RETHINKING LASER TRANSMISSIONS (5-21-04)
Things come to me at the oddest times.
I just finished doing a web search for any information pertaining to these ideas and came up short -- but that is not to say that the thoughts/ideas presented here are entirely new, or even not already being utilized somewhere in a corner of this vast holography community. So I will present my thoughts on this with a grain of salt to their originality in the hope of not stepping on any toes along the way.
In a previous post I began the introduction to color separation for full-color portraits. The advantages of using an LCD screen, along with a single-frequency laser, is that each color of RGB channel can be recorded separately. With an actual object, utilizing a single-frequency laser, this is not possible -- due to the fact that the object cannot be broken down into separate RGB channels. Of course, if you have RGB laser light, this is not an issue. Just record the object onto polychromatic holographic film or plates.
With this, we are using only one laser (HeNe or Diode, your choice), and monochrome film or plates. This is more in the realm of most amateurs and hobbyists than having red, green and blue lasers on the table.
Since we can bring a color photograph into Adobe Photoshop and extract the individual RGB channels, these 3 separate exposures can be made onto one plate. In order to play back this information so that each channel is truly an individual exposure (in all respects), 3 separate, single-frequency reference beams, and angles, must be used. These 3 reference beam angles will mirror the angles that we will use to view the resulting 3-exposure hologram upon reconstruction. Since each angle is association with an individual RGB channel, each reconstruction angle will illuminate that respective channel -- and not the other two.
For instance, our red-extracted LCD image will be recorded with a specific angle of reference beam. Then our green-extracted LCD image will be recorded with another. Finally our blue-extracted LCD image will be recorded with yet a 3rd reference angle. This will place all three channels -- RGB -- onto the laser transmission hologram. Since the LCD screen and plate has not moved, all three channels will be in registration with one another.
Upon reconstruction, RGB LED's are used to view the finished laser transmission hologram. The Red LED is placed so that it illuminates the hologram at the same angle as the red reference beam angle. The green LED is placed so that it illuminates the hologram at the same angle as the green reference beam angle. And the same for blue. If all works out properly, and the LED's turn out to exhibit a narrow enough band, then the laser transmission hologram will reconstruct all 3 RGB images, in registration with one another, and a full-color image will appear.
LED's have been used to very good effect in reconstructing reflection holograms (and also transmissions). This requirement is less taxing than what will be needed to reconstruct a laser transmission hologram however. If the LED shows properties of being too broadband, then spectral "smearing" of each channel will take place -- and the resulting image will not be sharp. However, if the narrow-band LED's on the market can truly provide performance close to single frequency, then this would provide a sharp image for each of the reconstructed RGB channels . . . resulting in a very sharp image indeed.
Much to think about and ponder yet however -- and as always, I thought I would post my initial thoughts and ideas. Once again, nothing is new under the Sun . . . so this may not only have been thought about before, but may actually be used in practice. Google searches for info. on this has not turned up anything however -- and perhaps that is due to the fact that something within these ideas and thoughts have already been tried . . . and failed.
I have already used an LED to illuminate laser transmission holograms in the past. The results were fair to poor. However these LED's were very early generation, "cheap", and definately not the narrow-band LED's of today.
For single-frequency work, Paulos Stylos has had breathtaking results with single-frequency LED reconstruction of laser transmission masters. Examples of his work can be found Here.
"Tom B" has also had breathtaking results illuminating his reflection holograms with LED's. Examples of his work can be found Here.
With the use of color-separated RGB LCD projected images, multiple reference beams (per RGB channel), and separate RGB LED's for reconstruction, the possiblity exists for full-color transmission holograms created with a single-frequency laser.
A display could be made that contained each individual RGB LED, so that they illuminated the hologram at the proper angles for each channel. Perhaps coming from above and behind at a 45-degree angle "down", then each LED illuminating "IN" towards the hologram at 45-degree from left, 90-degree from back, and 45-degree from right.
If so, then due to the color-separated nature of our recording, separate red, green, and blue images could be recorded using a single-frequency laser, then reconstrcuted using the actual colors of red, green, and blue for a full-color image.
There are a few speed bumps along the way -- most notably the usual shifts that occur when recording with one frequency and reconstructing with another. Rome wasn't built in a day -- but it was built!
A variation of this technique could be utilized for for H1 to H2 transfers as well -- where the resulting hologram would be an RGB reflection. This would have to totally rely on the clean separation of angles for reconstruction -- but would allow a wall-displayed, image-planed hologram as an end-result. More on this at another time.