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19. Latensification

In this lesson we shall discuss a very important property of photographic emulsions – dependence of sensitivity on exposure conditions.
One of the basic laws of photochemistry – the so called law of interchangeability (Grotguss-Dreper law [1] asserts that the amount of products of the photochemical reaction depends on the total exposure H, that is on the product of illumination E by the exposure time t and doesn’t depend on E and t taken separately. But soon after elucidation of this regularity it turned out that the law is true in a rather narrow range of exposures. As a rule under ordinary conditions of the day illumination the law is observed by exposure times from the thousandths of one second up to hundreds of seconds. Outside this range sensitivity of the photomaterial begins to fall. This phenomenon was called the Schwarzschild effect.
The Schwarzschild effect is especially noticeable by recording holograms using the pulse neodymium laser which radiates a supershort pulse with a duration of about 20 nsec. According to the Gurney-Mott theory [1] during this short period of time the photons practically simultaneously knock out from the crystal of silver bromide a big number of electrons. They form an ensemble of very small centers of latent image literally consisting of several silver atoms and these centers are located in the lattice defects of the crystal (dislocations) as in the potential wells. In connection with their small sizes these centers can’t create a required density of darkening by development and hence brightness of holographic image turns out to be rather small. Moreover small centers of the latent image are strongly subject to thermal fluctuations of the lattice and can be simply destroyed in the course of time. This happens with the hologram. If it is developed not immediately but the next day the latent image can disappear at all so that development gives nothing. We have to increase the pulse energy what negatively affects laser service life and safety of recording portraits of people.

But in photography another remarkable effect called latensification is known (from the latin word “latent”). Nature of this effect consists in amplification of the latent image created by a short light pulse with the following continuous and weak exposure. Gurney-Mott theory easily explains this effect. By weak exposure of the silver bromide crystal the electrons knocked out by photons in small amounts don’t have time to create new centers of latent image but they are quickly captured by the centers of latent image already created by the first exposure. These centers increase in their sizes (up to 100 and more atoms of silver) and so turn into active centers of development. Development of such centers occurs quickly and density of darkening of the photographic layer becomes normal. The most interesting fact in this effect is that such a weak exposure doesn’t in itself exert influence on the photographic layer that is it doesn’t cause darkening by development and in such a way it doesn't worsen quality of the hologram. It can only intensify the first pulse exposure.
Let us go over to determination of the optimum time of latensification [2]. For our setup a photographic lantern with a mat bulb 40 Watt and with a green filter or without it was used. The distance between the lantern and the hologram was equal to 80 cm. The photoplate was cut into several small pieces. All pieces but one were in turn placed at the prescribed distance from the lantern and were exposed during different time intervals. Not exposed part is necessary for evaluation of the photoplate density veil. Then all parts were developed in the same developing tray and fixed (formulas of solutions will be given in the next lesson). All parts will have different density of blackening. Optimum time of latensification is determined as a time by which density of blackening of the latensified photoplate begins to exceed the veil density which is determined using the not exposed plate.

In the fig. above the dependences of darkening density of the test photographic plates on the latensification time averaged by two series are shown. The horizontal line on the graph is a veil density of the photoplates used in the experiment. As can be seen from the graph optimum time of latensification is equal to 2 min. for the lantern with a green filter and 10 sec. for the lantern without a filter.

In the fig. above the dependences of darkening density of the exposed holograms on the latensification time averaged by two series are shown. Horizontal line is a darkening density of the exposed hologram without latensification. It follows from the graph that optimum latensification secures increase of opacity of the exposed photoplate approximately by a factor of ten (D=1). From this point of view we can speak about tenfold increase of sensitivity of the VRP photoplates. Visual evaluation of quality of the restored image on the whitened holograms has shown a significant increase of the image brightness by latensification of the hologram. Type of the latensification source didn’t affect quality of the hologram. And another advantage of latensification, which can't be totally explained, consists in the following: after latensification uniformity of the reference beam across the field of the photoplate increases. This fact has a positive influence on uniformity of brightness of the restored image when it is observed from various points of view within the observation zone.

1. T. H. James, The Theory of the Photographic Process, Macmillan Publishing Co., New York, 1977.
2. Sergey P. Vorobyov, The sensitization of VR-P Russian photoplates for recording pulsed holograms, SPIE, v. 3358, 1997, p. 67.