By: John F. Pecora
Baltimore, Maryland USA
e-mail:holograms3d@yahoo.com
February 20, 2006
Single Beam Reflection (SBR) holograms are commonly used by most novice and beginners as an easy way to make a white light viewable hologram. SBR holograms are also very appealing for display work in Dichromated Gelatin (DCG). While DCG is very efficient in replay brightness for SBR Holograms, there are times when the object may need to be holographed with its natural colors which may not be conducive to high reflectivity and thus the light reflecting off the object is lower then a white painted object. There are also times in which a particular object has protrusions or indentations such that other parts of the object are shadowed by a single overhead reference and thus object lighting beam. Another shortcoming of SBR holograms is the fact that the object to reference light ratios cannot be controlled. In this paper an easy, novel and straightforward technique is described that addresses these issues without compromise to the simple nature of a SBR hologram.
Single Beam Reflection (SBR) Holograms are widely used in the starter holography kits due to fewer optics, less stringent stability requirements and ultimately less monetary investment as compared to more elaborate geometries. SBR Holograms are viewable with white light which makes them ideal for display holography and with the increased availability of inexpensive green laser I suspect we will see the beginner holographer venture into the DCG experience. Furthermore, even the seasoned DCG Holographer likes to maintain a degree of control over Object to Reference light ratios which seems bulky and difficult at times with additional mirrors placed behind the film in a SBR Hologram.
The basic design of the SBR hologram has built within it the inability to control the Object beam to Reference beam light ratios. This leads to 2 scenarios. First, an object much be chosen that has a very high reflectance to the wavelength of laser being used. Most of the time an object choice will be a flat, bright, white painted object and the natural color of an object is usually less then ideal. Second, mirrors may be added behind the plate (object side) to capture some of the wasted overflow light and redirect it to the object. The downfall with this technique is that it is difficult to place the mirrors such that they are out of the scene of the hologram yet still capture and redirect enough light. Additionally, two additional mirrors, one on each side of the object, needs to be used in order to evenly illuminate the “front” of the object.
The technique described in this paper addresses the need to add additional lighting in an SBR by using a front surface mirror in front (Reference Beam Side) of the hologram. The additional mirror captures overflow reference light before it enters the film and redirects it through the film directly to the front of the object. Because the additional light is entering the plate at a great angle from the original Reference Beam, there is no crosstalk or degradation of the final hologram between the Overhead Reference Beam and the Reflected Beam off the Front Surface Mirror.
Figure 1 illustrates a typical geometry for a SBR hologram. Although it is not depicted here, an optimal geometry for those that do not have a vibration isolation table is to secure the object to the table and lean the film plate right against the object insuring a static interface between the object and the film. The laser light enters from above at Brewster’s angle (56 degrees). The light travels through the holographic film and is reflected off the object and travels back to the holographic film where it interferes with the original reference beam to create a standing wave patter or fringes, which creates the hologram. As you can see there is little that can be done to control the amount of light that bounces off the object. When an ideal very white, flat (not glossy) paint is used the resultant hologram is nicely bright but still has room for improvement. When a naturally colored object is used, which may have grays or colors not ideally reflective for the wavelength of light being used, the resultant hologram is most likely going to be dim. There can also be very dark areas caused by shadows as seen in the diagram.
Figure 1 – A standard Single Beam Reflection Hologram does not allow alterations to the light ratio of Object to Reference Beam. In addition the single overhead lighting from the Reference beam that illuminates the object causes shadows on the object. This can be seen by the green shading on the front of the object which represents parts of the object illuminated from the reference beam and black shading on the front of the object which represents parts of the object that does not receive illumination light from the reference beam.
Figure 2 Illustrates the technique used to make brighter SBR holograms. A front surface mirror approximately the same size as the plate is added in a horizontal or flat position directly in front of the plate. As this mirror is lying flat on the surface, there is little room for movement. The beam is expanded to not only cover the plate but to cover the front surface mirror also. As the light enters from above at Brewster’s angle to the film there is some overflow which hits the front surface mirror and is reflected back up to the plate at Brewster’s angle again, but from the bottom of the plate. This light also, travels through the film, reflects off the object and is allows to interfere with the Reference Beam that strikes the film from above. This moves the light ratio more toward an ideal ratio for a SBR hologram which can be as low as 1:1 but typically 1:2 (Object to Reference) is best.
Figure 2 – With the simple addition of a Front Surface Mirror placed on the reference side of the plate the standard Single Beam Reflection Hologram receives additional Object light to interfere with the Overhead Reference Beam. The Object to Reference Beam ratio is total light reflected off the object from both the overhead beam and the reflected beam off the Front Surface Mirror with respect to just the Overhead beam as the Reference beam. In addition the dual lighting from the both the straight Reference beam and the reflected beam off the mirror more evenly illuminates the object which causes less shadows on the object. This can be seen by an increase in the total area shaded by green representing the parts of the object that receive light.
