JPH0958048A - Light source driver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to correct the suitable exposure amount at a pixel unit in response to the light output change characteristics of a light source by providing means for correcting the emitted output of the source based on the characteristics obtained by means for obtaining the characteristics. SOLUTION: A controller 151 inputs the digital image gray level data 11 from an image reader, decides the correction coefficient based on the light output stored in memory means 9, and decides the correction value by using it. The image gray level data 71 after the correction based on the corrected value is inputted to a laser driver 6 as digital-analog image gray level data 51. Further, the controller 151 can input the predetermined previously light emission pattern signal 141 emitted in a predetermined pattern out of the image area to correct the exposure amount. Further, the controller 151 has correction coefficient deciding means 101 for deciding the coefficient.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light source driving device, and more particularly to a light source driving device for modulating and driving a light source such as a semiconductor laser for forming an image on a photosensitive image carrier based on image information.

[0002]

2. Description of the Related Art In the field of image forming apparatuses such as digital copying machines and printers, a light source scans a photosensitive image carrier such as a photoconductor or a photosensitive film, and the light source is turned on based on document image information during the scanning. The image corresponding to the original image is formed on the image carrier by controlling the light output of the light source and the off state.

Today, a typical example of such a light source is a semiconductor laser light source. Semiconductor lasers are widely adopted because they are small and inexpensive. However, the semiconductor laser has a drawback that the light output with respect to the drive current is easily influenced by the temperature, that is, even if the drive current is kept constant, the light output decreases due to the temperature rise of the semiconductor laser chip. Particularly, the temperature rise due to the emission loss of the semiconductor laser itself becomes a problem.

When the light output of the semiconductor laser fluctuates, it becomes difficult to form a good quality image when controlling the light output of the semiconductor laser in forming a halftone image or the like. Therefore, as a means for maintaining the optical output of the semiconductor laser at a predetermined level, an automatic output control circuit (so-called APC) that monitors the emission level of the laser with a photodiode and controls the drive current based on the monitoring result.
Circuit) has been proposed.

Further, Japanese Patent Laid-Open No. 3-150964 discloses that
The effective value of the exposure amount within the predetermined time is obtained by detecting the light output of the semiconductor laser and integrating within the predetermined time, and the estimated exposure amount within the predetermined time is obtained from the desired exposure amount setting value. It teaches a light source driving device that can compare the values of (1) and (2) and obtain the correction value of the next exposure amount setting value from the result (difference or large or small), and thereby correct the exposure amount.
The light source driving device taught in this publication has a problem in the above APC circuit, that is, an image forming device using a semiconductor laser (for example, a laser beam printer) whose recording speed per pixel (pixel clock frequency) is as high as several MHz. In order to perform the intensity modulation of the semiconductor laser in the APC circuit, a considerably high control speed is required, but it is very difficult, and the APC circuit of the normal control speed is required.
If the circuit is used, the driving speed of the entire semiconductor laser driving circuit is reduced, and it is intended to solve the problem that high speed modulation cannot be performed.

[0006]

However, in the light source driving device disclosed in Japanese Patent Laid-Open No. 3-150964, the light output of the semiconductor laser is corrected by correcting the exposure amount of each pixel by integration. Therefore, even if the exposure amount can be corrected for the entire pixel, the correction does not work for the variation in the exposure amount in the pixel unit. That is, the semiconductor laser has a characteristic (thermal droop characteristic) in which the light output gradually decreases due to the temperature rise of the semiconductor laser chip due to the light emission. The output is different, and also when the semiconductor laser is turned on and off, there are so-called overshoot and undershoot due to a sharp rise and a sharp fall of the optical output when the semiconductor laser is turned on and when it is turned off. There is a problem that the light output varies within the pixel.

Further, there is a problem of optical output fluctuation due to overshoot / undershoot depending on the difference in exposure amount between the current pixel and the next pixel. Although the semiconductor laser has been described above as an example, such a problem related to the fluctuation of the optical output is not limited to the semiconductor laser and may occur with other light sources having thermal droop characteristics, overshoot / undershoot characteristics, and the like. .

