Measurements in Fluid Mechanics 058:180 (ME:5180) Time & Location: 2:30P - 3:20P MWF 3315 SC Office Hours: 4:00P – 5:00P MWF 223B-5 HL Instructor: Lichuan Gui Phone: (Lab), (Cell)

2 Lecture 9. Optical experimentation: Recording

3 Light transmission, sensing, and recording Fiber optics 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 Fiber optic cables n 1 – refractive index in core n 2 – refractive index in cladding n 0 – refractive index of surrounding medium n 2 < n 1 n 0 < n 1 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 - shutter - photographic film (sensor) f – focus length D f-number (f/#): Camera for motion pictures

5 Light transmission, sensing, and recording Photodetectors Photomultiplier tubes (PMTs) Photodiodes (PDs) - gas-filled or vacuum tube that is extremely sensitive to light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. - photoeletric effect: incoming photons strike a photocathode, generating electrons, which are attracted to an anode. Photoelectric effect photons electrons - semiconductor diodes - capable of converting light into either current or voltage Quantum efficiency: N p – number of absorbed photons N e – number of emitted electrons - internal amplification - 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 - High resolution (up to 5000×7000 pixels or more) - Low pixel read out rate - Very low frame rate (e.g. <1 Hz) Light transmission, sensing, and recording Full-frame CCD (charge coupled device) camera

8 - 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 Light transmission, sensing, and recording Frame transfer CCD

9 - 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) Light transmission, sensing, and recording Interline transfer CCD

10 Color CCD - Color filter on top of each pixels - Reduced digital resolution 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 Light transmission, sensing, and recording

11 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) Light transmission, sensing, and recording Timing diagrams for PIV recording based on CCDs

12 - 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: Light transmission, sensing, and recording Digital High speed digital imaging system

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

14 Homework - Questions and Problems: 13 on page Read textbook on page Due on 09/14

Determine positions (x,y) and values G(x,y) of the 9 pixels in the 3x3 neighborhood of the maximal gray value with a linear coordinate trans formation Start to write a Matlab program x y 0 i j A(i,j) for i=1,2,3 , M; j=i=1,2,3 , N x = j - N/2 y = M/2 - i G(x,y)