This invention relates to a copying machine having two drums.
A copying machine having two drums, namely a memory drum and a transfer drum, has been disclosed in Japanese Patent Application No. 37289/1980 filed by the present applicant. In such a copying machine, the memory drum 1 is spaced a predetermined distance from the transfer drum 2, and both are arranged on the optical axis of an image forming lens 3 which is interposed between the two drums 1 and 2, as shown in FIGS. 1 through 3. A light source 4 for exposure is provided near the memory drum 1. A mirror 5 is interposed between the memory drum 1 and the transfer drum 2 and is on the central optical axis of the lens 3 (hereinafter referred to as "an optical axis 9"). The mirror 5 can be retracted from the path of the optical axis 9. An original 6, an original stand 7 and a light source for illuminating the original 6 are shown in the upper part of each figure.
In operation, the light source 8 irradiates the original 6 placed on the transparent original stand 7. As the transparent original stand 7 is moved while the light source 8 remains stationary, light from the light source 8 irradiates the original 6 on the transparent original stand 7 from one end to the other end and provides the optical data of the original to the memory drum 1 through the mirror 5 and the image forming lens 3. As the memory drum 1 rotates in synchronization with the movement of the original stand 7, an electrostatic latent image of the original 6 is formed on the memory drum 1. The latent image is developed with toner into a memory image.
Subsequently, the illuminating device 4 applies light to the transfer point 1a of the memory drum 1 as the latter is rotated. The incident light is reflected, in the form of optical data of the memory image formed on the memory drum 1, along the optical axis 9, and is then applied to the transfer drum 2 through the image forming lens 3.
As the transfer drum 2 rotates in synchronization with the memory drum 1, a secondary electrostatic latent image is formed on the transfer drum 2 in accordance with the optical data of the memory image received from the memory drum 1. The latent image is developed with toner into a toner image which is transferred onto a sheet or the like to complete the copying operation.
An important feature of this type of copying machine resides in that, once the secondary original is formed through the first image forming operation, a plurality of copies of the original can be obtained in a short time by repeating the second image forming operation. For this purpose, the reflection coefficient of the memory drum 1 is made high, and its surface smooth, so that the overall reflection characteristics of the memory drum 1 will be high. However, the copying machine is inefficient in that light from the light source 4 is regularly reflected by the memory drum as shown in FIG. 2, and a part of the energy of the light source 4 consequently is applied to the lens 3. The difficulty may be eliminated by employing a method in which the reflection characteristic of the surface of the memory drum 1 is made more diffuse to allow a part of the relfected light to pass through the lens. However, the method is still disadvantageous in that the efficiency of diffuse reflection is generally low, resulting still in a loss of energy of the light source 4. Part of this problem is due to the fact that the central optical axis of the lens 3 and the center of at least one of the two drums lie on the same straight line.
A further cause of inefficiency will be explained with reference to FIG. 4. When the copying machine of FIG. 1 is viewed from above as shown in FIG. 4a, a light beam 21 reflected from the center of the memory drum 1 is applied to the entrance pupil 3a of the image forming lens 3, thus contributing to the formation of an image on the transfer drum 2; however, some of the light beams applied to the peripheral portion 22 of the memory drum 1 are not reflected to the entrance pupil 3a of the lens 3 as indicated by the broken line 23 but are instead reflected outside of the lens. These light beams cannot contribute to the formation of an image on the transfer drum 2, thus further contributing to inefficiency.
The copying machine may be improved by employing a method in which the smooth surface of the memory drum 1 is modified into a rough surface but the improvement will generally not be sufficient.
In another method of improving the copying machine, the illuminating device 4 can be replaced by an elongated light source 25 as shown in FIG. 4b. In this method, a memory image formed on a peripheral portion of the memory drum 1 is irradiated by a ray 27 from a point 26 on the light source 25 and the incident light is reflected, as optical data, to the image forming lens 33. However, the light beam from the point 26 is scattered in various directions 27', 27", and so forth, different from the direction of the ray 27, and therefore the loss of energy is still considerable due to the rays indicated by 27', 27", etc. If the light source 25 is nonuniform in luminance because of an unsatisfactory filament arrangement, then an image formed on the transfer drum 2 will include bright stripes, and accordingly the resultant copy is irregular.