How do i get a uniform intensity field???

[TaurusTheTenacious]

Hello All,
I am an intern at a company looking to do schlieren photography and putting me in charge of developing a benchtop system. I've been at it for 4 weeks, and I think I've made some progress, but one problem I keep getting yelled at for is that the field isnt uniform.
The setup I was given is a laser with a small lens with a short focal length. This was all sent to a larger pair of lenses where the beam passes through the sonic field we're trying to study. The beam that comes out of the first lens is more bright in the middle than on the edges. Is this what they mean when they say "gaussian beam" ?
I've been presented with 2 solutions to the same problem, the one I've been going with since it was presented in my schlieren book is the "slit". I was first told to try to implement an iris to behave like a slit, but the result was actually worse. I've been trying to put a pinhole in thin objects like copper films and peices of floppy disk, but I have gotten similar results.
I believe the slit approach is correct, otherwise it wouldnt be in the book, but I dont have the right gear to be trying to make something good. I was told taht the slit has to be really thin or there would be a diffraction pattern instead of a uniform field.
What does the slit do to make the field uniform?
Is there a better way to get a uniform field?

 

[IRstuff]

"more bright in the middle than on the edges" could be a description of a gaussian beam, or just that it's non-uniform. A gaussian beam has a specific mathematical relationship of intensity vs. position.

I don't see that a slit is going to improve your beam uniformity.

But, you need to determine how uniform you need. 1%?? 0.1%??

One standard approach to produce a uniform field is an integrating sphere, but the intensity will be uniformly lower.



TTFN

 

[TaurusTheTenacious]

I believe it is a gaussian beam, it's driving me nuts trying to figure out what they're trying to accomplish by having the slit there, one detail that I forgot to mention though, the slit is at the focal length of the lens.

I do not have a quantified parameter for the degree of uniformity, all I know is that what's coming out of the laser isnt uniform enough.

I can tell you that the 2d images will undergo 3 dimensional image reconstruction, I'm assuming, based on that information that we can get away with a uniformity of 5-1%, but I'd say, as long as it's cheap, the higher the better.

how much intensity are we giving up with an integrating sphere?

 

[IRstuff]

Depends on how big a beam you need and how much loss your'e willing to accept. See:

http://www.labsphere.com/data/userFiles/A Guide to Integrating Sphere Theory&Apps.pdf

TTFN

 

[cev]

It sounds like you are using a laser diode, so your beam is probably something like gaussian. If you can confine your field of view into a small enough portion of the laser beam, you will get good uniformity. This comes at the expense of losing a lot of light. Along similar lines, you can expand your beam using a shaped diffuser (as in

http://www.poc.com/),

but you won't gain a whole lot. The latter approach is best if your laser beam is of poor quality.

A much more efficient approach is to get a diffractive optic to convert your gaussian beam into a uniform beam (

http://www.memsoptical.com/prodserv/products/bshapers.htm).

This will be overkill for your application.

Curtis

 

[JJSX]

put a pinhole at the first lens back focal length position and use another lens with the pinhole at its front focal length point, I think it will work if i got your question.

 

[TaurusTheTenacious]

I'd like to thank everyone for their input. I found out why the slit works. It turns out that its operation is based on fraunhofer diffraction.

As the pinhole approaches the beam spot size at the focal point, it experiences a broadening that makes the feild uniform while still letting plenty of power through.

Bioengineer, that solution takes out the dispersion too, is that right?
Right now I have just a laser going into a lens into the pinhole and out to the rest of the system.

 

[JJSX]

laser beam is a Gaussian beam. At its waist postion (inside the laser cavity), the optical wave of this beam is close to a plane wave. After a gaussian beam passes through a lens, it's changed into another Gaussian beam. under certain conditions(one condition is a lens with short focal length ), its wasit locats in the back focal point of this lens. so using a pin hole at this lens back focal point to achieve a better plane optical wave and cut out those deflected wave, just like a low pass filter. The plane wave will have uniform amplitude. In addition, i don't get your mean about the "dispersion"? why?