Measurements in Fluid Mechanics058:180:001 (ME:5180:0001)
Time & Location: 2:30P - 3:20P MWF 218 MLH
Office Hours: 4:00P – 5:00P MWF 223B-5 HL
Instructor: Lichuan [email protected]
http://lcgui.net
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Lecture 8. Optical experimentation: Illumination
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Background for optical experimentation: Illumination
Point light source
- idealized source of electromagnetic radiation
- concentrated at a point in space
- radiates uniformly in all directions
Plane light source - emits energy uniformly from all pints on a plane surface
Radiance Le: A – plane source area
– angle of solid angle axis
Spectral radiance Le : – wave length
Irradiance Ee :
Radiant intensity Ie : – solid angle,
e – radiation power
units: steradian (sr)
Background for optical experimentation: Illumination
Luminous power (flux) v
- power of visible radiation sensed by standard human eye measured by lumens (lm)
Luminous intensity Iv : unit: candela (cd , 1 cd=1 lm/sr)
Luminance (brightness) Lv :
Spectral luminance Lv :
Illuminance Ev :
Human eye
- 3 membranes: cornea-sclera, choroid, and retina
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- a lens images received radiation onto retina
- 7 million cones on retina respond to bright light and are sensitive to colors. (photopic or bright-adapted vision)
- 100 million rods on retina are sensitive to dim light but cannot separate different colors. (scotopic or dark-adapted vision)
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Background for optical experimentation: Illumination
Luminous efficacy - ratio of luminous power to radiant power v / e (lm/W)
Solid curve: photopic vision Dashed curve: scotopic vision
Luminous efficacies of standard human eye
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Background for optical experimentation: Illumination
Thermal radiation
Wien’s radiation law:
for infrared-visible-ultraviolet & T<104K
Wien’s displacement law:
Plank’s radiation law: Le – spectral radianceh – plank’s constantKB – Boltzmann’s constant (=1.3804210-23J/K)
for blackbody
Radiation power emitted by blackbody:
– Stefan-Boltzmann constant (=5.6703310-8 W/m2K)
Radiation power emitted by other than blackbody:
– total emissivity, e.g. 0.02-0.03 for shiny metallic surface, 0.95 for black flat surface
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Background for optical experimentation: Illumination
Thermal light sources - emit electromagnetic radiation when heated to high temperature
- available in visible, ultraviolet and infrared ranges- line source: one or more narrow spectral bands continuum source: wideband radiation
Incandescent lamps- contain electrically heated tungsten filament in evacuated container - smooth continuous spectrum across visible range
- peak at =900 nm with T=2854 K
- filled with halogen for longer life and higher T
Electric discharge lamps- filled with mercury vapor at low pressure
- produce ultraviolet range light by electric discharge- convert to visible range through fluorescence
- e.g. mercury lamp:
sodium lamp:
- continuous spectrum & spectral lines
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Flash lamps
Background for optical experimentation: Illumination
- single-flash or stroboscopic devices
- light pulse typically between 1 s – 1 ms xenon flashtube1909 flash-lamp
- tubes containing noble gas, e.g. xenon, krypton, or argon
- high voltage discharge
Lasers - Light amplification by stimulated emission of radiation
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Background for optical experimentation: Illumination
Helium-Neon lasers– Continuous wave laser
– Extremely monochromatic with wave length of =632.8 nm
– High temporal coherence (typical coherence length of 1030 cm)
– Spatially coherent
– Unidirectional, parallel to the body of the laser
– Beam of Gaussian intensity distribution
– Low cost but not very powerful
– Used for flow visualization
– Traditionally used for evaluation of PIV images
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Background for optical experimentation: Illumination
Argon-ion lasers– Gas laser
– Continuous wave
– Multiple wavelengths with very narrow bandwidths
– two dominant wavelengths, 514nm and 488nm, make up about 67% of the total beam output power
– Single line operation possible by inserting prisms, diffraction gratings and other optical devices to "filter out" the unwanted wavelengths
– Powerful enough to illuminate particles in PIV tests
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Background for optical experimentation: Illumination
Copper-vapor lasers (Cu lasers)
– High pulse speed, can be considered either CW or individual pulses for PIV particle illumination
– Wavelength within the yellow and green spectrum
– High average power (Typically 130 W)
– Properties of a commercial Cu laser
Wavelength: 510.6 nm and 578.2 nmAverage power: 50 WPulse energy: 10 mJPulse duration: 15 ns – 60 nsPeak power: <300 kWPulse frequency: 5 kHz – 15 kHzBeam diameter: 40 mmBeam divergence: 0.6·10-3 rad
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Background for optical experimentation: Illumination
Nd:YAG laser
– Most popular solid-state laser for PIV
– Available wavelengths: 1064, 532, 355, 266 nm etc.
– Short laser pulses (~5 ns)
– Slow repeat rate (10-15 Hz)
– Operated in triggered mode with quality switch (Q-switch)
– Dual-cavity configuration enables short time interval between laser pulses
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Background for optical experimentation: Illumination
Illumination with white light
Front & back lighting
- view direction perpendicular to seeded flow
- front & back lighting inclined by 120
Collimators- combinations of lenses and mirrors
- cylindrical or slightly diverging light beam
- backlighting for high-speed imaging
- sheet of white light
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Background for optical experimentation: Illumination
Illumination with lasers
- Laser beam diameter 1mm
Laser light sheet
- created with cylindrical lenses
Laser wide beam
- created with lens group
- for volume illumination e.g. MPIV, HPIV
- for PIV etc.
- created with rotation mirror
- for PIV etc.
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Background for optical experimentation: Illumination
Light scattering behavior
MIE’s scattering (dp>) for spherical particles
Light scattering by a 1 m oil particle in air with 532 nm laser
Back scattering Forward scatteringSide scattering
Factors influencing the scattered light power - Light source power - Ratio of refractive index of particles to that of surrounding medium- Particle size- Particle shape and orientation- Polarization and observation angle- Others
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Background for optical experimentation: Illumination
Light scattering behavior
MIE’s scattering (dp>) for spherical particles
1 m glass particle in water
10 m glass particle in water
30 m glass particle in water
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Background for optical experimentation: Illumination
Light scattering behavior
Rayleigh scattering (dp</10) for spherical particles
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Homework
- Questions and Problems: 11 on page 143
- Read textbook 5.3-5.4 on page 107-128
- Due on 09/12
(optional, but may add credit to midterm examination )