1. Field of the Invention
A certain aspect of the present invention relates to an image scanning device, an image forming apparatus, and a light source failure detection method.
2. Description of the Related Art
In these days, light-emitting diodes (LED) are more commonly used than xenon lamps as light sources for scanning a document in apparatuses such as a multifunction peripheral (MFP) and a scanner. Using LEDs as light sources makes it possible to reduce the start-up time, to save energy, and to lengthen the service lifetime of an apparatus. When using LEDs in a reducing optical system, the illuminance on a document surface needs to be higher than that in an 1X optical system because, compared with an 1X optical system, the distance between the document surface and an image sensor is longer and the attenuation of light is larger due to the longer distance. Therefore, in a reducing optical system, multiple LEDs are arrayed in the main-scanning direction to increase the luminance on the document surface.
Typically, in such an LED illumination system, serially-connected LEDs are arranged in the main-scanning direction to illuminate a document in the main-scanning direction. With this configuration, however, the document is not correctly scanned and an inaccurate image is formed if just one of the LEDs fails.
There are various known methods for detecting failure of LEDs. In one method, a moving body on which a light source is mounted is moved to an area where visual inspection is carried out. In another method, light reflected from a white reference part used as a white reference in shading correction is detected and failed LEDs are determined based on abnormal distribution of the detected light (see, for example, Japanese Patent Application Publication No. 2007-150934 and Japanese Patent Application Publication No. 2009-10830)
Japanese Patent Application Publication No. 2007-150934 discloses an LED failure determining, method where all LEDs are turned on, light reflected from a reference plate used for shading correction is detected to obtain reference plate data, and failed LEDs are determined based on abnormal distribution of the reference plate data.
Japanese Patent Application Publication N 2009-10830 discloses an LED failure determining method where each group of adjacent LEDs connected to the same light intensity control unit are defined as a block and blocks including failed LEDs are identified based on reference plate data obtained by scanning a reference plate used for shading correction.
In another LED failure determining method, LEDs are separately turned on and off to determine failed LEDs (see, for example, Japanese Patent Application Publication No. 2007-243580 and Japanese Patent Application Publication No. 2002-320070). Japanese Patent Application Publication No. 2007-243580 discloses a technology where LEDs are selectively turned on during pre-scanning to reduce power consumption. Japanese Patent Application Publication No. 2002-320070 discloses a technology where LEDs are sequentially turned on and light reflected from a reference white plate is detected to determine failed LEDs and thereby to compensate for decrease in light intensity.
Also, there is a technology for allowing the user to determine failed LEDs by visual inspection (see, for example, Japanese Patent Application Publication No. 2008-199247). Japanese Patent Application Publication No. 2008-199247 discloses a technology where a carriage holding LEDs is moved to a position at which the LEDs can be seen from the outside and the LEDs are turned on sequentially or selectively.
Further, there is a technology for separately mounting LEDs (see, for example, Japanese Patent Application Publication No. 2008-172389). Japanese Patent Application Publication No. 2008-172389 discloses a technology where LED arrays each including at least one light-emitting element are separately held on a mounting table so that LEDs can be efficiently replaced.
However, if all LEDs are turned on at the same time as disclosed in JP2007-150934, it is difficult to detect failure of a small number (e.g., 1 through several) of LEDs. Similarly, if all LEDs in each block are turned on at the same time as disclosed in JP2009-10830, it is difficult to detect failure of a small number (e.g., 1 through several) of LEDs in the block.
In an LED illumination system, the pitch. between LEDs is set at a small value to increase the number of LEDs and thereby to achieve a high light intensity. Therefore, even if one to several LEDs in a large number of LEDs are at fault and their light intensity is low or they are not emitting light, it is difficult to distinguish the resulting decrease in the light intensity from that caused, for example, by variation in the sensitivity of image sensors (e.g., CCDs), variation in the drive current, or blocking of light by a stain or dust in the optical path. Also, it is difficult to detect failed LEDs at both ends in the main-scanning direction because it is difficult to distinguish the decrease in the light intensity caused by the failed LEDs at the ends from that caused by inaccurate mounting positions of LEDs.
Meanwhile, it is possible to detect even one failed LED by turning on LEDs selectively or separately as disclosed in JP2007-243580, JP2002-320070, and JP2008-199247. However, with the related-art configurations, complicated wiring is necessary to selectively or separately turn on LEDs.