JP2010205445A - Light source device, projector, light volume correction method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light source device capable of obtaining a corrected light volume in a high accuracy even with a simple structure. <P>SOLUTION: The light source device 2 is provided with a light source 20, a first field effect transistor 213 for supplying a gradation current Id for making the light source 20 to generate light corresponding to a gradation value D to the light source 20, a second field effect transistor 216 for supplying a threshold current Id2 necessary at least for making the light source 20 to generate light to the light source 20, a light volume measuring part 22 for measuring the light volume generated by the light source 20, and a light volume correcting part 3 for correcting a gate voltage of the first field effect transistor 213 and a gate voltage of the second field effect transistor 216 so that the light volume measured value measured by the light volume measuring part can be drawn near a light volume target value as shown in a relation formula of a non-linear shape of the gradation value D. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a light source device, a projector, and a light amount correction method.

  In recent years, a laser scan display (scanning projector) that displays an image by raster scanning laser light on a surface to be scanned has attracted attention. According to the laser scan display, complete black can be expressed by stopping the supply of laser light. Therefore, the contrast can be remarkably increased as compared with, for example, a projector using a liquid crystal light valve. In addition, since the laser light has a single wavelength, the color purity is high, and since the coherence is high, the laser light has characteristics such as easy beam shaping (easy to stop). As described above, the laser scan display is expected as a high-quality display that realizes high contrast, high color reproducibility, and high resolution.

  As a laser light source of a laser scan display, a semiconductor laser element such as a laser diode is mainly used. Since the laser characteristics of a semiconductor laser element change due to temperature change, aging deterioration, etc., it is necessary to correct the laser light quantity so as to obtain a desired image luminance. Examples of a method for correcting the laser light amount include methods disclosed in Patent Documents 1 and 2.

In Patent Document 1, the bias current value of the semiconductor laser element is changed by two or more points, and the light emission power of the semiconductor laser element is detected by the received light power detection means. By detecting changes in the threshold current value and quantum efficiency of the semiconductor laser element, the DC bias current of the semiconductor laser element is set in the bias current control means. By setting a pulse current corresponding to a change in quantum efficiency or the like in the pulse current control means, the laser output level is kept constant.
In Patent Document 2, a relational expression between a detected light amount of a laser light source and a set value of light output is automatically calculated. Data of this relational expression is set in the optical recording medium drive device to adjust the output level of the laser light source.

JP-A-7-147446 JP 2003-91853 A

  By the way, the laser characteristics also change due to a temperature change during the operation of the laser light source. Therefore, in order to improve the correction accuracy, it is considered desirable to correct the laser characteristics in real time during the operation of the laser light source. However, in a laser light source used for a laser scan display or the like, since gradation (output) is changed with time during operation, it is difficult to correct laser characteristics by the techniques of Patent Documents 1 and 2.

  As a configuration of a light source device capable of performing light amount correction in real time, the following configuration is conceivable. A field effect transistor (hereinafter referred to as FET) and a resistance element are connected in series with the semiconductor laser element. Depending on the laser current supplied to the semiconductor laser element, a voltage drop occurs in the resistance element. A potential difference between both ends of the resistor element is input to the differential amplifier, and an output of the differential amplifier, that is, a voltage corresponding to the laser current is input to the gain amplifier as a gain. A voltage obtained by D / A converting the gradation value is input to the gain amplifier to adjust the gain. The voltage after gain adjustment is input to the gate electrode of the FET. Thereby, a constant current circuit that feeds back the laser current is configured, and the light quantity can be adjusted by two parameters of the threshold current of the semiconductor laser element and the differential coefficient of the light quantity with respect to the driving current. However, in the above configuration, since the resistance element is connected in series with the FET, the source-drain voltage of the FET is lowered. If the FET is driven in the saturation region from the viewpoint of ensuring the detection accuracy of the laser current, the drive voltage increases, leading to overheating of the FET and increased power consumption.

  Further, since the gradation is changed for each pixel in the laser scan display, it is necessary to set the operating band of the constant current circuit to several tens of MHz or more, and it is difficult to ensure the stability of the feedback loop. In the past application (Japanese Patent Application No. 2007-268004), the inventor of the present application has proposed a light source device of a system in which currents from a plurality of constant current sources are switched and supplied to a semiconductor laser element. According to this light source device, it is possible to achieve both high-speed driving and stability of the feedback loop, but there is also a point that should be improved from the viewpoint of simplifying the driving circuit.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for correcting the light amount of a light source with high accuracy. Another object is to provide a light source device that can obtain a high-precision light amount with a simple configuration. Another object is to provide a projector that can obtain a high-quality image with a simple configuration.

  The light source device of the present invention includes a light source, a first field effect transistor that supplies the light source with a gradation current that generates light according to a gradation value, and a minimum for generating light in the light source. A second field-effect transistor that supplies a necessary threshold current to the light source; a light amount measurement unit that measures the amount of light generated by the light source; and a light amount measurement value measured by the light amount measurement unit as the gradation value. A light amount correction unit that corrects the gate voltage of the first field effect transistor and the gate voltage of the second field effect transistor so as to approach a light amount target value represented by a nonlinear relational expression. It is characterized by being.

