Joe Farina wrote:I'm wondering if gelatin transparency can become an issue.
There are two issues here...no wait, there are three issues here....no wait, there are {The Spanish Inquisition has two weapons: fear and surprise. Our weapons are fear, surprise and ruthless efficiency...no wait, the Spanish Inquisition has three weapons.... )
Seriously, though, there are three effects: actinic reaction, absorption and scattering. The scattering issue is comprised of two types of scattering: Mie and Rayleigh. The light passing through the system of dichromate and
gelatin can be considered independent, in that, the loss/unit distance of both the actinic reaction and the absorption of
gelatin is just the sum of the individual losses I(x) = I(0) - I(actinic) - I(
gelatin). The actinic part just concerns the laser wavelength. We can model this in terms of the activation energy of the cross linking process. Thus, if each cross linking event causes a loss of hf Joules, and the density of cross linking locations is N, then I(actinic) = hfNA/unit distnce, where A is the beam cross section area. The question then is: what is I(
gelatin).
You have two processes occurring: scattering and absorption, So I(
gelatin) = I(absorption) + I(scattering). The absorption will be lambda dependent, so a transmission plot in a spectrophotometer will give you the absorption loss. The absorption will probably depend on the constituents of the
gelatin, since absorption implies absorption of a photon, then a change of energy in the molecule. This process depends on the molecular structure of the
gelatin. You may be able to use the Kramer-Kronigs relations to model this, if you knew the
gelatin density. I think the real problem is the scattering. If the molecule is large, there will be Mie scattering, but Mie scattering is essentially a white light phenomenon, and so is irrelevant in terms of recording. However, the Rayleigh scattering is proportional to the fourth power of the frequency, and you generally use high frequencies, optically speaking, to record a dcg hologram. Thus, the scattering is exacerbated by the fact that it's necessary to use blue-green to record. However, this also will depend on the
gelatin density, but, in addition, it will depend on the smoothness of the
gelatin, since the smoother the
gelatin, the fewer the scattering centres. One way to test this would be to shine your recording wavelength into a cube of the
gelatin and note the light coming off sideways.