes technique - Fully digital evaluation after digitizing the film recording

Young’s fringes evaluation system

laser

PC

2D traversesystem

CCD camera frosted glass


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PIV camerasVideo standard camera- Low cost;- Low digital resolution: 640480 ~ 768576 pixels; - Frame rate: 25 Hz (PAL) or 30 Hz (NTSC);- 2 interlaced fields per frame with time interval 1/50s or 1/60s;- Frame separation necessary before evaluation


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PIV camerasFull-frame CCD (charge coupled device) camera- High resolution (up to 5000×7000 pixels or more) - Low pixel read out rate- Very low frame rate (e.g. <1 Hz)


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PIV camerasFrame transfer CCD

-Mega pixel full frame CCD

-Two halves of CCD array for imaging and storage, respectively

-Rows shifted down at high rates (e.g. 1s per row)

-Time gap between frames within 0.5 1 ms

-Low frame rate


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PIV camerasInterline transfer CCD- Mega pixel full frame CCD- One masked storage area for each pixel - Charge shift from light sensitive area to storage area at high very high rates - Time gap between frames as low as 200 ns- Low frame rate (e.g. 15 & 30 fps)


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PIV cameras


Color CCD

- Color filter on top of each pixels - Reduced digital resolution- Not suitable for PIV application

CMOS (Complementary metal–oxide–semiconductor) sensors- higher image capture speed - lower price vs. CCD- lower image quality

ultima APX CMOS camera1024 × 1024-pixel resolution Pixel size 17 × 17 µm²10 bit dynamic range8 GB image memory in camera 2000 fps at full resolution (up to 120,000fps)Minimal inter-framing time 8333 ns

PCD2000 CCD camera2048 × 2048-pixel resolution Pixel size 7.4 × 7.4 µm²14 bit dynamic range4 GB image memory in camera frame rate of 14.7 fps at full resolutionInter-framing time for PIV 180 ns

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PIV cameras

Full-frame CCD

- Double/multi exposures

- Low and high velocity

Frame transfer CCD

- Single exposures

- Low to medium velocity

Interline transfer CCD

- Single exposures

- t down to 75 ns

- High velocity

Time Exposure Read-out

Camera frame rate

Laser light pulses

Camera frame rate

Laser light pulses

Charge transfer period (>1s)

Camera frame rate

Laser light pulses

Charge transfer period (<1s)

Timing diagrams for PIV recording based on CCDs


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PIV camerasHigh speed digital imaging system- Mega pixel full frame possible at >2000 fps

- 10,000 fps available at standard video resolution (i.e. 640480 pixels)

- High intensity light source required

- Commercially available high-speed imaging systems:


2020

1. Read EDPIV help manual pages:“Particle image simulation” and “Image noise simulation”

2. Create synthetic PIV images of LID, LS and HID modes with EDPIV simulation tools. Random background noise is suggested with intensity of 10 and mean value of 80. Following menu choices and buttons may be used.

In start window: menu choice “ File \ New image” and “Processing” button;

In “Image processing” window: menu “Tools \ Simulation settings \ Particle”menu “Tools \ Simulation settings \ Noise” menu “Tools \ Single frame”

(EDPIV software and help manual are available at http://www.edpiv.com)

HomeworkHomework

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1 Imaging Techniques for Flow and Motion Measurement Lecture 5 Lichuan Gui University of Mississippi 2011 Imaging & Recording Techniques