Image reading apparatus with a diffraction grating

Claim 1

An image reading apparatus, comprising: a heat radiation type light source for irradiating a document to cause the document to reflect image light, the light source having a light intensity distribution in which light having a wavelength in a blue light region has a lower intensity than light having a wavelength in either a red light region or a green light region; a slit member having a slit, for receiving the image light reflected from the document and for allowing only a line of the image light to pass through the slit; a diffraction grating element including a transparent plate for diffracting the line of the image light into three lines of zero order light, -1 order light, and +1 order light, and for directing each of the three lines of light onto one of three predetermined positions that are displaced from one another, the transparent plate having a thickness that varies in consecutive steps, wherein the consecutive steps are arranged so that zero order light has a wavelength in the blue light region and +1 order light and -1 order light have wavelengths either in the green light region and the red light region, respectively, or in the red light region and the green light region, respectively; and three image reading means each for reading light in a respective one of the blue, red, and green light regions, wherein the image reading means for reading light in the blue light region is positioned to read only zero order light and the image reading means for reading light in the red light region and green light region are positioned to read only +1 order light and -1 order light, or -1 order light, and +1 order light, respectively.