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

2. 2 Particle Imaging Geometric imaging 2D standard PIV imaging system -Light sheet coordinate (X,Y,Z) -Image coordinate (x,y,z) -Constant magnification factor M=z 0 /Z 0 -Relation between particle and image position: X=x/M, Y=y/M

3. 3 Particle Imaging Geometric imaging Thin lens formula -Focal length f -Image distance z 0 -Object distance Z 0 -Focus criterion

4. 4 Particle Imaging Imaging of small particles Circular Aperture Diffraction -Airy pattern of a point light source -Image of sub-micron particle -Airy disk diameter

5. 5 Particle Imaging I0I0 dpdp 0 r I Intensity distribution of particle image: I p (x,y) = I 0  exp(-2r 2 /d p 2 ) r 2 =(x-x 0 ) 2 +(y-y 0 ) 2 x 0, y 0 — particle position I 0 — brightness at particle center d p — particle image diameter 3D-function viewimage view Imaging of small particles Gaussian image profile

6. 6 Particle Imaging Imaging of small particles Particle image size -Particle image diameter d  -Example - Measurement area: 100  100 mm 2 - Image area: 9  9 mm 2 - F-number (f#): 8 - Particle diameter: 0~100  m - Laser wavelength: 532 nm

7. 7 Particle Imaging Imaging of small particles Depth of field  Z -Unsharp imaging due to misalignment -Acceptable diameter bias =d diff -Estimation of  Z

8. 8 - Particle image with Gaussian profile (I p ) - Single pixel random noise, e.g. thermal noise (I sp ) - Low frequency background noise (I lf ): non-uniform illumination, flow boundary etc. - Total intensity distribution (I tot ): root-sum-square (RSS) of I p, I sp and I lf Particle Image Recording Imaging of small particles Gray value (intensity) distribution

9. 9 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

10. 10 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 Particle Image Recording

11. 11 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 Particle Image Recording

12. 12 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 Young’s fringes evaluation system laser PC 2D traverse system CCD camera frosted glass Particle Image Recording

13. 13 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 Particle Image Recording

14. 14 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) Particle Image Recording

15. 15 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 Particle Image Recording

16. 16 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) Particle Image Recording

17. 17 PIV cameras Particle Image Recording 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 pricevs. CCD - 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 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

18. 18 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 TimeExposureRead-out Camera frame rate Laser light pulses Camera frame rate Laser light pulses Charge transfer period (>1  s) Camera frame rate Laser light pulses Charge transfer period (<1  s) Timing diagrams for PIV recording based on CCDs Particle Image Recording

19. 19 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: Particle Image Recording

20. 20 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) Homework