DIGITAL HOLOGRAM PORTRAITS #5 -- 4/05/04
The first test shot utilizing a computer workstation and LCD panel for image generation has been completed. I set this up to be a very simple shot -- a standard split-beam reflection -- so as to not add too many variables into the system in case of the need to troubleshoot. The image was a 2-D logo from my studio, which was shot as a real image floating out 4-inches from the final 4 x 5 hologram. This image was created in Adobe Photoshop, then fed to the LCD panel on the holo table. It was/is also very important that this be completed using only hand-built equipment. In this case, all hand-bulit optical mounts/holders and a sand table.
This time around, I ran the video camera to capture this in progress, then edited down to a 5 minute presentation in RealPlayer format. You will find the link to the video at the bottom of this page, and several stills from the video throughout this text. I tried to take into consideration both dial-up and broadband connections, so the video is the combination of both: higher-quality and frame rate, but a smaller 160 x 120 size video window. A significant number of HoloWorld E-mail subscribers (out of 3,000+ total world-wide) are still accessing the Internet via 56k modem (a few even 28.8!), according to the questions answered during the time of their subscription activation.
As stated before in a previous update, the LCD panel exhibits quite a polarization effect on the laser light being transmitted through it. Several spots during a 360-degree rotation of the panel will block transmitted laser light entirely. So it is important to have the laser oriented properly for maximum transmission. If you're working with a cylindrical head or diode barrel, it is much easier to rotate the laser than to rotate the LCD panel in order to achieve this. Once this has been achieved however, transmission properties fall within an acceptable level. I will include actual transmission efficiency numbers in a later post. It has been noted in the literature that LCD panels reduce the amount of transmitted laser light to only 10%, but I am not finding this to be the case -- which is noticeable even without measurement at present -- although having the LCD panel improperly oriented, as noted above, can certainly drop transmssion to nearly 0%. You need to find the sweet spot -- and it is a very narrow spot at that.
Update evening: Having already received an Email on the transmission efficiency of the panel, I just finished doing a test using the Science & Mechanics photometer with a 6-inch diameter diverged beam. I have found the percentage to be 55% transmission at its greatest, and 0% transmission at its least (depending on orientation of the panel). While 55% is not a high percentage, it is certainly not 10%. Greyscale testing will be forthcoming as I begin to fine-tune this process and results.
One very important adjustment to the LCD panel is what is termed "dot crawl". This is an adjustment that locks the liquid crystals in snyc with the voltage output of the imaging source. Some panels have this feature, and others do not. You can visibly see the crystals locking into place. Overall, this is important to the resulting diffraction efficiency of the recording hologram. More detail on this, and other aspects (and tips) that improve efficiency overall, and beyond historical reviews of utilizing LCD panels, in later posts.
I managed to get around the bulkiness of the panel by opening up the table set-up. This may (or may not) prove troublesome for the portrait tests. I previously attempted to remove the actual internal LCD screen from my original panel, but one quick slip made that a disaster. With once bitten twice shy being the motto this time around, the panel went onto the table as is. I want to get a few successful shots of this before making another attempt at removal. Having the panel removed will provide for much tighter table set-ups -- and the possibility of having this entire system table-top in size (rather than on an 4 x 8-foot holo table). That however, is further down the road.
I was quite pleased with the resulting hologram -- shot onto an AGFA8E75HD glass plate. I choose this plate since it is one that I am most familiar with when it comes to exposure and processing -- although it no longer provides the diffraction efficiencies of the newer recording materials. Despite this drawback, the hologram was a screamer, much to my surprise, and I'll be interested in seeing just how bright the results will be when testing on my favorite plates: BB640's.
Processing was done with the older PyroChrome processing technique -- still a great stand-by to do initial testing with (and actually a preferred method with many final holograms as well). The plate bloomed to the proper density in approx. 5 minutes. Despite the high-contrast between the transmission areas and black areas of the LCD image, there was no burning-in noted after bleaching -- which can be a potential problem doing line-art logos. If you've ever worked on these, you'll know what I'm referring to as far as high concentration of deposited silver that matches the line-art on the post-processed holographic plate.
Overall, this was a successful test of all parameters, and holds great potential to be matched with the portrait process in general -- and not just with people portraits, but holograms of objects and scenes not possible to do directly in the lab, along with computer-generated imaging and medical imaging as well (see my fluorescein angiography hologram in the 20 years/20 holograms section of HoloWorld).
Unfortunately, the clear LCD screen (no image) blurs what is seen beyond it, or it would work out well for sync'd slits too. I'm currently looking into a stepping-motor driven slit arrangement to sync with the workstation-controlled main images (or something to that affect). But this is much further down the road.
Next steps will be to set up a laser-transmission hologram test -- similar to this initial one in that it will be one image recorded to keep troubleshooting simple. I plan on doing this using the mini-portrait geometry, so that further testing can proceed right in step if initial results are positive. One step at a time. So I am two steps away from the first laser transmission portrait master at this point.
Now is the time to watch the video if you choose to do so. This was fun to do, and I plan on additional video "records" of progress as I continue on. When the time comes, I will also have several of these showing snippets of each step from the portrait digital photo shoot, to the resulting final holo portrait itself.
Web Video Notes: This video is approx. 3 MB in file size. If you are accessing from a long distance, you may experience rebuffering due to congestion -- especially if you are on a dial-up connection. Those of you who are closer to the Philadelphia, PA area, and/or have broadband should experience the best playback. Best I can do, all things considered.
You will need the FREE RealPlayer from RealNetworks in order to watch this video. If you do not have the player, you may obtain it here. They offer a player for sale, so you'll have to look closely for the free one.
Click above to watch video.
More reports coming up.