1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image and outputting an image signal, and a light source unit used with the image forming apparatus.
2. Related Background Art
As one of conventional image forming apparatus, a light source switching type color image sensor is known which reads a color image by applying light beams having three different spectral characteristics to the same color image and outputting image signals. FIGS. 1 to 4 show an example of such an image forming apparatus. This image forming apparatus is constituted by LEDs of red, green, and blue (hereinafter abbreviated as R, G, and B) colors, a short focal point focussing element array, and a sensor array with a plurality of line sensors being disposed in line.
FIG. 1 is a perspective view showing the image forming apparatus, and FIG. 2 is a cross sectional view of the image forming apparatus. Referring to FIGS. 1 and 2, in the fundamental structure of the image forming apparatus, a light beam 212 outputted from an LED array 211 on an LED substrate 210 mounted on a frame 200 is applied to an original which is in contact with a transparent glass plate 201 mounted on the upper area of the frame 200, and a light beam 213 reflected from the original is applied via an optical system 209 to a sensor array 1 on a sensor substrate 19.
As shown in FIG. 3, the LED array 211 has a plurality set of LED chips 211R, 211G, and 211B alternately disposed in line on the LED substrate 210. The LED chips 211R, 211G, and 211B emit RGB light beams, and light of each color of RGB can be independently turned on and off. The optical system uses a short focal point focussing element array, for example, a product name "Selfoc Lens Array" manufactured by Nippon Sheet Glass Co., Ltd.
As shown in FIG. 4, the sensor array 1 has a plurality of line sensors 2-1, 2-2, . . . , 2-15 disposed in line on the sensor substrate 19, the line sensors being covered with a protection film 206. A tight contact type multi-chip image sensor fundamentally reads an image by applying a light beam reflected from an original to a sensor array and focussing an image of the same size as an original. Therefore, the length of the sensor array 1 is required to be equal to or longer than the width of an original.
The length of the sensor array 1 changes with the size of an original to be read, and the number of line sensors of the sensor array 1 changes. For example, for reading of an A3 size original, the sensor array has fifteen line sensors assuming that the length of each line sensor is 20 mm.
The sensor substrate 19 is coupled via a flexible substrate 208 to another substrate 203 on which a connector 202 for input/output of a power source and control signals is mounted. The substrate 203 is fixedly mounted on the frame 200 by means of screws 207.
Next, the read operation of the image forming apparatus will be described. First, data for correcting shading error is read, the shading error being generated by a variation of line sensor sensitivities and a variation of emission of a light source. In reading shading correction data, LED chips 211R, 211G, and 211B are sequentially turned on to read a white reference plate built in the image forming apparatus, and the output signals of the image sensor are temporarily stored in memories provided for respective colors.
By reading the sensor output signals r1 for LED 211R, g1 for LED 211G, and b1 for LED 211G obtained by independent emission of RGB light sources and stored in the memories, the gain of each color is adjusted to satisfy the condition of r=g=b where r, g and b are sensor output signals for RGB colors obtained when the white reference plate is again read.
In reading an original with a light source switching color image sensor, it is necessary to independently apply RGB light beams to the original in order to obtain three RGB signals. To this end, a frame sequential method and a line sequential method are used. With the frame sequential method, LEDs of one color among RGB colors are turned on to sub-scan the whole frame of the original, this operation being repeated for the other two colors. With the line sequential method, LEDs of three colors are sequentially turned on for each line of an original to sub-scan the whole frame. Both the methods can obtain RGB signals of the whole area of an original to reproduce a color image.
The ideal spectral characteristics of RGB light sources for a light source switching color image sensor will next be described. A G light source is used by way of example. As shown in FIG. 5, it is assumed that an original image is read by using three G light sources each having a different spectral characteristic. A light source G6 does not contain light in the wavelength ranges from near 480 to near 500 nm and from near 570 to near 590 nm, as compared to the light source G7.
