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HelicopterOff Topics › Need help from optics experts
05-01-2004 06:03 AM  13 years agoPost 1
helicopter34

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New Jersey, exit 82

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I'm working on an undergraduate experiment for one of my proffessors, he wants me to recreate an experiment he read about, working with a suspension of particles in a solution that hopefully align when in a magnetic field and will polarize light. He's given me freedom to design the experiment myself, but I am far from an expert in optics, so I figured I ask for a little free consulting from the RR experts.

I need some kind of light source that has relatively constant luminosity.
I was thinking about a laser (maybe even a laser pointer) but was unsure how constant its luminosity. Also I need some kind of sensor to use to detect the luminosity, like a photoresistor, but hopefully something relatively linear.

What I need to do is device some type of simple test to tell whether these particles are indeed aligned (method will be used in further experiments to) when an magnetic field is applied. I plan on putting a polarized film in the path of the light leaving the suspension. If the light was polarized by suspension, then when it hits the film which would be 90 degrees out of phase, it should cut out a percent of the luminoscity.

Thanks in advance guys.

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05-01-2004 06:13 AM  13 years agoPost 2
Jagboy69

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Miami

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Sorry dude.... I am still mystified that I cant read the gas pump LCD thru my pair of 265dollar sunglasses...

Jason /// Sceadu50/9chp WWW.Jagboy69.com

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05-01-2004 06:49 AM  13 years agoPost 3
Texas1

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Tyler, Texas

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When light passes through two indentical polarized filter at ninty degrees to one another no light passes. Your experiment should be so easy. If you use a laser which is polarized by nature, then all you would have to do is rotate the laser and measure any intensity change of the beam due to a rotation angle through the solution. If the particals in the solution are polarized as you said, there will be a reduction in the amount of light getting through. A cheap laser pointer should work. . Off hand I could not give you a device number, you could use a phototransistor, photodiode or photoconductive cell. Do a google search for data on any one of the above devices. Which ever one you go with it should come with a data sheet that gives the current output to light intensity. I know I haven't been much help. Its been to long since I completed my physics degree.

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05-01-2004 08:58 AM  13 years agoPost 4
Sar

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Saugeties, NY

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Avoid laser pointers, buy a cheap polarized he-ne(helium neon) from e-bay. Avoid random polarization he-ne's. Laser pointers are kind of hit and miss and the beam quality is pretty sad. For the price a he-ne is a better deal with and the beam quality is usually great and works better with optics. It's pretty bad when you have to spend over $100 in optics just to make your laser beam round

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Jon

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05-02-2004 05:00 AM  13 years agoPost 5
Texas1

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Tyler, Texas

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You are right that laser pointers are hit and miss. Most do focus about a foot from the diode lens. Could you not use the pointer within this limit? It would be a cheaper way to go. You will get some refraction and diffision as the beam passes through the liquid any way.

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05-02-2004 06:30 AM  13 years agoPost 6
Sar

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Saugeties, NY

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If you decide to go with a diode laser, you can get one with known specs at Midwest Laser Products for pretty cheap. They sell modules which should provide what you need. Polarization ratio of 100:1. Polarized He-Ne's commonly found cheap are 500:1 polarization. Selection depends on what you need I guess. This would eliminate the need to add an additional polarizing filter after the sample cell. If you'd rather use the filter behind the sample cell, skip the laser and just use a highly collimated light source with a color filter that corrosponds to the peak spectral range of your photosensor/photodetector.

http://www.midwest-laser.com/html/l...de_modules.html

They also have optics:
http://www.midwest-laser.com/html/optics.html

Sensor:
Consider a photocell, and check it's specs/datasheet before setting it up. Photocells can be linear if the supporting electronics are set up properly with respect to the load and supply voltage.

JagBoy: http://travel.howstuffworks.com/sunglass4.htm If you read this, keep in mind that LCD displays have polarizing filters on them as well.

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Jon

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05-02-2004 07:22 AM  13 years agoPost 7
helicopter34

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New Jersey, exit 82

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inexperienced

Im only in my sophmore year of ME, so I am really inexperienced in optics. 500:1 polarization, I'm guessing thats the ratio of polarized light emitted to non polarized? Does a polarizing filter produce 100% polarized light.

If I used a diode laser with a polarizing filter in front, would that be good choice. Would it be constant output, or would it flucuate with factors such as heat or time?

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05-02-2004 08:58 AM  13 years agoPost 8
Sar

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Saugeties, NY

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A laser diodes primary polarization is parallel to the junction of the semiconductor. They also produce random polarization and/or polarization perpendicular to the junction. This is the smaller number on a polarization ratio.

Polarizing filters are not 100% efficient at polarizing light, their efficiency depends on the quality of the filter as well. Keep in mind that the first pass of unpolarized light(non laser, or randomly polarized laser)through a polarizing filter will attenuate the amount of light that gets through by quite a bit. You're not changing the polarization of the light, you are throwing out the components of the light which you do not want. If you get a LD with at least a 100:1 polarization ratio you shouldn't need an additional polarization filter in front of it. You will need to rotate the LD to position the polarization how you want(horizontal or vertical unless your solution polarizes at 45 degrees, etc.)

Laser diodes change their output power and wavelength according to temperature and supply voltage. Since photodetectors of various types all have sensitivity to wavelength which falls off to either side of the optimum wavelength, changes in wavelength will alter the response of the device.

In the case of these small diode modules with built in supplies, your best bet is to start the laser and allow it time to warm up and stabilize before performing your experiment. This warmup wavelength shift is common with many types of lasers. In high precision applications they use microcontrollers, temperature sensors, and thermoelectric coolers to stabilize the temperature of the diode down to 0.05 Deg C. I don't think you need this kind of precision for your application

Here's another link for optics and polarizing filters, not cheap, but good quality product.

http://www.edmundoptics.com/US/
http://www.edmundoptics.com/onlinec...?productid=1558

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Jon

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05-02-2004 02:06 PM  13 years agoPost 9
Sealerman

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Long Island, New York.

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Hey Jag,
I am still mystified that I cant read the gas pump LCD thru my pair of 265dollar sunglasses...
It must be the mist from the tears you cry knowing that money could have been spent on heli stuff. Oh try turning your head sideways.
Sealerman

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