VRP-M Lessons Learned

For almost two months now, I’ve been working to figure out the exposure and processing steps to use in order to get bright and clean transmission holograms using my Coherent 315M and VRP-M film.

After blowing through roughly 68 pieces of film (mostly 1.5×1" in size) I think I’ve finally nailed the process to use for transmission masters.

For those of you in the same situation, hopefully the following will shorten your experimentation phase.

1. Laser controller startup is key

After
doing a number of preliminary tests with VRP-M I thought that I’d hit
on a successful combination of exposure time, development and bleach to
use and was ready to get down to the making of "real" holograms again.

Then my holograms started coming out dim and I had to head back to square one to find out what was wrong.

During
this period I’d been doing all sorts of experiments such as changing
the developer from JD2, a CW-C2 variant, to TJ1 and using them
undiluted and diluted to find out the effect of each change on the
final hologram. I’d also experimented with bleaches from EDTA to JD2,
JD4 and Potassium Ferricyanide.

Each time I got a bright
hologram I thought "great, now I’m ready" but the next hologram was
likely to be dim and ugly. The dim and ugly that with my HeNe would
have said that I’d forgotten to measure beam lengths and would find
they were way off once I went and measured them.

As it turns out, the problem was in the controller for the 315M.

But the controller wasn’t the problem. I was.

I’d
discovered that if I left the controller turned on and running and
instead turned off the power to it "at the wall" I could turn the power
back on and the controller would appear to go through its startup
routine and hit the target power level each time. Normally it ends up
10mW lower than the target setting I’d chosen and I thought that this
new startup method was a good thing.

I was wrong.

It
turns out that starting up the laser this way results in unstable
output. I haven’t tested it in an interferometer, it’s on my todo list
now, but I’ve found through hologram making that by, at the very least,
hitting the target power level switch the controller will go through a
complete startup and I can make consistently bright holograms as long
as I do that and follow up by using the right chemicals.

Which leads me to the next section.

2. VRP-M hates EDTA bleach

The
high fog levels I was seeing in the first set of bleached holograms led
me to believe that the film I’d bought back in January, which has a
made on date of Oct 2005, was ruined by the high humidity we had here
in North Texas this Spring and Summer.

It’s not nearly as bad as I’d thought now that I’m not using EDTA bleach.

I’ve
experimented now with EDTA, JD2, JD4 and Potassium Ferricyanide and
while I can’t tell you why the PotFerr bleach works the best, when
combined with JD2 developer, it provides the highest contrast results
of the four.

JD2 gives images with fairly low fog but also much
less brightness and a kind of dim look that makes the imagery
unpleasant to look at.

EDTA give similar results to JD2 in terms
of brightness and overall image quality but is quite a bit foggier than
I can work with and while an Ascorbic post-treatment increases the
contrast it also darkens the image more than I can accept.

JD4
is better but even with newer film (Jan 2007) there’s a bit of fog.
It’s usable though and with a light Ascorbic treatment the fog is wiped
out with only slight dimming of the image.

Potassium
Ferricyanide gives the best results of the all of them. Right out of
the bath the holograms are slightly brighter than JD4 and the contrast
is much better. Black areas are black and "white" areas are cleaner.

Note
that this bleach result is likely dependent on the developer used so
I’ll update this post once I’ve done some additional tests with the TJ1
developer.

3. VRP-M prefers low light ratios

I’ve also
found that VRP-M works very well with low light ratios. I’ve gone as
low as 1.1:1 and couldn’t see any increase in noise in the final
hologram.

At the moment I’ve settled on a 2:1 ratio mostly for
emotional reasons. I’ll do some more experimentation along this line
with other subjects as time goes by.

So to recap:

1. Make sure you put your laser through the proper startup procedure each time you power it up.

2.
Until something better is identified for transmission holograms, at
least use JD2 developer and JD4 bleach. If you’ve got the chemicals,
use Potassium Ferricyanide bleach as the contrast is much better.

3.
If you’re doing split beam work, go with lower light ratios. The
images will be brighter and there’s no visible difference in noise
level of the final hologram.

Visual results

Below are
two images from three test runs to show the visual differences between
treatment methods. Click on them to see a full-size version

Holofog HoloTest65-67

This
first image shows the relative fog levels in three different
exposures. Hologram 65 was a test using JD2 developer and JD4 bleach
on the top half and JD2 bleach on the bottom. There’s less fog on the
JD2 but the image is darker than the image from JD4.

Hologram 66
was a comparison between JD4, Potassium Ferricyanide and EDTA. The JD4
hologram looks pretty good with only a bit of fog. The EDTA hologram
shows more fog and the image is muddy looking. The PotFerr image shows
almost no fog and the contrast levels are much better than either of
the first two.

Hologram 67 was a combination test of development
times and bleach. A & B were developed as one piece (cut up just
before bleaching) and developed in JD2 for 20 seconds. C & D were
developed for 40 seconds. A & C were bleached in JD4 and B & D
were bleached in PotFerr. There’s not a great deal of brightness
difference between the 20 and 40 second pieces but still a clear
difference in contrast between the JD4 and PotFerr pieces. There’s
actually less difference in brightness than the photos would imply.

It was then time to do a film batch test between the Jan 2007 and Oct 2005 batches of VRP-M film stock.

HologramOldNewHologramOldNew2

Once
again the first image shows the relative fog levels with the January
2007 film on the top and October 2005 on the bottom. 68a and 68c have
both been given a very light ascorbic post treatment. The older film
shows some fog but not when developed in JD2 and bleached in PotFerr
it’s not much at all and can be worked around by post treatment with
only minor darkening of the image.

Author: Michael Harrison

Husband, Programmer, Irish dancer, tinkerer, astronomer, layabout (as much as possible)

Leave a Reply