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To create portrait holograms, I used an overhead setup that combined a portrait type lighting technique used in photography with a holographic reflection setup. One beam acts like a strong spotlight that comes in from above the subject on one side at a high angle causing a slight nose shadow under the nose of the face and a weaker fill light from the opposite side to reduce the strong contrast. This glamor lighting technique is commonly used to contrast cheek bones etc. Sometimes a third light is used to highlight the subject's hair. The following diagram illustrates a design that utilizes a standard holographic reflection setup with this two light glamor lighting technique. For this technique to work correctly it is important that the beam splitters provide the correct lighting intensities for the spotlights. I opted not to use the transmission (H1) hologram and copy process known as H1/H2, allowing a simpler setup, shoot, and one film/ one develop process. I designed the setup for minimal optical components to reduce distortion and diffraction effects. Since the system is setup as an over head projection camera system, it is not at eye level for additional safety benefit.
holoreflect1.gif Diagram of Reflection Hologram Setup.
hhead.jpg Photo of the overhead laser and beam splitter setup. Red lines illustrate the beam directions.
hsetup2.jpg Photo of the mirrors and film plate setup. Red lines illustrate the beam directions. Where beams intersect is where the subject is placed. Typically their chin is located at that point. Note that the reference beam falls below the person's chin level. A height adjustable barstool (center screw and circular seat type) is used to adjust the person to the correct height and distance from the film plate.
supplies.htm Supply list for hard to find optics and supplies.
dev.htm Pulse development procedure
250 mj minimum holographic laser with built in diverging lens. -12.5mm focal length (-80 diopters) , Best results are with a 1 to 2 joule laser for better object beam lighting. Bare minimum portrait holography was done with a 90 to 112mj single pulse laser. Coherence length of the laser also is a critical factor and should be at least 10 to 20cm and best at 1 to 3 meter.
2 each cube beam splitters 40mm size. The beam splitters used were 78/16 split at 694nm with vertical polarization. 16% transmitted and 78% reflected. The beam splitters where arranged so that the diverging laser beam reflected 78% to object beam 1. Then the remaining 17% was again 78/16 split for Object 2 beam and the final reference beam was the remaining 17% from the second split.
2 each 13.5" x 14.5" x 3/4" flat front surface mirrors and stands
2 each 16"x20" ground glass diffusion screens (sand blasted variety)
1 each 13.1" F4.5 (1500mm focal length) parabolic telescope mirror 1.6" thick. Serves as reference beam and is required to collimate beam and eliminate photographic distortion of subject.
1 each 4"x9"x 0.5" flat front surface mirror for reference beam steering. Also a small 4x6" flat front surface mirror for the object beam 2 steering.
1 film plate made from 2 10"x13" window panes for 8x10 holograms. The top of the plate should tilt toward the subject at 15 degrees and the reference mirror should angle the beam down at 45 degrees, yielding a 30 degree difference. This is optimal both for recording and keeping ref light out of subject's eyes.
Various flat black beam block sheets to prevent reference beam falling on other side of film plate and hitting subject in eyes. Also side blocks to prevent object beams from hitting film plate on wrong side (the reference side).
Two 1 nanosecond photometers were used with digital storage scope to record light ratios from subject at film plate from reference and diffused object beams reaching film after reflecting off of subject. Ratio should be around 2 to 1 (ref: obj) Published studies indicate that skin reflectance can vary from 30% to 70%.
Film stock was Agfa 8E75HD film. Developed using a pulse specific developer: SM-6 formula.
SM-6 formula is basically
Some image brightness can be achieved by shifting the playback frequency of the reflection hologram. TEA (triethanolamine) as a presoak before exposure increases the sensitivity and later during development and bleach will shift toward green. I found it to be extra trouble to presoak and dry the film in a darkroom and then shoot it. Instead I went with a pulse developer and a reversal beach to shift toward green. (Honestly I like the red/orange to yellow colors better for people's faces than green). If you use the TEA method then use a rehalogenating beach, which doesn’t shrink the emulsion, and control the color by Tea percentage in water during the presoak.
A word of note, early versions of RW-MOPA Ruby Laser was able to create single beam reflection holograms of bright objects like toy helicopters etc with out the need for an isolation table, but the size of the image was 3x5 or less. These required 30-60mj output. In order to do portrait holography and multiple beam setups required a laser of 250 or more millijoule output. I found with laser outputs of 250 to 350 millijoule, optimizations like using Agfa 8e75HD, and immediate development in SM-6 that 8x10 portrait pictures could be developed. If instead of 8x10, you want to shoot 4x5 then the laser power level would be 1/4th the above minimum and for example my version 2 laser might have produced a multi-beam reflection hologram. When I tried using single beam method with version 2 and me holding the plate next to me, I could barely say "oh, a reflection of some glasses, or that looks like a nose". So, I didn't work that power level for long as my goal was getting to 8x10 and not working much with 4x5. Don't use single beam for portrait as the results will be poor and too great a risk to the subject's eyes. When I tried this my self I kept my eyes closed as a precaution.