Therefore, if colors a and b shown in FIG. 6 having different spectral characteristics only in the wavelength range near 500 nm are read by using the light source G6, a difference of the spectral characteristics between the colors a and b cannot be discriminated and generally a same G signal is obtained for the colors a and b.
If the B light source having a shorter wavelength than the G light source does not contain light in the same wavelength range as the light source G6, then the colors a and b cannot be discriminated. In order to improve color discrimination between various colors contained in a color original, the spectral characteristics of the RGB light sources are required to cover the whole visible light range.
Next, a difference of color reproduction between the light sources G7 and G8 will be described. Light of the light sources G7 and G8 covers the same wavelength range, and only the energy distribution in the wavelength range is different. Color spaces of a light source switching color image sensor using the light sources G7 and G8 are shown in FIG. 7.
The diagram shown in FIG. 7 is called a CIE-xy chromaticity diagram. In FIG. 7, all colors are contained in an area surrounded by a solid curve line representative of a spectrum locus or reddish-purple line. Triangles in this area represent color spaces of the color image sensor. An output G.sub.OUT of an image sensor when an original is applied with light from the light source G7 or G8 is given by the following equation. EQU G.sub.OUT =.intg.G7(.lambda.)(or G8(.lambda.))S(.lambda.)d.lambda.
where G7(.lambda.) represents a spectral emission characteristic of LED G7, G8(.lambda.) represents a spectral emission characteristic of LED G8, and S(.lambda.) represents a spectral sensitivity characteristic of a line sensor.
Color reproduction is made not by measuring the detailed spectral reflection characteristic of an original, but by using RGB signals. As seen from FIG. 7, the color image sensor using the light source G7 has a broader color space than the light source G8. The spectral characteristic of the light source G7 for a light source switching color image sensor is more desirable than the light source G8.
The ideal spectral characteristics of RGB light sources for a light source switching color image sensor are therefore required to have as broad color spaces as possible and cover the whole wavelength range. An LED light source has many advantages such as compact size, high response speed, and good reliability, over other tubular type light sources. Therefore, it is suitable for use with a light source switching color image sensor.
Color reproduction using RGB signals of an LED light source switching color image sensor is, however, associated with some problems. FIG. 8 shows an example of a color space of a conventional LED light source switching color image sensor. As seen from FIG. 8, the color space of the image sensor is rather narrow as compared to various colors in a natural world. This results from that the spectral characteristic of an LED of G color is positioned too near the long wavelength side and that there is a wavelength range having too small light emission. There are LEDs for three colors having the spectral characteristics which can solve the above problems and realize ideal color reproduction. However, such LEDs are very expensive and the manufacturing cost of an image forming apparatus becomes too high. In contrast, general LEDs used for display devices or other devices are mass-produced and relatively cheap. If such display LEDs are used as light sources of an image sensor, the cost can be reduced considerably.
However, the display LED has a sharp spectral characteristic with a small full width at half-maximum. It is necessary for a light source of a light source switching color image sensor to cover the whole visible light range by using LEDs of three RGB colors. Therefore, if display LEDs are used as the light sources for an image sensor, they are associated with the above problems such as a wavelength range with an extremely small emission amount and poor color reproduction, because the display LEDs have too narrow full width at half-maximum.
In a conventional image forming apparatus, LED chips of three RGB colors are disposed at an equal pitch. Therefore, as shown in FIG. 9, an incident angle of light from an LED for each color is different for each color at an arbitrary point on an original. As a result, the optical information of an original supplied to a sensor pixel train, i.e., the intensity of vertical components of a reflected light beam, is different at each point on the original. From these reasons, even if an original having a uniform density is read, the color component ratio is different at each point of the original to thereby result in color shade. As shown in FIG. 10, the size of shadow at the corner of a convex portion of an uneven original such as an original with a pasted sheet changes with the color component, coloring the shadow at the corner.