The first variation is changing the Object to Reference light ratios. One may initially think that the Object to Reference beam ratio is fixed at a specific ratio with the above technique but upon further explanation, we shall see this is not so. With a small repositioning of the laser’s Gaussian center, the ratio of Object Light to Overhead Reference Light can be changed. In Figure 3 on the left, the Gaussian center of the expanded laser beam is direct to the center of the plate. This dictates the part of the laser beam reflected off the front surface mirror is relatively low in intensity thus adding only a small addition to the object lighting. Now look at Figure 3 on the right, where the Gaussian center of the expanded beam is directed to the center of the Front Surface Mirror. This dictates there is very high addition lighting of the object with respect to the Overhead Reference beam. By moving the Gaussian center of the expanded laser beam to any position between these points, an ideal Object Beam to Overhead Reference Beam ratio can be achieved for a particular object. It has been found that the best lighting is with the Gaussian center at center of the film plate. This allows a more even illumination of Reference Beam to the plate with the advantage of additional object lighting. But each object needs to be evaluated individually as it has been found that some objects prefer a setup leaning toward the higher object lighting off the Front Surface mirror. It has also be found that DCG has such a latitude that when the Gaussian center is off center, that is when the distribution of reference light is not evenly directly on the film plate, it is minimally or not noticed in the final display hologram with proper exposure and processing.
Figure 4 – On the left the Gaussian center of the Reference beam is positioned at the center of the film plate. On the right the Gaussian Center remains at the center of the film plate but the Front Surface Mirror is tilted to alter the geometry of the reflected light off the Front Surface Mirror. This adjustability can benefit each individual object specifically by allowing lighting of otherwise shadowed areas and allowing a less oblique angle for the reflected secondary object illumination lighting which can direct more light off the Front Surface Mirror to the object. A wedge is used to hold fast the tilted mirror in place and both the mirror and wedge should be completely secure and stable. The angle of the Front Surface Mirror can be changed by using a different size wedge and/or moving the wedge toward and away from the end of the Front Surface Mirror.
To confirm this technique an object was designed that has a protrusion such that the light falling on the object from above causes a shadow below the protrusion and the light falling on the object from below causes a shadow above the protrusion on the object. This object also had a recess such that no light from either top or bottom lighting illuminated the back of the recess. The object was painted flat white. This is illustrated in Figure 5.
Figure 5 – An object was chosen to Holograph that not only had a Protrusion such that each object illumination beam could be seen interfering individually with the overhead reference beam (the other beam is shaded by the protrusion) but also a void area to provide a base line where no light from either beam would reach. Parts of the object that had no shading would receive the benefit of both the Overhead Reference Beam and the Reflected Lighting off the Front Surface Mirror.
A DCG hologram was then made of this object with the described technique (using the front surface mirror in front of the film) at a few different reference beam Gaussian center locations close to and including the Gaussian center to be at the center of the film. In each of the holograms it was clearly evident that the parts of the object that was illuminated with both the overhead and below lightings was brighter then either of the shadowed areas above or below the protrusion and the recess was dark.
An additional confirmation was noted with the below technique as illustrated in Figure 6. The same object was chosen which would provide shaded and unshaded areas. The object was mounted in the described configuration with the front surface mirror in position in front of the film. A flat black card blocker was made which would cover half the front surface mirror in the vertical direction. This was done such that a direct comparison can be viewed between the half of the hologram’s object that received both illuminations and the half that was only illuminated with the original Overhead Reference beam.
Figure 6 – The same object was used except this time half the front surface mirror was covered by a black opaque card blocker. This allowed the two sides to be exposed and processed exactly the same way thus a side by side comparison of the final hologram could be studied. It is clearly evident that some parts that would receive no light are now lighted (the black area on the right compared to its symmetrical dark green part on the left) and thus would be visible in the hologram. Furthermore, all parts that are light green on the left receive and thus reflect additional light back to the plate yielding a more optimal Object to Reference light intensity ratio and ultimately a brighter SBR Hologram..
A DCG hologram was then made of the object with the described technique. In comparing the two separate exposures side by side it was noted that the side receiving the additional lighting by using the front surface mirror as describe, was much brighter and had less harsh shadows compared to the object side that received the overhead lighting only.
· There was no problem of light from the Front Surface Mirror acting as a second reference beam which could cause "crosstalk" when an overhead white light at 56 degrees to the normal was used as long as the angles between the overhead beam and reflected beam off the mirror were far enough apart.
· There was no transmission grating or diffraction seen from the reference beam and the reflected beam off the mirror when the hologram was illuminate with an overhead white light at 56 degrees to the normal.
· Exposure and Processing plays a key role in balancing the additional lighting. If the secondary lighting off the mirror is strong and the exposure is strong, a burn in from the image created with the bottom reference beam becomes viewable even with overhead lighting at 56 degrees to the normal.
· It has been found that this technique works best with what would normally be dim objects and possibly non recordable with the SBR technique. Some objects that are gray or of a low reflectivity paint for the wavelength being used (example: a blue object being exposed with green light) do not reflect enough light to be usable as object for a SBR hologram. But this technique improves the reflected light ratio from object to reference beam allowing that object to be acceptable as a SBR model.
I would like to thank August Muth for getting me started in Dichromated Gelatin Holography. Without his teachings, expertise and patients it would have been years before I may have discovered the versatility of this material and ultimately this technique.
I would also like to thank Jeff Blyth for his very valuable input and suggestions directed at this paper. His suggestions helped me express more clearly my writings and illustrations to portray the essential idea and tests results of this technique.