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Holographic
Setup
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
Basic setup:
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.
