by Din » Sun Jan 17, 2021 1:56 pm
The difference is the way in which bleaches work.
The point of the bleach is to convert an amplitude hologram to a phase hologram. The emulsion is a silver halide, which is clear. When you develop the hologram, the silver halide - a clear substance - to silver which is black, an amplitude hologram. This works by robbing some of the light used to reconstruct the hologram. So, an amplitude hologram is weaker, because less light goes through the hologram. If you simply develop (and, maybe, fix) the hologram, the hologram is dark. A phase hologram does not rob any of the light used for reconstruction, ideally. In practice, there is always some loss. A bleached (phase) hologram works because it changes the density, very locally, inside the emulsion - some parts of the emulsion are denser than other parts. The denser the part of the emulsion, the more the light is "delayed", the more some parts are 'delayed with respect to other parts, the more the light is altered from the 'pure' reconstruction beam to a structured, image forming beam. It is this loss and creation of density variations that determines which bleach to use, and the loss and density variations work with the developer. So, as Petr says, you have to match the bleach to the developer.
There are two basic differences in the way bleaches work - rehalogenating bleaches and reversal bleaches.
In a rehalogenating bleach, the developer converts the exposed silver halide grains in the emulsion to silver, which is black. Hence, the plate goes dark. In order to change the dark plate, which, remember, absorbs light, to a clear plate which does not absorb light, it's necessary to make the plate go back to it's unexposed state, when it was clear; but, you cannot take it all the way back to its 'virgin' state, because you must retain the image creating, black, parts of the emulsion. So, you have to alter the black, exposed, silver back to the original silver halide - but only where there was image forming black silver, and wash away all the unexposed parts which did not go black on development. So, you 'rehalogenate', ie convert the silver back to a silver halide.
In a reversal bleach, you dissolve away the exposed, black silver, leaving only the unexposed silver halide. This does not convert the exposed black silver into a clear silver halide, it removes the black silver. In this case, the image 'data' is not in the exposed parts of the emulsion, it's in the unexposed, 'opposite' parts of the emulsion. You reverse the image forming data into it's exact opposite.
After these two major categories, the next part of the choice rests with the type of developer you use. Some developers are very aggressive, and convert silver halide into silver very quickly. If you don't stop development at the right time, unexposed parts will also go black. In this case, you want a bleach that may act slowly, so you allow the bleach to convert the exposed silver, but stop bleaching as the bleach attacks the unexposed part which have also gone black. In other situations, depending on the developer, you may have underdeveloped. In this case you may use a bleach that corrects underdevelopment. So, you have to 'marry' the correct development scheme to the development scheme.
The difference is the way in which bleaches work.
The point of the bleach is to convert an amplitude hologram to a phase hologram. The emulsion is a silver halide, which is clear. When you develop the hologram, the silver halide - a clear substance - to silver which is black, an amplitude hologram. This works by robbing some of the light used to reconstruct the hologram. So, an amplitude hologram is weaker, because less light goes through the hologram. If you simply develop (and, maybe, fix) the hologram, the hologram is dark. A phase hologram does not rob any of the light used for reconstruction, ideally. In practice, there is always some loss. A bleached (phase) hologram works because it changes the density, very locally, inside the emulsion - some parts of the emulsion are denser than other parts. The denser the part of the emulsion, the more the light is "delayed", the more some parts are 'delayed with respect to other parts, the more the light is altered from the 'pure' reconstruction beam to a structured, image forming beam. It is this loss and creation of density variations that determines which bleach to use, and the loss and density variations work with the developer. So, as Petr says, you have to match the bleach to the developer.
There are two basic differences in the way bleaches work - rehalogenating bleaches and reversal bleaches.
In a rehalogenating bleach, the developer converts the exposed silver halide grains in the emulsion to silver, which is black. Hence, the plate goes dark. In order to change the dark plate, which, remember, absorbs light, to a clear plate which does not absorb light, it's necessary to make the plate go back to it's unexposed state, when it was clear; but, you cannot take it all the way back to its 'virgin' state, because you must retain the image creating, black, parts of the emulsion. So, you have to alter the black, exposed, silver back to the original silver halide - but only where there was image forming black silver, and wash away all the unexposed parts which did not go black on development. So, you 'rehalogenate', ie convert the silver back to a silver halide.
In a reversal bleach, you dissolve away the exposed, black silver, leaving only the unexposed silver halide. This does not convert the exposed black silver into a clear silver halide, it removes the black silver. In this case, the image 'data' is not in the exposed parts of the emulsion, it's in the unexposed, 'opposite' parts of the emulsion. You reverse the image forming data into it's exact opposite.
After these two major categories, the next part of the choice rests with the type of developer you use. Some developers are very aggressive, and convert silver halide into silver very quickly. If you don't stop development at the right time, unexposed parts will also go black. In this case, you want a bleach that may act slowly, so you allow the bleach to convert the exposed silver, but stop bleaching as the bleach attacks the unexposed part which have also gone black. In other situations, depending on the developer, you may have underdeveloped. In this case you may use a bleach that corrects underdevelopment. So, you have to 'marry' the correct development scheme to the development scheme.