Therefore, the present invention is a drive device for a light source for forming an image on a photosensitive image carrier based on image information, and the exposure amount (light output of the light source) by the light source is determined by the light of the light source. An object of the present invention is to provide a light source drive device capable of appropriately correcting in pixel units according to output fluctuation characteristics (thermal droop characteristics, overshoot / undershoot characteristics, etc.).

[0009]

In order to solve the above-mentioned problems, the present invention is a drive device for a light source for forming an image on a photosensitive image bearing member based on image information, and a pixel based on the image information. A unit for turning the light source on and off, a unit for monitoring the light output from the light source when the light source is turned on and / or a light source is turned off, and the light source is turned on based on the monitoring result by the monitoring unit. And (or) a means for obtaining a light output variation characteristic when turned off, and a means for correcting the light emission output of the light source on a pixel-by-pixel basis based on the light output variation characteristic obtained by the means for obtaining the light output variation characteristic. The present invention provides a light source driving device characterized by being provided.

Here, the image information is, in other words, image data, which is one or more of the image density of pixels, the pixel formation time, and the like. Further, here, "turning on the light source" means both the case where the light source is switched from the extinguished state to the lit state and the case where the light source is switched from the already lit state to the lit state where the light output is further increased.

Further, "turning off the light source" means both the case where the light source is switched from the lighting state to the light-off state and the case where the light source is switched from the already lighting state to the lighting state in which the light output is reduced. In addition, "when the light source is turned on" means any of the above-mentioned initial state of the light source being turned on, the initial state of being turned on, and the on state thereafter.

Further, "when the light source is turned off" means any of the above-mentioned light source being turned off when it is turned off, when it is turned off, and after that. As a typical example, the means for obtaining the light output variation characteristic is for obtaining the light output variation characteristic from the thermal droop characteristic of the light source,
An example is one in which the optical output fluctuation characteristic is obtained from the overshoot / undershoot characteristic of the optical output in the off state.

When the means for obtaining the light output variation characteristic is for obtaining the light output variation characteristic from the thermal droop characteristic of the light source, the means for correcting the light emission output is, for example, the accumulated value of the thermal droop characteristic and the image data. The light emission output of the light source is corrected based on the above (for example, the light emission output of the (m + 1) th pixel is corrected based on the thermal droop characteristic and the cumulative value of the image data up to the mth pixel).

When the means for obtaining the light output variation characteristic is for obtaining the light output variation characteristic from the overshoot / undershoot characteristic of the light output, the means for correcting the light emission output is, for example, the overshoot / undershoot. The case where the light emission output of the light source is corrected based on the characteristics and the image data of the current pixel and the next pixel can be mentioned.

According to the light source driving device of the present invention, the light source is turned on / off based on the image information provided. Then, when the light source is turned on / off based on the image information, the light output variation characteristic when the light source is turned on and / or when the light source is turned off is obtained based on the monitoring result by the means for monitoring the light output from the light source in advance. The light emission output of the light source is appropriately corrected for each pixel based on the light output variation characteristic.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) First, a first embodiment will be described with reference to FIGS.
An embodiment will be described. FIG. 1 is a block circuit diagram of a light source drive device according to the first embodiment, FIG. 2 is a timing chart showing signal timings of respective parts shown in FIG. 1, and FIG. 3 is a view showing correction coefficient determining means in FIG. FIG. 4 is a diagram illustrating a process of determining a correction coefficient, and FIG. 4 is a diagram illustrating a process of determining a correction value in the correction value determining unit in FIG. 1.

The light source driving device of FIG. 1 drives the semiconductor laser 1, and is based on the thermal droop characteristic of the semiconductor laser 1 and the cumulative value of the exposure amount set value (in other words, the image data of the pixel) which are obtained in advance. The exposure amount (in other words, the light emission output of the light source) can be corrected. The device shown in FIG.
A laser driver 6 that drives the semiconductor laser 1, a beam splitter 2 that splits the light emitted from the semiconductor laser 1, a PIN photodiode 3 for monitoring the light output that has passed through the beam splitter 2, and a current detected by the diode 3 A current / voltage conversion circuit (I / V circuit) 4 for converting into a voltage, an optical output storage means 9 for storing an output (optical output converted into a voltage) 41 from the I / V circuit 4, and a controller 151 are provided. There is.

