LATENT IMAGE (Fr., Impression latente ; Ger.,,Latentes Bild) The action of light upon many sensitive sub stances is at once visible by a change of colour, as in the darkening of silver or bichromate salts on paper. This is known as a direct light action. In other cases there is no visible change, and the exposed material has to be treated with some agent, usually termed the developer, which renders the action of light visible. The action of light is thus said to produce a latent image, which is of such a nature that the result of the light action cannot be quantitatively or chemic ally recognised.
The exact nature of the latent image has been a subject of much dispute, but the theories may be divided into two sections, the physical and the chemical. Advocates of the former consider that the action of light is to produce some change in the physical character of the silver salts, whilst the adherents to the chemical theory assume that there is an actual chemical change and the formation of a lower haloid salt, which is usually called a suhhaloid.
Bearing in mind that it has been proved by Dewar that a photographic plate possesses the power of forming the latent image at tempera tures closely approaching absolute zero, whilst every other known chemical action ceases at a much higher temperature, it may be asked why the photo-sensitive salts of silver should be an exception. Dr. Bose, a well-known physicist, would liken the formation of the image to the strain of the silver salt under the electric force in the light-wave, a theory known as that of molecular disturbance or strain theory. Accord ing to this, the silver bromide is converted into an allotropic form, which is more readily reduced to the metallic state than the normal silver haloid, and the function of the sensitiser is then to retard the recovery from the strain. There are undoubtedly many parallels between the strain phenomena in metallic silver and other substances under the electromagnetic radiation of light and the effects of exposure of the photo graphic plate. But this theory hardly explains the various latent images which may be formed on the silver haloids, for instance with silver bro mide and iodide. In the former case, so far as experience goes, the latent image is permanent, whilst with iodide there is retrogression of the image or it fades away, and the sensitive salt returns to its non-developable original state. It is
stated above that the physicist looks upon the sensitiser as a substance which prevents the recovery from the strain, and a correlative action has been found in the case of calcium oxalate for pure electric response from mechani cal and light stimuli. When the action of light is permanent—that is to say, no matter how long the exposed sensitive material be kept the product of the light action, the latent image, may be developed. This is then known as an irrever sible action ; but if, on the other hand, the latent image reverts to its original and undevelopable condition, the action is reversible, and assuming that the formula for silver subiodide be taken as Ag,I, then this may be expressed by the follow ing equation :— In light 5 AgI = 2 In darkness That is to say, in light the five molecules of AgI are split up into two molecules of subiodide, and one molecule of silver triiodide, whereas if such a mixture be kept in the dark, the two molecules of subiodide and one molecule of triiodide rearrange themselves and reform five molecules of silver iodide. Now as this action takes place in the presence of gelatine, but does not take place in the case of silver bromide, it seems a somewhat difficult point to explain from the point of view of the physicist.
Another physical theory is that the silver salts are charged electrically, and that the action of light is to ionise them or set free the electrons, and in support of this theory is advanced the fact that the of silver are vigorously electric and in the order of bromide, chloride, and iodide, which is the same as their order of sen sitiveness to ultra-violet light. As considerable support to this theory is also adduced the fact that eosine, fuchsine, cyanine, and other dyes set free electrons under the stimulus of light, and that the particular wave-lengths absorbed by these substances are those which are most effective in liberating the electrons. In other words, the photo-electric activity displayed is dependent upon their colour absorption, and there is thus an exact parallel with the sen sitising power of these dyes for the photo salts of silver.