Lecture09 - Lcgui.net
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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 β 5:00P MWF 223B-5 HL
Instructor: Lichuan Gui
[email protected]
http://lcgui.net
Lecture 9. Optical experimentation: Recording
2
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 β refractive index of surrounding medium
n1 β refractive index in core
n2 β refractive index in cladding
n0 < n1
n2 < n1
Total internal reflection required at interface between core and cladding
π2 β₯ ππ = arcsin π2 /π1
Numerical aperture of the fiber
π0 β€ ππππ₯ = arcsin
NA = π0 sin πmax =
π1 2 β π2 2
π0
π1 2 β π2 2
3
Light transmission, sensing, and recording
Conventional photography & cinematography
Still camera
- lens
f β focus length
- aperture
f-number (f/#): π =
D
π
π·
- shutter
- photographic film (sensor)
Camera for motion pictures
4
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 β number of absorbed photons
Ne β 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)
5
Light transmission, sensing, and recording
Video standard camera
-
-
Low cost;
Low digital resolution: 640ο΄480 ~ 768ο΄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
6
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)
7
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. 1οs per row)
Time gap between frames within 0.5 οΎ 1 ms
Low frame rate
8
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)
9
Light transmission, sensing, and recording
Color CCD
- Color filter on top of each pixels
- Reduced digital resolution
CMOS (Complementary metalβoxideβsemiconductor) 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
10
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 (>1οs)
Frame transfer CCD
- Single exposures
- Low to medium velocity
Interline transfer CCD
- Single exposures
- οt down to 75 ns
- High velocity
Camera frame rate
Laser light pulses
Charge transfer period (<1οs)
Camera frame rate
Laser light pulses
11
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. 640ο΄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# β f-number
M β magnification factor
ο¬ β wave length
- Particle image diameter
dp β particle diameter
- Depth of field
13
Homework
- Read textbook 5.5-5.6 on page 128-141
- Questions and Problems: 13 on page 143
- Due on 09/14
14