School of Holography
School of Holography

LASER POINTER / SEMICONDUCTOR LASER HOLOGRAPHY
by Frank DeFreitas Holography Studio
Allentown, Pennsylvania
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Creative Holography Using
Inexpensive Laser Pointers

My magical journey of making
holograms with a $7.99 laser pointer
and inexpensive laser diodes.




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35mW Diode Test Shot (4/23/99)



Digital photo of test hologram using a 35mW, 658nm diode

Well, the 35mW diode worked! Here's the coral again, but this time it's special. It was shot using a beam spread for a 30 x 40cm plate. Everything looked so good with this laser diode, and I didn't have time to do several test shots, so I combined a few aspects into one. All I did was sandwich the film between two glass plates (no index matching fluid) and got the shot off and processed. Next few shots will be done on glass plates and carefully done. Stop back for those shots. (see new angel hologram below, it is the best image yet!).

If you've ever worked with 30 x 40cm plates (or larger) you know that you cannot afford to use that size to see if your shot is going to work or not (at least most of us can't afford it). So what you do is set up for your 30 x 40cm shot as usual, but for the initial test shots you use a cheap piece of 4 x 5-inch film to make sure that everything is OK. Since everything is set up for 30 x 40cm, you'll know by the 4 x 5 results exactly how bright, deep, etc. your 30 x 40 will be when you shoot it and if your exposure time needs to be adjusted.


Second "higher-quality" shot using the 35mW diode laser.
30x40cm spread light on 4x5-inch BB-Plate. 5% TEA.
Single-beam using my exposure formula below.

What I did with these shots is spread the light out to cover a 30 x 40cm card in my large-format holder and then replaced everything with the 4 x 5 holder and card and lowered the center of the spread beam down to the card. This put quite a bit of light into the sand, so I had to use a black T-shirt to stop this reflection from bouncing back up onto the film holder and causing problems.


Close-up of the 35mW laser diode module.

ABOUT THE DIODE
This hologram was shot with a 35mW(!), 658nm laser diode module. It was purchased from Power Technology, Inc. in Little Rock, Arkansas (US). This module has a few added "goodies" that I will share with you below. I'm nearly 100% certain that the diode itself was manufactured by Hitachi, but I will have to call PT to confirm this. I think Hitachi is the only manufacturer of a 35mW, 658nm diode at present. I just found out yesterday that Hitachi also has a 50mW diode that operates at the same wavelength. The lowest price I have found for the 35mW raw diode on the web has been $33 (US). My best guess is that you could put this entire module together for a little over $100. ($33 for the diode, $30 or so for the driver, $20 or so for collimating lens and $20 for the brass barrel). This module has a longer aluminum barrel to hold the extra optical goodies.

To start things off, Sam of Bell Labs was with me on Thursday when we powered this baby up. We both were in shock. The module contains anamorphic prisms which gives a circular beam output in combination with the collimating lens. The beam was beautiful. We began running the diode at 5-volts, which gives the spec'd. 658nm output at 25-degrees C. Running the diode at the lower spec'd end of power (4-volts) gives an output of 645nm at 25-degrees C. For the hologram, I used 4.5-volts, which would have put me somewhere between the 645 and 658nm range.


Circular output beam profile of the 35mW laser diode module.
Actual diameter of beam is approx. .5mm (guesstimate).

The module has the optional AR-coated anamorphic prisms, which decreases the amount of light lost through reflection off of approx. 8 surfaces (prisms and collimating lenses). Power drop is estimated at 30% with coated optics, 50% or greater for standard. This diode was extremely bright with the spread beam reflecting off of a white card in the plate holder. I'd say it was slightly uncomfortable to look at for any length of time. Very bright. Intense.

The spread laser light was perfect. What a wonderful spatially filtered spread of light! Polarization was incredible. With the pointers, I have found areas of the spread light that were not polarized in the same plane as the majority of the output -- almost like islands on the white card. This light, however, went totally OUT with our polarization test. Nada. Nothing. Not even the slightest indication on the white card (in a darkened room) that any light was left on the white card at all. A stellar performance!


