R. Rallison "Notes and considerations" for DCG
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Joe Farina
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Steven
R. Rallison "Notes and considerations" for DCG
Interesting info on glass bowing.
I wonder what method the big boys use to produce very large format holograms?
I suppose they are using high power lasers (Watts) and short exposures?
I can't imagine that they would be using a very thick glass substrate to resist bending.
Steven.
I wonder what method the big boys use to produce very large format holograms?
I suppose they are using high power lasers (Watts) and short exposures?
I can't imagine that they would be using a very thick glass substrate to resist bending.
Steven.
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Joe Farina
- Posts: 805
- Joined: Wed Jan 07, 2015 2:10 pm
R. Rallison "Notes and considerations" for DCG
Hi Steven,
I've never seen glass bowing, but all of my holograms have been small and done on 3mm (1/8") glass. According to what Rallison said, the effect is more likely to occur with dry, old film on thinner glass. I guess there's no reason why really thick glass can't be used. For example, 3/16" or 1/4." Seldom do DCG holograms get very large, for a number of reasons. There is some more information on bowing in the Rallison section of the wiki, in the "Control of DCG" paper, which has the following:
"The glass may be soda lime plate or float glass or any most any other kind but it has to be thick enough to withstand the shrinking forces generated during exposure. This means that it should be double strength or thicker(3 to 6 mm) for 8 x 10 shots, single strength (2 to 3 mm) for 4 x 5 and 5 x 7, and may be picture glass or as thin as 1 mm for 2 x 2 exposures."
I've never seen glass bowing, but all of my holograms have been small and done on 3mm (1/8") glass. According to what Rallison said, the effect is more likely to occur with dry, old film on thinner glass. I guess there's no reason why really thick glass can't be used. For example, 3/16" or 1/4." Seldom do DCG holograms get very large, for a number of reasons. There is some more information on bowing in the Rallison section of the wiki, in the "Control of DCG" paper, which has the following:
"The glass may be soda lime plate or float glass or any most any other kind but it has to be thick enough to withstand the shrinking forces generated during exposure. This means that it should be double strength or thicker(3 to 6 mm) for 8 x 10 shots, single strength (2 to 3 mm) for 4 x 5 and 5 x 7, and may be picture glass or as thin as 1 mm for 2 x 2 exposures."
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Steven
R. Rallison "Notes and considerations" for DCG
I have a large quantity 2mm picture framing glass which I obtained quite cheaply and a smaller supply of 3mm float glass.
I have observed glass bowing using the 2mm glass, but only at exposures at about 200mJ/square cm and using glass larger than 4inch x 7inch.
The exposures are long, at about 40 minuets. During that long time period significant cross linking would be taking place in the emulsion.
I guess that if you were to hit the 2mm plate with 200mJ/square cm, but over a ten second time period, significant cross linking would not have had time to take place during the exposure.
Steven.
I have observed glass bowing using the 2mm glass, but only at exposures at about 200mJ/square cm and using glass larger than 4inch x 7inch.
The exposures are long, at about 40 minuets. During that long time period significant cross linking would be taking place in the emulsion.
I guess that if you were to hit the 2mm plate with 200mJ/square cm, but over a ten second time period, significant cross linking would not have had time to take place during the exposure.
Steven.
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Dinesh
R. Rallison "Notes and considerations" for DCG
I would suggest a little caution in all this talk of bowing. Rallison says that the glass "may" bow under certain conditions, so it does sound like it's speculation, not supported by any theoretical calculation or experimental data.
