In electrophotography, known processes for converting an electrostatic latent image formed on a photoconductive photoreceptor or the like into a visible image with a toner include, for example, magnetic brush development, cascade development and powder cloud development. Toners for use in these development processes are generally mixtures of a thermoplastic resin and a colorant. The toner image formed on a photoconductive photoreceptor or the like by the above or other development processes is transferred to a support such as paper and fixed thereto by pressing and/or heating. With the recent increasing requirement to provide higher image quality in copies, various improvements have been tried to make in both copiers and developers. In particular, a technique frequently used for improving image quality is to employ a toner having a reduced average particle diameter. The use of a toner having a reduced average particle diameter is an effective means for improving image quality. However, this technique is disadvantageous in that triboelectric charging amount is enhanced, resulting in difficulties in obtaining a desired color density, and that the amount of charges which the toner can have per particle decreases, resulting in blurring. Various limitations are hence imposed on the use thereof.
Employed for improving the developing ability are, e.g., a technique of using a carrier having a reduced diameter so as to have an enlarged chargeable surface area and/or a reduced electrical resistance or a technique for development in which toner particles are flown by means of an AC electric field. These techniques are effective in enabling toners having a high-triboelectric charging amount and a small diameter to have a highly improved developing ability and to satisfactorily ensure a maximum color density in solid parts of from 1.5 to 1.9. With respect to the line density in images having a width of from 150 to 500 .mu.m, the above described conventional techniques are capable of ensuring a line density of 1.0 or higher to enable line or character images made up of such thin lines to be clearly recognizable.
However, the conventional techniques described above have the following problems. In the reproduction of lines or characters (6 to 8 points) having a line width of 100 .mu.m, which reproduction is recently required to attain further higher image quality in terms of line density and character image density, the reproduced line or character images have a image density of 1.0 or lower and are hence light and unclear. Even when it is attempted to form black toner particles in two or more layers so as to form a thin line having a high image density, the resulting black toner particles are only formed in almost a single layer. This is because the electrostatic latent images on the photoreceptor which correspond to the thin lines have a smaller charge amount unlike solid parts.
If the content of carbon black as a colorant is increased in order to obtain a high density, use of the resulting black toner for development in an AC electric field poses a problem that blurring is caused by charge injection due to combinations of the black toner containing a large proportion of carbon black and having a low electrical resistance with a carrier having a low electrical resistance, resulting in image defects.
Furthermore, if an electrostatic latent image corresponding to thin lines or small characters (6 to 8 points) having a line width of 100 .mu.m is formed on a photoreceptor in such a manner that the resulting image has a line width larger than 100 .mu.m in order to heighten only the image density of these lines and characters, there are problems, for example, that the resulting images are deformed although a heightened image density is obtained. In this method, repeated copying operations result in an increase in the line widths of both the thin lines and the characters.