Extreme care should be taken to make sure that the laser light is of known quantity and care taken to reduce amounts that will enter a person's eyes. No guessing here. Exposure limits for direct viewing are set around 500 nanojoules/centimeter squared for exposures of less than 10 microseconds. 10 times this value or 5 microjoules increases the risk of lesions to 50% accordingly. US army's distance vs power level graph http://chppm-www.apgea.army.mil/rfup/website/FIG16.HTM on the M60A3 tank employs the AN /VVG-2 ruby rangefinder (a 50mj/40nsec single pulse) also shows the MPE at 500 nanojoules/cm2 and the lesion level at 25 microjoules/cm2 and hemorrhage level at 25millijoules/cm2. Specifically large dilation of pupils, looking directly at the beam, and focusing on distant objects are the risk factors. Single large q switch pulses produce the best holograms due to short duration time but at a increased risk to eyes. Knowledge of the laser design with pulse measurements and measured light levels are needed to understand the exposure. Some design issues that was considered: Diverge the laser beam therefore reduce that risk from a Class IV laser. The other is to downward direct the beam energies, MUST use diffusers and MUST avoid the reference beam entering the eyes and instructing the person where to look and visual monitoring of the subject before exposure is taken by using a dim green light in the room to see by. For example telling the person don't look up (in this setup), look straight ahead.
For various skin reflectivity's see http://www.vision.auc.dk/~mst/Publications/sirs99html/node4.html - SECTION000.Typical reflectives off of subjects and their clothing can be about 1/20 to 1/50 of the object beam.
The Agfa 8E75HD has a sensitivity of 10 and 25 uj/cm2 at 694 for optical densities of 1 and 2 and a resolution of 3000 to 5000 line pairs/mm. The Agfa 8E75HD is still the best reflection film for low level exposures. Due to the short nature of the beam pulses having them directed downward through the diffuser panels help minimize possible damage to critical sight areas like the Macula and Fovea. If calculated using the formula for extended source diffused light, based on the acceptance angle of the eye, a 7mm pupil would only be 3.5mm (Lamberts cosine law) while looking straight ahead of the diffusers which are located above and to either side of the subjects eyes. The eye's 17mm depth gives a value of 11.6 degrees of acceptance) and E=pi*L*sin squared (theta/2) gives a reduction factor of 30 for someone looking straight ahead in this setup.
According to ANSI Z136.1 yr2000 Table 3 would indicate that a 1.6joule laser spread out to 10cm diameter and viewed from 20 cm away after striking a diffuser would be at the limit of the allowed MPE. So if the laser is spread out to 40cm diameter then an even safer margin is provided for a diffused source. In the setup the object beams spread out to 16" diameter or 40cm and then 30-40cm later strike the subject. Care was taken to insure the reference beam doesn't strike the subject's eyes and the reference laser beam is directed downward. In addition any reflection off the glass film plates would also be directed downward. Avoid any possible small source beam exiting the laser from striking a subject as it is best not to allow collimated beams to exit the laser. But instead allow only highly diverging beams to exit the laser. The only allowable light to strike the subject must be diffused and the beam diameter large enough to illuminate the subject and a minimal size of 5" diameter or larger for a 1 joule laser. This diffused light was also directed downward in the setup.
The MPE calculated for the object beam is as follows from the ANSI Z136.1 2000 reference: MPE for the visible laser at 1 nanosec to 10 microseconds is 500nanojoules x Correction factor for the extended source (wider than 1.5mrad viewing angle). To calculate the correction factor lets first calculate the viewing angle. Beam diameter in cm/viewing distance in cm = number of radians. Example 20cm beam dia/40cm view distance = 500 milliradians. A 1cm beamdia/50cm viewdist is 20mrad. The correction factor for millirads greater than 100 is a^2/amax*amin. a in our case is 500, amax is 100mrads and amin is 1.5mrad. so 500squared/150 = 1666.6 correction factor. If the beam diameter/view dist ratio is less than 100mrad then the formula given is a/amin. A 20mrad viewing angle would be 20/amin or 20/1.5 or a correction factor of 13.3. The MPE then is 500nanojoules/cm2 * 1666.6 in the first case = 833ujoules and in the second case at 500nj/cm2 * 13.3 = only 6.65 microjoules/cm2!, The radiant energy from most Class IV lasers at this 20mrad view angle would make this is a very dangerous beam viewing condition and goggles would be required! .
An OHSA Instruction PUB 8-1.7 dated 1991 http://www.osha-slc.gov/OshDoc/Directive_data/PUB_8-1_7.html indicated that there may be even other potential issues with diffused light that may affect angle acuity or color and may require even much lower levels MPEs due to speckle. Recommended lowering MPEs levels due to use of these extended source diffusions. These studies suggested near direct viewing of the light (affecting the fovea) but I would definitely take these exposures seriously even if they appear okay and take all precautions as possible. The larger the beam diameter, the larger the spot on the retina which is why the correction factor can be used for extended sources. But due to other studies regarding long term issues with diffused light and speckle, I would take OSHA's pub's recommendation of reducing this calculated MPE by 1/10. So based on the first example of 833ujoules, It would be best to limit the exposure to 83ujoules/cm2. Using photodiode detectors with known sensitivities and specific neutral density filters installed and a digital storage oscilloscope to record the test setup of a person (with laser goggles on) would allow you to measure the object beam intensity at the subject's eye positions for the allowable MPE. In addition, the ratios at the film plate of the reference beam and the reflected object beams from the subject can be recorded and adjustments made to achieve the 2 to 1 ratio of reference to object power density and the required power density to properly expose the film.
H.I. Bjelkhagen's suggestion of shining a green light into the subject's eyes to reduce the size of the pupils is a good suggestion. http://www.holonet.khm.de/Holographers/Bjelkhagen_Hans/text/Pulse_Portrait.html
Basic info regarding holography: http://www.holo.com/holo/book/book1.html
Brief info on films: http://www.hololight.net/materials.html
A great reference book below can be ordered from integraf http://members.aol.com/integraf/
H.I. Bjelkhagen "Silver-halide Recording Materials" for holography and their processing.