Consider however, the conditions under which the glass will bow. In order for this to happen, there must be skew forces on the edges of the glass in a plane perpendicular to the glass. What then may these skew forces be? There is a possible lateral force due to shrinkage along the glass if the "pull" of the receding emulsion were counter-acted by the bonding between the glass and the receding emulsion.In other words, there is a surface bond between the glass/emulsion surface which would resist any "pulling in" of the emulsion along the surface of the glass. However, this alone would not cause any bowing since the forces are parallel to the glass surface. In order to bow the glass, there must be forces perpendicular to the glass also. In addition, if you assume a coordinate system where the z axis is perpendicular to the glass, then any shrinkage of the emulsion must occur in both the x and the y direction. Thus, there may be a shearing force in the plane of the glass causing a couple acting on the edges of the glass, but still no force in the z direction, and so no bowing forces. The shear forces would, at best (or perhaps at worst) cause delamination of the emulsion from the glass. To make this clear, let's assume you have a cardboard sheet. Then you get a thin sheet of plastic or paper and apply glue to it. Then you drag the plastic (glued) sheet across and at an angle along the face of the cardboard. I think you can see that the cardboard would tear along it's surface, depending on the strength of the cardboard, but would not "bow" or tear "outward". There would be no separation of the cardboard surface. In order for the cardboard to 'bow', I must not only drag the plastic (glued) surface, I must also simultaneously pull it perpendicular to the cardboard. So, the question arises, are there any perpendicular forces on the glass as a result of emulsion shrinkage? I think not, since shrinkage is caused by extraction of water from the emulsion as a result of alcohol dessication. Thus, the forces are all on the surface of the emulsion, not at the emulsion/glass interface. The only force at the emulsion/glass interface is the 3rd law force, which is exactly equal and opposite to the force at the surface. There are no "leftover" forces causing the glass to have both a lateral, shear and perpendicular force.
Excess exposure will harden the emulsion, but, again, would not cause any forces to act on the glass. Think, for example, of making jello (jelly, the the Brits). If you make the jello in a thin glass bowl, the setting - "hardening" of the jello does not cause forces perpendicular to the bowl and so cause the bowl to break or crack. Excess exposure may cause lateral forces, if the plate is partially exposed, so that if a beam only about 25 mm (~1 in) exposes a 10 cm x 12 cm (4 x 5 in) plate, then the central exposed area might "pull in" on the surrounding softer emulsion, but the exposure needs to be quite high to overcome the strain forces on the glass.
Consider however, the conditions under which the glass will bow. In order for this to happen, there must be skew forces on the edges of the glass in a plane perpendicular to the glass. What then may these skew forces be? There is a possible lateral force due to shrinkage along the glass if the "pull" of the receding emulsion were counter-acted by the bonding between the glass and the receding emulsion.In other words, there is a surface bond between the glass/emulsion surface which would resist any "pulling in" of the emulsion along the surface of the glass. However, this alone would not cause any bowing since the forces are parallel to the glass surface. In order to bow the glass, there must be forces perpendicular to the glass also. In addition, if you assume a coordinate system where the z axis is perpendicular to the glass, then any shrinkage of the emulsion must occur in both the x and the y direction. Thus, there may be a shearing force in the plane of the glass causing a couple acting on the edges of the glass, but still no force in the z direction, and so no bowing forces. The shear forces would, at best (or perhaps at worst) cause delamination of the emulsion from the glass. To make this clear, let's assume you have a cardboard sheet. Then you get a thin sheet of plastic or paper and apply glue to it. Then you drag the plastic (glued) sheet across and at an angle along the face of the cardboard. I think you can see that the cardboard would tear along it's surface, depending on the strength of the cardboard, but would not "bow" or tear "outward". There would be no separation of the cardboard surface. In order for the cardboard to 'bow', I must not only drag the plastic (glued) surface, I must also simultaneously pull it perpendicular to the cardboard. So, the question arises, are there any perpendicular forces on the glass as a result of emulsion shrinkage? I think not, since shrinkage is caused by extraction of water from the emulsion as a result of alcohol dessication. Thus, the forces are all on the surface of the emulsion, not at the emulsion/glass interface. The only force at the emulsion/glass interface is the 3rd law force, which is exactly equal and opposite to the force at the surface. There are no "leftover" forces causing the glass to have both a lateral, shear and perpendicular force.
Excess exposure will harden the emulsion, but, again, would not cause any forces to act on the glass. Think, for example, of making jello (jelly, the the Brits). If you make the jello in a thin glass bowl, the setting - "hardening" of the jello does not cause forces perpendicular to the bowl and so cause the bowl to break or crack. Excess exposure may cause lateral forces, if the plate is partially exposed, so that if a beam only about 25 mm (~1 in) exposes a 10 cm x 12 cm (4 x 5 in) plate, then the central exposed area might "pull in" on the surrounding softer emulsion, but the exposure needs to be quite high to overcome the strain forces on the glass.