  In this way, the gate voltages of the first and second field effect transistors are adjusted so that the light quantity measurement value actually generated by the light source is close to the light quantity target value defined by the gradation value. Even when the gradation value changes with time, the amount of light can be corrected in real time.

  In general, the drain current of a transistor is expressed by a nonlinear relational expression of the gate voltage, and the light amount of the light source is expressed by a linear expression of the drain current. Since the gate voltage is adjusted so that the light intensity of the light source, that is, the light intensity measurement value, approaches the light intensity target value represented by the nonlinear relational expression of the gradation value that defines the gate voltage, the sensitivity of the light intensity target value to the gradation value The sensitivity of the gradation value with respect to the light quantity measurement value can be made uniform, and the gradation can be corrected satisfactorily.

Compared to the case where the current supplied to the light source is directly extracted to configure a feedback loop for correcting the amount of light, the effect of the circuit components used for light amount correction on the current supplied to the light source is reduced and the light amount correction is stable. Can be done. In short, the amount of light can be corrected by two transistors, and the circuit can be made simpler than when the currents from a plurality of constant current sources are switched.
As described above, according to the light source device of the present invention, it is possible to obtain a highly accurate light amount with a simple configuration.

The nonlinear relational expression may be a secondary expression of the gradation value.
It is known that the drain current of a transistor can be accurately approximated by a quadratic expression of the gate voltage. If the nonlinear relational expression is a quadratic expression of the gradation value, the sensitivity to the gradation value of the light amount target value and the sensitivity of the gradation value to the light amount measurement value can be substantially matched, and gradation correction is performed satisfactorily. be able to.

  The recording light amount correction unit is based on a differential coefficient of an evaluation function defined by a difference between the variable light amount measurement value and the light amount target value with respect to a first variable that defines the gate voltage of the first field effect transistor. Preferably, the first variable is adjusted, and the second variable is adjusted based on a derivative of the evaluation function with respect to a second variable that defines a gate voltage of the second field effect transistor.

  By using the derivative of the evaluation function for the first variable, it is possible to evaluate whether the value of the evaluation function increases or decreases with respect to the change of the first variable, and the evaluation function value is successively decreased so as to decrease. The value of one variable can be adjusted. Further, when the derivative of the evaluation function for the second variable is used, the value of the second variable can be adjusted so that the value of the evaluation function becomes small. Thereby, the gate voltage can be adjusted so that the value of the evaluation function becomes small, that is, the light amount measurement value approaches the light amount target value.

Further, the light amount correction unit sequentially changes the first variable in a direction to decrease the absolute value of the derivative of the evaluation function with respect to the first variable, and the absolute value of the derivative of the evaluation function with respect to the second variable. The second variable may be sequentially changed in the direction of decreasing the value.
A function having a minimum value when the value of a variable is changed within a predetermined range, that is, a downward convex function, has a function coefficient of 0 at the value of the variable having the minimum value. In this way, since the absolute value of the derivative of the evaluation function for the first variable approaches 0 sequentially, the value of the first variable at which the evaluation function becomes the minimum value with respect to the change of the first variable is obtained. It is done. Similarly, the value of the second variable having a minimum evaluation function with respect to the change of the second variable is obtained. As a result, the gate voltage can be adjusted so that the difference between the light quantity measurement value and the light quantity target value is minimized.

The evaluation function is preferably a function in which a function defined by a difference between the light quantity measurement value and the light quantity target value is normalized using the gradation value.
As the gradation value increases, the absolute value of the light amount generated by the light source increases, so the absolute value of the difference between the light amount measurement value and the light amount target value increases. By doing so, the influence of the absolute value of the light amount generated by the light source on the absolute value of the evaluation function can be reduced, and the light amount can be corrected with high accuracy for a wide range of gradation values.

A power source for applying a light emission voltage to the light source; and detecting the amount of change in the light quantity measurement value before and after the voltage is reduced, and the amount of change is less than or equal to a predetermined set value A power supply voltage adjusting unit that sets the light emission voltage to a minimum value within a range may be included.
In this way, the light emission voltage is set to the minimum value within a range in which the amount of change in the light quantity measurement value is not more than a predetermined set value, so that the driving power of the light source device can be reduced while ensuring the accuracy of light quantity correction. Can be minimized.

  In addition, a threshold voltage of the first field effect transistor and the second field effect transistor is calculated using a power source that applies a light emission voltage to the light source and a correction result of the light amount correction unit, and A power supply voltage that calculates a condition in which one field effect transistor and the second field effect transistor are saturated based on the threshold voltage, and sets the light-emitting voltage to a minimum value under the saturation condition An adjustment unit may be included.

  In this way, since the threshold voltages of the first and second field effect transistors are calculated using the correction result of the light amount correction unit, the first and second fields are taken into account by taking into account variations in the threshold voltage due to manufacturing intersections and the like. Conditions under which the field effect transistor is saturated can be evaluated. Since the voltage for light emission is set to the minimum value under the condition that the first and second field effect transistors are saturated, the driving power of the light source device can be minimized while ensuring the accuracy of light amount correction. .

