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Instructor: Lichuan Gui lichuan-gui@uiowa lcgui

Measurements in Fluid Mechanics 058:180:001 (ME:5180:0001) Time & Location: 2:30P - 3:20P MWF 218 MLH Office Hours: 4:00P – 5:00P MWF 223B-5 HL. Instructor: Lichuan Gui lichuan-gui@uiowa.edu http:// lcgui.net. Lecture 9. Optical experimentation: Recording.

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Instructor: Lichuan Gui lichuan-gui@uiowa lcgui

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  1. Measurements in Fluid Mechanics058:180:001 (ME:5180:0001)Time & Location: 2:30P - 3:20P MWF 218 MLHOffice Hours: 4:00P – 5:00P MWF 223B-5 HL Instructor: Lichuan Gui lichuan-gui@uiowa.edu http://lcgui.net

  2. Lecture 9. Optical experimentation: Recording

  3. Light transmission, sensing, and recording Fiber optics Fiber optic cables Core - Thin glass center of the fiber where the light travels Cladding - Outer optical material surrounding the core that reflects the light back into the core Buffer coating - Plastic coating that protects the fiber from damage and moisture n0– refractive index of surrounding medium n1– refractive index in core n2– refractive index in cladding n0<n1 n2<n1 Total internal reflection required at interface between core and cladding Numerical aperture of the fiber

  4. Light transmission, sensing, and recording Conventional photography & cinematography Still camera - lens - aperture D f – focus length f-number (f/#): - shutter - photographic film (sensor) Camera for motion pictures

  5. Light transmission, sensing, and recording Photodetectors Photomultiplier tubes (PMTs) - gas-filled or vacuum tube that is extremely sensitiveto light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. electrons photons - photoeletric effect: incoming photons strike a photocathode, generating electrons, which are attracted to an anode. - internal amplification Np – number of absorbed photons Photoelectric effect Quantum efficiency: Ne – number of emitted electrons Photodiodes (PDs) - semiconductor diodes - capable of converting light into either current or voltage - external amplification required - smaller size, less expensive but lower signal-to-noise ratio (vs. PMTs)

  6. Light transmission, sensing, and recording 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

  7. Light transmission, sensing, and recording 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)

  8. Light transmission, sensing, and recording 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

  9. Light transmission, sensing, and recording 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)

  10. Light transmission, sensing, and recording Color CCD • Color filter on top of each pixels • Reduced digital resolution CMOS (Complementary metal–oxide–semiconductor) sensors • higher image capture speed • lower price vs. CCD • lower image quality 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 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

  11. Light transmission, sensing, and recording Time Exposure Read-out Camera frame rate Laser light pulses Charge transfer period (>1s) Camera frame rate Laser light pulses Charge transfer period (<1s) Camera frame rate Laser light pulses Timing diagrams for PIV recording based on CCDs 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

  12. Light transmission, sensing, and recording Digital 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:

  13. Light transmission, sensing, and recording Small particle imaging Circular aperture diffraction - Airy pattern of a point light source - Image of sub-micron particle - Airy disk diameter f#–f-number M – magnification factor - Particle image diameter – wave length dp– particle diameter - Depth of field

  14. Homework - Read textbook 5.5-5.6 on page 128-141 • Questions and Problems: 13 on page 143 - Due on 09/14

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