Transcript Background for optical experimentation: Illumination

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 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
Radiant intensity Ie :
e – radiation power
 – solid angle, units: steradian (sr)
Plane light source - emits energy uniformly from all pints on a plane surface
A – plane source area
Radiance Le:
 – angle of solid angle axis
Spectral radiance Le :
 – wave length
Irradiance Ee :
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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
- 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)
Luminous efficacies of standard human eye
Solid curve: photopic vision
Dashed curve: scotopic vision
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Background for optical experimentation: Illumination
Thermal radiation
Le – spectral radiance
Plank’s radiation law:
h – plank’s constant
KB – Boltzmann’s constant (=1.3804210-23J/K)
for blackbody
Wien’s radiation law:
for infrared-visible-ultraviolet & T<104K
Wien’s displacement law:
Radiation power emitted by blackbody:
 – Stefan-Boltzmann constant (=5.6703310-8 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
- continuous spectrum & spectral lines
- e.g. mercury lamp:
sodium lamp:
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Background for optical experimentation: Illumination
Flash lamps
- single-flash or stroboscopic devices
- tubes containing noble gas,
e.g. xenon, krypton, or argon
- high voltage discharge
- light pulse typically between 1 s – 1 ms
1909 flash-lamp
xenon flashtube
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 1030 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 130 W)
– Properties of a commercial Cu laser
Wavelength:
Average power:
Pulse energy:
Pulse duration:
Peak power:
Pulse frequency:
Beam diameter:
Beam divergence:
510.6 nm and 578.2 nm
50 W
10 mJ
15 ns – 60 ns
<300 kW
5 kHz – 15 kHz
40 mm
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
- backlighting for high-speed imaging
Collimators
- combinations of lenses and mirrors
- cylindrical or slightly diverging light beam
- 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
- for PIV etc.
- created with rotation mirror
- for PIV etc.
Laser wide beam
- created with lens group
- for volume illumination
e.g. MPIV, HPIV
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Background for optical experimentation: Illumination
Light scattering behavior
MIE’s scattering (dp>) for spherical particles
Back scattering
Side scattering
Forward scattering
Light scattering by a 1 m oil particle in air with 532 nm laser
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
- Read textbook 5.3-5.4 on page 107-128
- Questions and Problems: 11 on page 143
(optional, but may add credit to midterm examination )
- Due on 09/12
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