1. 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 lichuan-gui@uiowa.edu http://lcgui.net

2. 2 Lecture 8. Optical experimentation: Illumination

3. 3 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 L e : A – plane source area – angle of solid angle axis Spectral radiance L e : – wave length Irradiance E e : Radiant intensity I e : – solid angle, e – radiation power units: steradian (sr)

4. Background for optical experimentation: Illumination Luminous power (flux) v - power of visible radiation sensed by standard human eye measured by lumens (lm) Luminous intensity I v : unit: candela (cd, 1 cd=1 lm/sr) Luminance (brightness) L v : Spectral luminance L v : Illuminance E v : Human eye - 3 membranes: cornea-sclera, choroid, and retina 4 - 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)

5. 5 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

6. 6 Background for optical experimentation: Illumination Thermal radiation Wiens radiation law: for infrared-visible-ultraviolet & T<10 4 K Wiens displacement law: Planks radiation law: L e – spectral radiance h – planks constant K B – Boltzmanns constant (=1.38042 10 -23 J/K) for blackbody Radiation power emitted by blackbody: – Stefan-Boltzmann constant (=5.67033 10 -8 W/m 2 K) 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

7. 7 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

8. 8 Flash lamps Background for optical experimentation: Illumination - single-flash or stroboscopic devices - light pulse typically between 1 s – 1 ms xenon flashtube 1909 flash-lamp - tubes containing noble gas, e.g. xenon, krypton, or argon - high voltage discharge Lasers - Light amplification by stimulated emission of radiation

9. 9 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

10. 10 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

11. 11 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:510.6 nm and 578.2 nm Average power:50 W Pulse energy:10 mJ Pulse duration:15 ns – 60 ns Peak power:<300 kW Pulse frequency:5 kHz – 15 kHz Beam diameter:40 mm Beam divergence:0.6·10 -3 rad

12. 12 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

13. 13 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

14. 14 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.

15. 15 Background for optical experimentation: Illumination Light scattering behavior MIEs scattering (d p > ) for spherical particles Light scattering by a 1 m oil particle in air with 532 nm laser Back scatteringForward 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

16. 16 Background for optical experimentation: Illumination Light scattering behavior MIEs scattering (d p > ) for spherical particles 1 m glass particle in water 10 m glass particle in water 30 m glass particle in water

17. 17 Background for optical experimentation: Illumination Light scattering behavior Rayleigh scattering (d p < /10) for spherical particles

18. 18 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 )