The controller 151 is adapted to receive the digital image density data 11 from an image reader (IR), a printer controller, etc., and also, based on the optical output stored in the storage means 9, a correction coefficient to be described later. Is determined, a correction value is determined based on the correction coefficient, and the corrected image density data 71 based on the correction value is determined.
Is input to the laser driver 6 as corrected analog image density data 51 via a digital / analog converter (D / A converter) 8.

Further, the controller 151 can also input a predetermined pre-emission pattern signal 141 for causing the laser driver 6 to emit light in a predetermined pattern outside the image area, in order to correct the exposure amount. The controller 151 also includes a correction coefficient determining unit 101 that determines a correction coefficient based on the light output stored in the light output storage unit 9 as described later, cumulative data of image density data up to the m-th pixel, and m + 1 pixels. An image density data storage unit 121 for storing image density data of the eye, and a correction value determination for determining a correction value as described later from the correction coefficient determined by the correction coefficient determination unit 101 and the data stored in the storage unit 121. Means 131 are included.

As the PIN photodiode 3, a PIN photodiode for monitoring the rear surface light emission of the chip built in the semiconductor laser 1 may be adopted. This light source drive device has a recording speed per pixel (pixel clock) of several MHz, and FIG. 2 shows the waveform of the pixel clock. The time for one pixel is T.

In order to synchronize the writing position of the image, the image area starts a few μsec after the SOS (Start of Scan) signal shown in FIG. 2 is issued.
Scanning of the image carrier for image formation begins. According to this light source driving device, the light output emitted from the semiconductor laser 1 is split by the beam splitter 2 and a part thereof enters the PIN photodiode 3. The other light output split by the beam splitter 2 is used for forming an image on a photosensitive image carrier such as a photosensitive drum or a photosensitive film by using an optical system, an optical deflector or the like not shown here. Used for scanning.

The current detected by the PIN photodiode 3 is converted into a voltage in the I / V circuit 4 and stored in the optical output storage means 9. To correct the exposure amount, first, a pre-emission pattern signal 141 determined in advance is input to the laser driver 6 from the controller 151 outside the image area. As shown in FIG. 2 and in FIG. 3, this signal causes the light to be emitted at the concentration K for the time t1, the light is subsequently extinguished for the time t2, and then the light is again emitted at the concentration K.

The light emission of the semiconductor laser 1 due to this pattern signal is monitored by the PIN photodiode 3, and the light output thereof is stored in the light output storage means 9 from the I / V circuit 4. The correction coefficient determination means 101 determines the correction coefficients H1 and H2 based on the stored light output. 2 and 3
The inside indicates the output state of the I / V circuit 4 at this time.

The correction coefficient H1 is an attenuation coefficient related to the light output, that is, the light emission density during light emission × the light output attenuation rate per time, and is calculated by the following equation (1a). H1 = (k2-k1) / t1 · K (Equation (1a)) where k1: light output at the beginning of light emission k2: light output at the end of light emission t1: lighting time K: light emission density Further, the correction coefficient H2 is here. It is a recovery coefficient related to the light output at the time of re-emission after the light is turned off, that is, the recovery rate of the light output per time at the time of light off, and is calculated by the following formula (1b).

H2 = (k3-k1) / t2 (1b) where k1: light output at the beginning of light emission k3: light output at the beginning of re-emission t2: turn-off time Next, in the image area, IR, printer The uncorrected digital image density data 11 is transmitted from the controller or the like and is input to the controller 151. In the controller 151, the cumulative data of the image density data up to the mth pixel and the image density data of the (m + 1) th pixel are stored in the storage unit 121.

