Transcript Lecture09 - Lcgui.net

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
[email protected]
http://lcgui.net
Lecture 9. Optical experimentation: Recording
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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
𝜑2 ≥ 𝜑𝑐 = arcsin 𝑛2 /𝑛1
Numerical aperture of the fiber
𝜑0 ≤ 𝜑𝑚𝑎𝑥 = arcsin
NA = 𝑛0 sin 𝜑max =
𝑛1 2 − 𝑛2 2
𝑛0
𝑛1 2 − 𝑛2 2
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Light transmission, sensing, and recording
Conventional photography & cinematography
Still camera
- lens
f – focus length
- aperture
f-number (f/#): 𝑁 =
D
𝑓
𝐷
- shutter
- photographic film (sensor)
Camera for motion pictures
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Light transmission, sensing, and recording
Photodetectors
photons
electrons
Photomultiplier tubes (PMTs)
- 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
- internal amplification
Quantum efficiency:
Np – number of absorbed photons
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)
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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
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Light transmission, sensing, and recording
Full-frame CCD (charge coupled device) camera
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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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Light transmission, sensing, and recording
Frame transfer CCD
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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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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)
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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
- 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
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Light transmission, sensing, and recording
Timing diagrams for PIV recording based on CCDs
Full-frame CCD
- Double/multi exposures
- Low and high velocity
Time
Exposure
Read-out
Camera frame rate
Laser light pulses
Charge transfer period (>1s)
Frame transfer CCD
- Single exposures
- Low to medium velocity
Interline transfer CCD
- Single exposures
- t down to 75 ns
- High velocity
Camera frame rate
Laser light pulses
Charge transfer period (<1s)
Camera frame rate
Laser light pulses
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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:
12
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
 – wave length
- Particle image diameter
dp – particle diameter
- Depth of field
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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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