Laser on DLP "screen door effect"

Justin W · Post by **Justin W** » Fri Feb 11, 2011 4:59 pm

I'm hoping Ed may be willing to elaborate some on the particulars of the DMD arrangement he used.

I too have a Texas Instruments DLP chip here, and I'm wondering if - in order to make it work for holography - I will want to remove it from its original housing and beam laser light directly on it, then gather that light and focus it with my own lensing arrangement.

Is that pretty how much yours was, Ed? Did the Micro-lens array that I see in the projectors' optical train play a role in the system you used?

Jeffrey Weil · Post by **Jeffrey Weil** » Fri Feb 11, 2011 6:54 pm

Johnfp wrote:To be honest with you. I assume you are planning on making stereograms. WIth that being said, once you make the sets of slits and then the final hologram, a healthy person (with two eyeballs that work) in viewing the finaly hologram, in essence by viewing through two of the slits at one time to see the 3D image, will be looking at two different images at once from slightly different angles. So I would imaging most of that screen door effect will not be seen. In essence there will be rare occasions that the two dark boxes (the wires on a screen door) will overlap exactly.

I am not sure how much of the negative effect will go away but I can bet it will dimminish.

Hello John,

Only the data displayed on the projection screen will change from shot to shot, the display device itself, and it's diffraction, will be stationary. With a moving slit and stationary film every "dark box ( wires on a screen door)" will overlap on every shot perfectly. Not rarely. With moving film and a stationary slit the screen door effect would be smeared across the hologram horizontally. It would be a perfectly regular pitch though. You would still see it.

Ed W used a dlp development board when he was at CFC, not a consumer product. It didn't have optics, or mirrors (other than the dlp), or anything. It's just a board with the dlp and associated electronics and a power supply. Its not even in a case.

Jeff Weil

Justin W · Post by **Justin W** » Sat Feb 12, 2011 4:47 am

Hey fellas

A few notes.
Just a quick clarification on something I said before: I misspoke when I said that defocusing the image reduced the mesh-like shadow pattern. Not quite sure why I thought that to be the case... perhaps I just assumed...
Anyway - as it turns out - The intricate shadowgrid pattern stays just as sharp at any focus.

Another interesting point: I went all Hardy Boys on this mystery and began a little sleuthing; I placed a large plano-convex lens with a long FL in the output of the unit and focused the image down to a "point" on the wall. I imply irony on the matter of focusing it to a point because it in fact did not focus to a single point. Instead I bore witness to a grid of laser points that reminded me of the glimpse of micro-lens array that I saw shortly upstream of the DLP in the units' optical module. Apparently, the whole while I've been projecting laser light from this thing, it's been possibly 10 or so images all mixed together - several primary bright ones and numerous peripheral images.

Is the little rectangular flies-eye micro-lens responsible for this? In an effort to evenly cover the DMD with the LED light it expects, is it inadvertently handing the micro-mirror chip multiple laser beams to interfere with one another?

favalora · Post by **favalora** » Sat Feb 12, 2011 7:21 am

Hi Justin - Feel free to post a few photos of your setup.

g

Justin W · Post by **Justin W** » Sat Feb 12, 2011 7:45 am

Hi Gregg!

I'd post some photos except - for all the optical whatnot I've got going here - I don't have a working camera at this time.

Anyway: Long story short - problem solved. Micro-lens array removed and mesh pattern gone. Now I've just got some old-fashioned diffraction artifacts from the internal lensing of the unit. Bullseye rings and dust motes I can handle. Apparently that micro-lens jobbie evened it all out inside and allowed for what I'll guess to be less-than-premium optics.

With that ghastly mesh pattern gone now I can see enough detail in the images to make out pixelization. It's really not that bad, and will diminish some when I score a long HDMI cable.

Score.

BobH · Post by **BobH** » Sat Feb 12, 2011 12:28 pm

You might try a light diffuser where the fly's-eye integrator was placed, or near it. If you mount the diffuse on something that will move it to a randomly different place for each exposure, you'll de-speckle the image and the patterns made through the optics system won't ad up coherently.

Ed Wesly · Post by **Ed Wesly** » Sun Feb 13, 2011 1:16 pm

Sounds like the problem's solved, and in one of my earlier posts I did mention the types of lenses I have used successfully in film and DMD based projection systems.

Keep up the good work!

holorefugee · Post by **holorefugee** » Mon Feb 14, 2011 9:17 pm

Nice work! There are lots of ways to despeckle an image. Post a new thread if that becomes a problem.

Johnfp · Post by **Johnfp** » Tue Feb 15, 2011 7:19 am

Ah yes Jeff. Thank you.