Here, as shown in FIG. 4, the image density data of the m-th pixel is set as km, and the accumulated data is Σkm (m = 1.
-M), the image density data of the (m + 1) th pixel is k (m + 1). Then, the correction value determining means 13 in the controller 151
1 is the cumulative data Σkm (m = 1 to m) and the image density data k (m + 1) of the m + 1 pixel and the correction coefficient (attenuation coefficient H1 and recovery coefficient H2) from the correction value k ′ (m + 1 of the m + 1 pixel. ) Is determined.

The correction value k '(m + 1) is calculated by the following equation (1c). k ′ (m + 1) = k (m + 1) + T × Σkm (m = 1 to m) × H1−Sm × H2 Equation (1c) where Sm is the semiconductor laser 1 from the image start position to the m-th pixel It is the time when no light is emitted.

The original data can be used as the image density data up to the m-th pixel, and the corrected image density data can also be used. The corrected digital image density data 71 is output from the controller 151,
The D / A converter 8 converts the analog data, and the corrected analog data 51 is input to the laser driver 6. The driver 6 sends a drive current corresponding to the corrected analog image density data to the semiconductor laser 1 to cause it to emit light. A light emission output emitted from the semiconductor laser 1 is split by a beam splitter 2 and is used for image forming scanning of an image carrier using an optical system, an optical deflector and the like not shown here.

2 shows the waveform of the corrected analog image density data 51 of the m + 1 pixel, and FIG. 2 shows the output waveform of the I / V circuit 4 after the correction of the m + 1 pixel. . For reference, FIG. 2 shows the waveform of the analog image density data without correction as I / O without correction.
The output waveform of the V circuit 4 is shown by, and the output waveform is too large or too small as compared with the desired output waveform. As can be easily understood by comparison with the waveform without correction, it can be seen that a desired optical output is obtained with the correction according to the present invention. (Second Embodiment) Next, a second embodiment will be described with reference to FIGS.
An embodiment will be described.

FIG. 5 is a block circuit diagram of a light source driving device according to the second embodiment, FIG. 6 is a timing chart showing the timing of signals of respective parts shown in FIG. 5, and FIG. 7 is a correction coefficient in FIG. It is a figure explaining the process of the correction coefficient determination in a determination means, and FIG. 8 is a figure explaining the process of the correction value determination in the correction value determination means in FIG. The light source driving device of FIG. 5 also drives the semiconductor laser 1, but this device takes into consideration so-called overshoot / undershoot due to a sharp rise / fall of the optical output when the semiconductor laser 1 is turned on / off. The exposure amount (light output) can be corrected by using the above-mentioned method, and the exposure amount setting value (image data) of the current pixel and the exposure amount setting value (image data) of the next pixel can be calculated based on the overshoot amount and the undershoot amount obtained in advance. The exposure amount is corrected in consideration of the above.

The apparatus shown in FIG. 5 comprises a laser driver 6 for driving the semiconductor laser 1, a beam splitter 2 for branching the light emitted from the semiconductor laser 1, and a PIN photodiode for monitoring the light output passing through the beam splitter 2. 3, a current / voltage conversion circuit (I / V circuit) 4 for converting the current detected by the diode 3 into a voltage, an I / V circuit 4
The optical output storage means 9 for storing the output (optical output converted into a voltage) 41 and the controller 152.

The controller 152 is adapted to receive digital image density data from an image reader (IR), a printer controller or the like, and to apply a correction coefficient, which will be described later, based on the light output stored in the storage means 9. Determine, and determine the correction value based on the correction coefficient,
The image density data 72 after correction based on the correction value is input to the laser driver 6 as the analog image density data 52 after correction via the digital / analog converter (D / A converter) 8. There is.

Further, the controller 152 can also input a predetermined pre-emission pattern signal 142, which causes the laser driver 6 to emit light in a predetermined pattern outside the image area, in order to correct the exposure amount. Further, the controller 152 includes a correction coefficient determining unit 102 that determines a correction coefficient based on the light output stored in the light output storage unit 9 as described later, image density data of the nth pixel, and n +.
An image density data storage unit 122 that stores the image density data of the first pixel, and a correction that determines a correction value as will be described later from the correction coefficient determined by the correction coefficient determination unit 102 and the data stored in the storage unit 122. Value determining means 13
2 is included.

