1. Field of the Invention
The present invention relates to a light-source control device, a light-source control method, an image reading device, and an image forming apparatus.
2. Description of the Related Art
A conventional method is known which uses a chip light emitting diode (LED) as a light source that is included in a scanner device and emits white light. LEDs have a superior responsiveness than conventional xenon lamps; therefore, the lighting intensity of an LED can be controlled by using a pulse width modulation (PWM) drive to control the lighting-up time. A method of controlling LEDs using PWM drives has already been put to practical use.
For example, Japanese Patent No. 3368890 discloses a technology in which, when a color is represented by using multiple LEDs that each have a different color, the pulse width of a PWM signal for the PWM drive is adjusted, a drive current is reduced for an LED for which color deviation is large, and the drive current is allocated to the other LEDs. According to Japanese Patent No. 3368890, it is possible to reduce color variation within a plane or line where the variations are due to variations in the chromaticity of individual LEDs, or the like.
An LED that emits white light can be configured from a blue LED that emits blue light and a yellow fluorescent material that is installed around the blue LED and emits a yellow fluorescence. With this configuration, the yellow fluorescent material is excited by the emission of the blue LED so that it emits light, and pseudo white light emission can be obtained by using the emission of the yellow fluorescent material and the emission of the blue LED. In the following, a light emitter that is configured from a blue LED and a yellow fluorescent material is simply referred to as a white LED.
If the white LED is lighted up by using a PWM drive according to a conventional technology, there is a problem in that the color of light emitted by the white LED changes in accordance with any change in the duty ratio of the PWM signal during adjustment of the lighting intensity.
A detailed explanation is given of a change in color in accordance with a change in the duty ratio of the PWM signal during the PWM drive. FIGS. 18A and 18B represent the emission response characteristics of a white LED with respect to the drive current. A blue LED and a yellow fluorescent material of the white LED have different emission response characteristics with respect to the drive current. Specifically, as illustrated in FIG. 18A, the blue LED responds to the drive current instantaneously, and the intensity of emitted light reaches its maximum light intensity at almost the same time as the drive current rises. Conversely, as illustrated in FIG. 18B, the response of the yellow fluorescent material with respect to the drive current is governed by a predetermined time constant, and a certain amount of time is required from when the drive current rises to when the intensity of emitted light reaches its maximum light intensity.
FIG. 19 illustrates an exemplary configuration of a circuit for driving an LED that is the light source of a scanner device. In this example, groups 600a, 600b, . . . , 600n, in which multiple LEDs are connected in series, are driven (hereafter, a group having multiple LEDs connected in series is referred to as an LED array, and the groups 600a, 600b, . . . , 600n are described as LED arrays 600a, 600b, . . . , 600n). An input voltage is boosted by a booster circuit that includes a coil L100, a zener diode ZD100, a transistor Q100, and a capacitor C100 in accordance with a booster clock, and the boosted voltage is applied to one end of each of the LED arrays 600a, 600b, . . . , 600n. The other end of each of the LED arrays 600a, 600b, . . . , 600n is connected to a constant current circuit.
FIG. 20 illustrates an exemplary circuit configuration of a constant current circuit. For example, a constant current source that includes an operational amplifier OP100, a reference voltage E100, a load resistor R100, and a transistor Q101 is often used for a PWM drive for a white LED, as illustrated in FIG. 20. The PWM drive for the LED arrays 600a, 600b, . . . , 600n (abbreviated as LED D600 in the drawing) is performed by controlling a switch circuit SW100 so that it is turned on/off in accordance with a PWM signal and by switching on/off the transistor Q101.
FIGS. 21A and 21B illustrate an example of the characteristics of the constant current circuit illustrated in FIG. 20. In this constant current circuit, when the current falls, the current pathway is instantly blocked due to switching of the transistor Q101; therefore, sharp characteristics are obtained, as illustrated in FIG. 21B. This example shows that the current falls to zero about 60 nanoseconds (nsecs) after the transistor Q101 is turned off.
In this constant current source, the load resistor R100 acts as a current-limiting load; therefore, the characteristics obtained when the current rises are slower than the characteristics obtained when the current falls. FIG. 21A illustrates an example of a time change in the current when the current rises. This example shows the current reaches a constant current about 2.8 μsec after the transistor Q101 is turned on.
If the difference between the current values obtained before and after the current is changed is 100%, the time it takes to change from 10% to 90% when the current rises is referred to as the rise transition time. Similarly, if the difference between the current values obtained before and after the current is changed is 100%, the time it takes to change from 90% to 10% when the current falls is referred to as the fall transition time.
FIG. 22 illustrates an example of the light emission behavior obtained when the white LED that has the characteristics illustrated in FIGS. 18A and 18B are driven by using a constant current source that has the characteristics illustrated in FIGS. 21A and 21B. The drive current slowly rises, and the emission by the yellow fluorescent material in the white LED slowly responds to the drive current. Therefore, during the rise transition time, the blue LED and the yellow fluorescent material in the white LED enter a light-on state over the same period of time (the period A in FIG. 22).
Conversely, during the fall transition time, as the blue LED has superior emission response characteristics with respect to the drive current, the blue LED enters a light-off state at almost the same time as the drive current is changed. However, the yellow fluorescent material has inferior emission response characteristics with respect to the drive current as compared to the blue LED; therefore, it takes time to follow a change in the drive current (the period B in FIG. 22). During the fall transition time, a state occurs in which, although the blue LED has already entered a light-off state, the yellow fluorescent material still emits light (residual light), which is illustrated as the area Ye in FIG. 22. In a state where there is residual light from the yellow fluorescent material, the white LED emits light of a different color (yellow) to the original emitted light, which was white.
This indicates that the light emission time of the yellow fluorescent material becomes longer than that of the blue LED during one cycle of lighting on/off of the white LED. Therefore, if the duty ratio of the drive current becomes lower or if the cycle of the drive current becomes shorter, the ratio of the light emission time of the yellow fluorescent material to the light emission time of the white LED relatively increases.
Thus, the color of the white LED is changed in accordance with a change in the duty ratio of the drive current or the cycle of the drive current. FIG. 23 illustrates an example of the actual measured values of changes in the color of the white LED, where they are represented on a CIE-xy chromaticity diagram. As illustrated in FIG. 23, when the duty ratio of the drive current becomes lower or when the cycle of the drive current becomes shorter, the color of light emitted by the white LED is changed such that the values on the chromaticity coordinates increase in both the x direction and the y direction. This means that the actual change in the color is from white to a yellowish color.
As described above, in the case of a white light source that includes a combination of light sources that each have different response characteristics with respect to the drive current, it is difficult to keep the same chromaticity during the lighting intensity adjustment that is performed by changing the duty or the cycle of the drive current. Therefore, there is a problem in that a change in the duty or the cycle of the drive current causes a change in the color of the light source.
Japanese Patent No. 3368890 described above is used to reduce color deviation that is caused by an LED unit that includes LEDs in colors R, G, and B and is not used to reduce a change in the chromaticity that occurs when being driven by PWM signals. Therefore, according to the technology disclosed in Japanese Patent No. 3368890, it is difficult to reduce a change in the chromaticity of individual LEDs.
Therefore, there is a need for correct control of a light source that includes a combination of light emitters that each have different response characteristics with respect to a drive current.