BobH, i can certainly imagine these alignment issues. I'm happy to follow the expert consensus, and I will order 10 micron and 25 micron pinholes this week.
Ed, Dinesh, it is end of semester and I've got to get some work done! Already I spend too much time mulling over the good ideas y'all have provided.
numerical aperture
Re: numerical aperture
Bob, yes, normally, I'd agree with you. However, in this case, as I understand it, this is a physics professor intending at some stage to teach physics students. To that end, I'd suggest that a theoretical basis be presented (why a pinhole at all? If a pinhole, what determines it's diameter? and so on). In my case, when I deal with tech students (say, physics or optics engineers from UCSD), I usually say that while a calculation of the pinhole size is a good exercise, as is understanding the reasons for such and such a diameter, in practice you'll never get pinholes commercially of the calculated diameter, and the calculations refer to an ideal situation, which you'll never get. So, I usually say, you can use a LUT, but be aware of the reasons and trade-offs. I think there's a difference between teaching physics students about holography and having an art workshop.BobH wrote: Pictorial holography is not a physics experiment, unless you want it to be.
Re: numerical aperture
So this week was alignments and getting used to the feel of new pieces. Eventually I made everything work perfectly for the green laser. A glass plate tilting down in a kludged-together mount allowed me to see when I had vertical polarization. Fast axis of half wave plate at exactly 45 degrees from horizontal, and plate tilted so angle of incidence is Brewster angle, and reflection spot on the table vanishes. Why do manufacturers even bother to orient laser polarization horizontally?
Next, beam through pinhole, then align objective lens and push it along rails toward pinhole. First attempt, can see the Fraunhofer diffraction pattern from objective and Fraunhofer diffraction from pinhole and a mess in between. Try, try again until ... pushing objective toward pinhole, Fraunhofer pattern from objective that gets brighter, then changes to Fresnel diffraction (dark spot at center), then just a bright perfect circle of light over (what feels like) a long range, then back to pinhole's Fresnel diffraction and Frauhofer diffraction. Textbook example of what should happen.
So then I expanded the beam as described in this thread. Much easier to align, works great and now I get a big circle of light except... there is a shadow of some kind of teeny fiber thread or eyelash in the circle. Take objective off the rail and clean it.
Okay, all fixed, nice circle with angle of divergence matching that of my little LED recon light. Time for a Denisyuk hologram of my favorite shiny coin. Plate blacked along its edges. Coin and plate tilted at Brewster angle. Boom... the best result I ever had. No woodgraining. With recon at same angle, perfect green reproduction of coin. Then tilting recon to smaller angles of incidence, image goes yellow and orange and red before it starts elongating... that specular reflection surface behaving exactly as Dinesh explained in vocabulary thread. How fun!
In tandem I also worked to set up the blue laser. And everything going right with the green went very badly wrong with the blue. For example, finally fixed its polarization problem today. But all that big mess will await another post.
Next, beam through pinhole, then align objective lens and push it along rails toward pinhole. First attempt, can see the Fraunhofer diffraction pattern from objective and Fraunhofer diffraction from pinhole and a mess in between. Try, try again until ... pushing objective toward pinhole, Fraunhofer pattern from objective that gets brighter, then changes to Fresnel diffraction (dark spot at center), then just a bright perfect circle of light over (what feels like) a long range, then back to pinhole's Fresnel diffraction and Frauhofer diffraction. Textbook example of what should happen.
So then I expanded the beam as described in this thread. Much easier to align, works great and now I get a big circle of light except... there is a shadow of some kind of teeny fiber thread or eyelash in the circle. Take objective off the rail and clean it.
Okay, all fixed, nice circle with angle of divergence matching that of my little LED recon light. Time for a Denisyuk hologram of my favorite shiny coin. Plate blacked along its edges. Coin and plate tilted at Brewster angle. Boom... the best result I ever had. No woodgraining. With recon at same angle, perfect green reproduction of coin. Then tilting recon to smaller angles of incidence, image goes yellow and orange and red before it starts elongating... that specular reflection surface behaving exactly as Dinesh explained in vocabulary thread. How fun!
In tandem I also worked to set up the blue laser. And everything going right with the green went very badly wrong with the blue. For example, finally fixed its polarization problem today. But all that big mess will await another post.