As for the PIN photodiode 3, a PIN photodiode for monitoring the rear surface light emission of the chip built in the semiconductor laser 1 may be adopted in this example as well. This light source drive device has a recording speed per pixel (pixel clock) of several MHz, and FIG. 6 shows the waveform of the pixel clock. The time for one pixel is T.

In order to synchronize the writing position of the image, the image area starts a few μsec after the SOS (Start of Scan) signal shown in FIG. 6 is issued.
Scanning of the image carrier for image formation begins. According to this light source driving device, the light output emitted from the semiconductor laser 1 is split by the beam splitter 2 and a part thereof enters the PIN photodiode 3. The other light output split by the beam splitter 2 is used for forming an image on a photosensitive image carrier such as a photosensitive drum or a photosensitive film by using an optical system, an optical deflector or the like not shown here. Used for scanning.

The current detected by the PIN photodiode 3 is converted into a voltage in the I / V circuit 4 and stored in the optical output storage means 9. To correct the exposure amount, first, a pre-emission pattern signal 142 determined in advance from the controller 152 is input to the laser driver 6 outside the image area. As shown in (1) in FIG. 6 and (7) in FIG. 7, this signal is pre-lighted at the density K for the time T of one pixel, and then continuously for the time T of one pixel at the density K ′ lower than the density K.
Is to pre-light.

The light emission of the semiconductor laser 1 due to this pattern signal is monitored by the PIN photodiode 3, and its optical output is stored in the optical output storage means 9 from the I / V circuit 4. 6 and 7 show the output state of the I / V circuit 4 at this time. The correction coefficient determining means 102 determines the correction coefficients H and H ′ based on the stored light output.

The correction coefficient H is an overshoot amount of light energy per density difference between the current pixel and the next pixel, and is calculated by the following equation (2a). H = (E−K × T) / K Equation (2a) where K: density difference of the pre-emission pattern signal (in this case, one has zero density, so K) T: 1 pixel Of time E: Actually applied light energy, which is obtained by integrating the actual light output waveform from the I / V circuit 4 at time T to obtain the light energy applied to one pixel.

K × T: desired light energy to be given The correction coefficient H ′ is an undershoot amount of light energy per density difference between the current pixel and the next pixel, and is calculated by the following equation (2b). H ′ = (E′−K ′ × T) / (K−K ′) Equation (2b) where K−K ′: concentration difference of pre-emission pattern signal E ′: actually applied light energy K ′ × T: desired light energy to be given Next, in the image area, the digital image density data 11 before correction is transmitted from the IR, the printer controller, etc., and is input to the controller 152. In the controller 152, the image density data of the nth pixel and n +
The image density data of the first pixel is stored in the storage unit 122.

Here, as shown in FIG. 8, the image density data of the nth pixel is x and the image density data of the n + 1th pixel is y.
Then, the correction value determining means 13 in the controller 152
2 determines the image density data x of the n-th pixel, the image density data y of the n + 1-th pixel, and the corrected image density data z of the n + 1-th pixel from the correction coefficients H and H '.

As shown in FIG. 8, the correction value z is calculated by the following equation (2c) when x <y, and by the following equation (2d) when x> y. When x <y (see FIG. 8A): y × T = (z−x) × H + z × T Therefore, z = (y × T + x × H) / (T + H) ... Formula (2c) x > Y (see FIG. 8B): y × T = (x−z) × H ′ + z × T Therefore, z = (y × T−x × H ′) / (T−H ′) Formula (2d) The corrected digital image density data 72 is the controller 1
The analog data 52 output from the data 52 is converted into analog data by the D / A converter 8 and the corrected analog data 52 is input to the laser driver 6. The driver 6 sends a drive current corresponding to the corrected analog image density data to the semiconductor laser 1 to cause it to emit light. A light emission output emitted from the semiconductor laser 1 is split by a beam splitter 2 and is used for image forming scanning of an image carrier using an optical system, an optical deflector and the like not shown here.

