Bulletins of Indian Industrial Research Improved Reflex-Copying

When exposure takes place the black parts of the original absorb the incident light almost completely while the -white parts ci:Ifuse and reflect the light on to the corresponding parts of the sensitive film. [...] For the sake of clearness let the original to be copied be taken to consist say of a black area B and a white area W. When the exposure is made the light traverses the support (paper glass or celluloid) of the emulsion coated material then the layer of the emulsion and encounters the original in intimate contact with the emulsion. [...] Since t ' is a constant and the same for all the factors to be treated we can deal interms of E instead of I. Let t1 and t2 be the transmission coefficients of the support and the emulsion respectively and let the reflectivity of the black and white portions of the original be B and W respectively. [...] Assuming that the exposures K and L corresponding to the black and the white areas lie on the straight region of the characteristic curve which ought to be the case when faithful rendering of tonal values is to be attempted the representative points P and Q corresponding to the exposures K and L will lie as in the figure when the plates are developed to the correct contrast. [...] A much more scientific method of increasing the transparency of the emusion is to choose the quality of the light used for exposure such that the plate transmits most of the light the transmitted light being actinic.