Imaging Techniques for Flow and Motion Measurement
Lecture 5
Imaging & Recording Techniques
Lichuan Gui
University of Mississippi
2011
1
Particle 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 Imaging

Geometric imaging
Thin lens formula
-
Focal length f
-
Image distance z0
-
Object distance Z0
-
Focus criterion
1
z0

1
Z0

1
f
3
Particle Imaging

Imaging of small particles
Circular Aperture Diffraction
-
Airy pattern of a point light source
Image of sub-micron particle
Airy disk diameter
d diff  2 . 44
D
d
 2 . 44 f #  M  1 
( f#  f / d )
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Particle Imaging

Imaging of small particles
Gaussian image profile
I
I
0
dp
r
0
3D-function view
image view
Intensity distribution of particle image:
Ip(x,y) = I0exp(-2r2/dp2)
r2=(x-x0)2+(y-y0)2
x0, y0 — particle position
I0 — brightness at particle center
dp — particle image diameter
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Particle Imaging
Imaging of small particles
Particle image size
-
d 
-
50
Particle image diameter d
Md 
p
2
 d diff
M dp
40
2
Example
- Measurement area: 100100 mm2
- Image area:
- F-number (f#):
- Particle diameter:
- Laser wavelength:
d
99 mm2
8
0~100 m
532 nm
Im age diam eter [ m ]

30
20
10
0
0
1 00
2 00
300
4 00
50 0
P article diam eter ( m )
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Particle Imaging

Imaging of small particles
Depth of field Z
-
Unsharp imaging due to misalignment
-
Acceptable diameter bias =ddiff
-
Estimation of Z
 Z  2 f # d diff  M  1 / M
2
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Particle Image Recording

Imaging of small particles
Gray value (intensity) distribution
- 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
I p (i,j)
I sp (i,j)
I lf (i,j)
I tot(i,j)
(a)
(b)
(c)
(d)
I tot i , j  
I p  I sp  I lf
2
2
2
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Particle 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
9
Particle Image Recording

PIV recording type
Multi frame recordings
-
Velocity determined with particle image displacement between frames
Double/Multi exposure used to increase image number in steady flow
10
Particle Image Recording

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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Particle Image Recording

PIV cameras
Photographic camera
-
High resolution black & white film
Single frame recording
Direct evaluation with Young’s fringes technique
Fully digital evaluation after digitizing the film recording
laser
PC
2D traverse
system
frosted glass
CCD camera
Young’s fringes evaluation system
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Particle Image Recording

PIV cameras
Video standard camera
-
Low cost;
Low digital resolution: 640480 ~ 768576 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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Particle Image Recording

PIV cameras
Full-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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Particle Image Recording

PIV cameras
Frame 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. 1s per row)
-Time gap between frames within
0.5  1 ms
-Low frame rate
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Particle Image Recording

PIV cameras
Interline 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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Particle Image Recording

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
- lower image quality
ultima APX CMOS camera
1024 × 1024-pixel resolution
Pixel size 17 × 17 µm²
10 bit dynamic range
8 GB image memory in camera
2000 fps at full resolution (up to 120,000fps)
Minimal inter-framing time 8333 ns
vs. CCD
PCD2000 CCD camera
2048 × 2048-pixel resolution
Pixel size 7.4 × 7.4 µm²
14 bit dynamic range
4 GB image memory in camera
frame rate of 14.7 fps at full resolution
Inter-framing time for PIV 180 ns
17
Particle Image Recording

PIV cameras
Timing diagrams for PIV recording based on CCDs
Time
Exposure
Read-out
Full-frame CCD
Camera frame rate
- Double/multi exposures
Laser light pulses
- Low and high velocity
Frame transfer CCD
Camera frame rate
- Single exposures
- Low to medium velocity Laser light pulses
Charge transfer period (>1s)
Interline transfer CCD
- Single exposures
- t down to 75 ns
- High velocity
Charge transfer period (<1s)
Camera frame rate
Laser light pulses
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Particle Image Recording

PIV cameras
High speed digital imaging system
- Mega pixel full frame possible at >2000 fps
- 10,000 fps available at standard video resolution (i.e. 640480 pixels)
- High intensity light source required
- Commercially available high-speed imaging systems:
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Homework
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)
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