Visible beam exiting the diode optics.

The intensity reading using my Science & Mechanics PhotoMeter averaged around 30 or so on the 2 scale. This worked out to 1.12 seconds with a formula I use that takes object brightness into account (even for single beam work). This seemed rather short for a 30 x 40cm exposure, but since it was a single beam and the intensity was very bright I just boosted the time up to 2 seconds. I'll have to put the forumula up for others to give it a try (you'll need the S&M meter). Believe it or not, you actually take a reference AND an object beam reading -- even for a single beam. Then you put your numbers into the equation to find out your exposure time. This gives you a variable exposure time which is determined (partially) by the reflective properties of the object itself (in combination with the intensity of the reference beam).


Single Beam Exposure Formula:

Exposure times for single beam hologram using the Science and Mechanics A-3 PhotoMeter (emulsion facing object). You must take a reading of BOTH the reference beam AND the laser light reflected off the object. This will allow you to compensate your exposures for varying object reflectivity.

Scale on Meter:
(1) = 6
(2) = 60
(3) = 600
(4) = 6000

Equation:
(Or + Rr) x .10 = n

t= [(0r + Rr) - n] / (1), (2), (3) or (4) from above.

Or = object reading.
Rr = reference reading.
n = total reading with substrate reflection adjustment
t = exposure time.
.10 = compensates for 10% (variable/avg.) initial reflection off of glass substrate (plate surface facing reference beam, emulsion facing object). This will be more or less depending on the reference angle and orientation of polarization. Put a glass plate into your plateholder. See the light reflected onto the wall? That light is never even getting to your emulsion. This value (which is sadly missing from many exposure formulas) makes up for that loss. If you're REAL careful with your reference angle and polarization, you can get down to nearly zero reflection (I'll save that for another page). You may wish to put a clear glass plate into your plate holder and take note of your intensity readings with and without the glass. Figure this number out as a percentage and use it as your precentage (ie: .01, .04, .10, .14, etc.) in the formula above to give you "n".

The above forumula is for AGFA film and plates. For BB-Plates you should multiply your final exposure time by a factor of at least 3 or 4.


Surprisingly, the development time was right around 2 minutes using straight Pyro (around 4 minutes or so for the BB-Plate). Nice density and very even coverage across the area of the film. I watched very carefully as the image started to appear and was quite shocked at how bright the image was -- remember, I didn't know if there would even BE an image at all. I do not feel that the diode's longer wavelength had any adverse effect on the image quality. Actually, the brightness of this hologram exceeds the 15mW coral test shot by a few factors.

Laser light at 658nm is much more beautiful than at the HeNe's 633nm. It is an intoxicating red beam, compred to the HeNe's somewhat more yellowish-red. This would be a great diode for small laser light shows -- the 50mW Hitachi even more so.


Size comparison: The 35mW and 15mW diodes sitting
on top of a 15mW Spectra-Physics 124B HeNe Laser.

So, there you have it folks. A laser diode capable of large-format holography for just slightly over $100 (this module was actually closer to $500 with the coated anamorphic prisms, collimator and adjustable driver). Personally, I don't see the need for prisms, as the 15mW test (see page 13) showed that the beam can be accurately controlled with the standard collimating lens alone. I'm going to see if I can get the 50mW without the prisms to do a few tests. But, as you know, you can do some SERIOUS holographic work with 35mW. Especially with the stability of these diodes. They are rock-steady.


My current laser diode module collection (left to right):
35mW/658nm; 10mW/635nm; 15mW/635nm;
500mW/660nm (problems); 5mW/650nm pointer; 5mW/650 pointer (internal)

Let's get out there and make some holograms . . . the best is yet to come!




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Raw Diode Testing Begins


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Frank DeFreitas Holography
Allentown, Pennsylvania
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School of Holography