Reversal development in electrophotography usually requires an external biasing source, especially in the case of continuous tone reproduction. It is further desirable to controll the value of the biase voltage following the surface potentiai, attenuation of the latent image. Such control is quite difficult in liquid development because the dark attenuation of the image potential, during development is rapid and complicated.
This paper deals with a method which requires no external biasing source but, instead, utilizes a latent image bearingsheet which has a uniformly charged frame zone outside the image area. In this method the bias voltage generating from the charge on the sheet, attenuates strictly in the identical manner with the image potential and this can form an excellent quality continuous tone image with low background.
As another modification, a uniformly charged sensitive sheet was used instead of development electrode, giving a similar result.

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In order to investigate the dominant factors of spectral dye sensitizing efficiency in PVK, we have studied the sensitizing behaviors by carbocyanine dyes, styryl dyes and dicarbazolyl methane dyes.
We observed the absorption spectra in various solvents and in PVK film, and investigated the relation between the photosensitivity and these spectral characteristics. Then, we calculated the electronic energy levels of each dye by HMO method and investigated the relation between the photosensitivity and (ε_h0+ε_lv)/2 values. The results thus obtained led to the following conclusions.
(1) The more stable the excited energy state of a dye is in PVK film, the higher its sensitizing efficiency is.
(2) The more electron-acceptive the excited dye is and the more electron-donative PVK or its derivative is, the higher is the sensitizing efficiency.
(3) In the case of the dye sensitization of PVK, the interaction between PVK and sensitizing dye is weaker than in the case of the dye sensitization of ZnO.
Finally, the dye-sensitization model in PVK has been proposed.

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