FIG. 6 shows the waveform of the corrected analog image density data 52 of the (n + 1) th pixel, and FIG. 6 shows the output waveform of the I / V circuit 4 after the correction of the (n + 1) th pixel. . For reference, FIG. 6 shows the waveform of the analog image density data without correction as I / O without correction.
The output waveform of the V circuit 4 is shown in, and overshoot and undershoot are noticeable in the output waveform.

In comparison with, the output a at the first start-up (see in FIG. 6) is made slightly lower in order to prevent the first overshoot, while the subsequent output b is prevented in order to prevent the subsequent light output reduction. (Refer to FIG. 6) is slightly raised. As can be easily understood by comparing with the uncorrected waveform, it can be seen that in the case where the correction according to the present invention is applied, overshoot and undershoot are reduced and a desired light output is obtained.

[0044]

According to the present invention, there is provided a driving device for a light source for forming an image on a photosensitive image carrier based on image information, wherein the exposure amount (light output of the light source) by the light source is It is possible to provide a light source drive device capable of appropriately correcting in pixel units according to the optical output fluctuation characteristics (thermal droop characteristics, overshoot / undershoot characteristics, etc.) of the above.

[Brief description of drawings]

FIG. 1 is a block circuit diagram of a light source driving device according to a first embodiment.

FIG. 2 is a timing chart showing timings of signals of respective parts shown in FIG.

FIG. 3 is a diagram illustrating a process of determining a correction coefficient in a correction coefficient determining unit in FIG.

FIG. 4 is a diagram illustrating a process of determining a correction value in a correction value determining unit in FIG.

FIG. 5 is a block circuit diagram of a light source drive device according to a second embodiment.

6 is a timing chart showing the timing of the signals of the respective parts shown in FIG.

FIG. 7 is a diagram illustrating a process of determining a correction coefficient in a correction coefficient determining unit in FIG.

FIG. 8 is a diagram illustrating a process of determining a correction value in a correction value determining unit in FIG.

[Explanation of symbols]

1 semiconductor laser (light source) 2 beam splitter 3 PIN photodiode (monitoring means) 4 current / voltage converter (I / V circuit) (means for obtaining optical output fluctuation characteristics) 41 output of I / V circuit 4 51, 52 correction Analog image density data after 6 Laser driver 71, 72 Digital image density after correction 8 Digital / analog converter (D / A converter) 9 Optical output storage means 11 Image density data before correction 101, 102 Correction coefficient determination Means 121 means for storing cumulative data of image density data up to m-th pixel and image density data for m + 1 pixel 122 means for storing image density data for n-th pixel and image density data for n + 1-th pixel 131, 132 correction value Determining means 141, 142 Pre-emission pattern signal 151, 152 Controller

Claims (3)

[Claims] 1. A driving device for a light source for forming an image on a photosensitive image carrier based on image information, comprising means for turning on / off the light source in pixel units based on image information,
Means for monitoring the light output from the light source when the light source is turned on and / or when the light source is turned off, and the light output when the light source is turned on and / or off based on the monitoring result by the monitoring means A light source driving device comprising: a means for obtaining a variation characteristic; and a means for correcting the light emission output of the light source on a pixel-by-pixel basis based on the light output variation characteristic obtained by the means for obtaining the light output variation characteristic. . 2. The light source drive device according to claim 1, wherein the means for obtaining the light output variation characteristic is for obtaining the light output variation characteristic from the thermal droop characteristic of the light source. 3. The means for obtaining the light output variation characteristic is for obtaining the light output variation characteristic from the overshoot / undershoot characteristic of the light output when the light source is on / off, and the means for correcting the light emission output comprises: The light source driving device according to claim 1, wherein the light emission output of the light source is corrected based on the overshoot / undershoot characteristic and the image data of the current pixel and the next pixel.