1. Field of the Invention
The present invention generally relates to document lighting devices used with image readers such as color document readers, image forming devices, and digital laboratories.
2. Description of the Related Art Recently, a light emitting diode (hereinafter, simply called an LED) has been developed and brightness of a LED element has been rapidly improved. The LED generally includes advantages of a longer operating life, a higher effectiveness, a higher G-resistance, a monochromatic luminescence, and a like. One application is a document lighting device of an image reader such as a digital copier, an image scanner, and a like.
FIG. 1 is a schematic diagram showing an image forming device including image reader.
In FIG. 1, an image forming device 1000 mainly includes an image forming part 100 and an image reader 200. Other parts will be cited with reference numbers in the following explanation.
The image forming part 100 includes a latent image carrier 111 being a drum shape, and an charging roller 112 functioning as an charging means, a developing device 113, a transfer roller 114, and a cleaning device 115 are arranged around the latent image carrier 111. A corona charger can be used as the charging means. Moreover, an optical scanning device 117 for conducting an optical scan by a laser beam LB when an original information is externally received is provided as the image reader, or a like, and an exposure by an optical writing process between the charging roller 112 and the developing device 113.
When an image formation is conducted, the latent image carrier 111 being a photoconductive photoreceptor is rotated at a constant speed, a surface of the latent image carrier 111 is uniformly charged, and exposed by the optical writing process with the laser beam LB of the optical scanning device 117, so as to form the static latent image. The static latent image being formed is called a negative latent image where an image is exposed. A cassette 118 accommodating transfer papers P is detachably connected to a main body of the image forming device 100. In a mounting state as shown in FIG. 1, one sheet at upper-most of the transfer paper P accommodated in the cassette 118 is fed by the paper sheet feed roller 120, and a tip of the transfer paper P being fed is caught by a pair of resist rollers 119. The pair of resist rollers 119 sends out the transfer paper P to a transfer part in exact timing with the pair of resist rollers 119 each other to move a toner image to a transfer position on the latent image carrier 111. The transfer paper P sent out to the transfer part is overlapped with the toner image at the transfer part so that the toner image is statically transferred onto the transfer paper P by an operation of the transfer roller 114. The transfer paper P on which the toner image is transferred is sent to a fixing device 116, and the toner image is fixed at the fixing device 116. The transfer paper P is passed through a carrying passage 121, and discharged onto a tray 123 by a pair of paper discharge rollers 122. After the toner image is transferred, the surface of the latent image carrier 111 is cleaned by the cleaning device 115, so that residual toner, paper dust, and a like are eliminated. The latent image carrier 111 is a photoconductive photoreceptor, the static latent image is formed by a uniform charge and an optical scan. The static latent image being formed is visualized as the toner image.
At the image reader 200, an original 202 is placed on a contact glass 201, and is illuminated by a illuminating part (not shown) mounted to a first traveling body 203 arranged at an underpart of a contact glass 201. A reflected light from the original 202 is reflected by a fist mirror 203a of the first traveling body 203. After that, the reflected light is reflected at a fist mirror 204a and a second mirror 204b of a second traveling body 204. The reflected light is led to a reduction image forming lens 205 in order to form an image on a line sensor 206.
In a case of reading the original 202 in a longitudinal direction, the first traveling body 203 moves toward a right direction at a speed V, and simultaneously, the second traveling body 204 moves toward the right direction at half speed ½V of the first traveling body 203, so that the entire original is read out.
In general, a document lighting device used for an image reader requires approximately the same length as a width of the original in order to illuminate the original. In a use method of the LED as the document lighting device, a plurality of LED elements are arranged in an array.
The LED has superior characteristics as described above. However, in order to use the LED as an lighting device of the image reader, each of the LED elements does not have sufficient brightness. The LED is mainly used for a low-speed image reader, a device placing great importance on compactness. Accordingly, mainly, a cold-cathode fluorescent lamp is used for a high-speed image reader, and a large-scale device.
In order to mitigate the above problems, in general, a light volume of the LED array is increased by using a large number of LED elements forming the LED array. However, a light diffuses wider, it is not effective, and it requires more power and cannot improve a power saving. Moreover, effectiveness can be improved by using a shell-shaped LED having less diffusion. However, directivity is higher and non-uniformity occurs in a main scanning direction.
For example, Japanese Laid-open Patent Applications No. 11-232912 (page 2, FIG. 1 and FIG. 9) and No. 8-111545 (page 4, FIG. 3) disclose document lighting devices in that an LED array and a long-sized lens are combined. These document lighting devices attempt to improve the effectiveness by converging a light of the LED on a cross-sectional surface of each LED.
However, if this method is applied, as described in Japanese Laid-open Patent Application No. 11-232912, a center of the converged light is brighter, but a light diffuses and becomes rapidly dark at an outer part of the center of the converged light.
Since almost all of the light of the outgoing light of the LED, having an angle with respect to a cross section in the sub-scanning direction, is wasted, if a large number of LEDs are not arrayed, non-uniformity occurs in the main scanning direction.
In order to eliminate the non-uniformity of the illumination in the main scanning direction, the inventor of the present invention provides Japanese Laid-open Patent Application No. 10-322521. However, Japanese Laid-open Patent Application No. 10-322521 does not disclose a converting method in the sub-scanning direction. The inventor proposes a configuration including an optical waveguide as an optical element, which includes an incident surface in a vicinity of an emitting surface of light flux of a point source and in which the emitting surface is faced to a read area (Japanese Patent Application No. 2003-140927). In this configuration, it is possible to obtain an aimed illumination distribution preferably. However, this configuration is complicated and expensive since a light reflector in addition to the optical waveguide is required.