  According to the light amount correction method of the present invention, a first gate voltage is applied to a first field effect transistor that supplies a gradation current that causes the light source to generate light corresponding to a gradation value. Applying a second gate voltage to a second field-effect transistor that supplies the light source with a minimum threshold current necessary for generating the light source, and a light source to which the gradation current and the threshold current are supplied. A step of measuring the generated light quantity, and a gate voltage of the first field effect transistor so that the light quantity measurement value measured by the light quantity measurement unit approaches a light quantity target value represented by a secondary expression of the gradation value And a step of correcting the gate voltage of the second field effect transistor.

  In this way, the gate voltages of the first and second field effect transistors are adjusted so that the light quantity measurement value actually generated by the light source is close to the light quantity target value defined by the gradation value. Even when the gradation value changes with time, the amount of light can be corrected in real time. In addition, the effect of the circuit components used for light amount correction on the current supplied to the light source is reduced compared to the case where the current supplied to the light source is directly extracted and light amount correction is performed, and the light amount correction is performed stably. Can do. As described above, according to the light amount correction method of the present invention, the light amount can be corrected with high accuracy.

The projector of the present invention includes the light source device of the present invention, a light modulation device that modulates light emitted from the light source device, and scanning optics that scans light to be scanned by the light modulation device on a surface to be scanned. And a system.
As described above, since the light source device of the present invention has a simple configuration and can obtain a high-accuracy light amount, the projector of the present invention has a simple configuration and can obtain a high-quality image. .

It is a schematic diagram which shows schematic structure of the projector which concerns on 1st Embodiment. It is explanatory drawing which shows the mechanism in which an image is displayed. It is a schematic diagram which shows schematic structure of the light source device which concerns on 2nd Embodiment. It is a block diagram which shows the structure of the light quantity correction | amendment part in 2nd Embodiment. (A), (b) is explanatory drawing which shows the adjustment method of a power supply voltage. It is a block diagram which shows the structure of the light quantity correction | amendment part in 3rd Embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings used for explanation, in order to show characteristic parts in an easy-to-understand manner, dimensions and scales of structures in the drawings may be different from actual structures. In addition, in the embodiment, the same components are illustrated with the same reference numerals, and detailed description thereof may be omitted.

[First Embodiment]
FIG. 1 is a schematic diagram showing a schematic configuration of a projector 1 according to the first embodiment of the present invention. As shown in FIG. 1, the projector 1 includes an image signal processing system 10, a laser light source device (light source device) 2, a relay optical system 12, and a scanning optical system 13. The laser light source device 2 is constituted by the light source device of the present invention.

  The projector 1 generally operates as follows. The electrical signal supplied from the signal source 8 such as a PC is processed by the image signal processing system 10 and output to the laser light source device 2 and the scanning optical system 13. The laser light source device 2 emits light L corresponding to the electrical signal, and the light L enters the scanning optical system 13 via the relay optical system 12. The light L incident on the scanning optical system 13 is emitted by the scanning optical system 13 in the direction corresponding to the electric signal, and scans the projection surface 9 such as a wall or a screen. Thereby, an image is drawn (formed) on the projection surface 9 by the light L. Hereinafter, the components of the projector 1 will be described in detail.

  The image signal processing system 10 includes an interface 101, an image signal processing circuit 102, a modulation circuit (light modulation device) 103, a timing generation circuit 104, and a mirror drive circuit 105. The interface 101 receives an electrical signal including an image signal and a synchronization signal from the signal source 8 and separates the electrical signal into an image signal and a synchronization signal. The separated image signal is output to the image signal processing circuit 102, and the separated synchronization signal is output to the timing generation circuit 104.

  The timing generation circuit 104 generates a timing signal indicating the display timing for each pixel for a plurality of pixels included in the image according to the resolution, switching speed (frame rate), scanning method, and the like of the image. With the timing signal, the image signal supplied from the signal source 8 can be converted into a format consistent with the scanning method of the projector 1. The timing signal is output to the image signal processing circuit 102 and the mirror driving circuit 105. The mirror drive circuit 105 generates drive signals for the first deflection mirror 131 and the second deflection mirror 132 constituting the scanning optical system 13 based on the timing signal, and outputs the drive signals to the scanning optical system 13.

  The image signal processing circuit 102 performs various image processing such as gamma processing on the image signal. In addition, the image signal processing circuit 102 adjusts the image signal based on the timing signal so that the pixel data included in the image signal is output to the modulation circuit 103 in time-sequential matching with the scanning method. For example, pixel data such as gradation is stored in a frame buffer for a plurality of pixels included in an image for one frame, and the pixel data is read sequentially in the scanning time and output to the modulation circuit 103. For example, in the image signal received from the interface 101, when the pixel data is arranged in one direction in the main scanning direction, the pixel data of a plurality of pixels corresponding to one scanning line is stored in the line memory. deep. The pixel data is read line by line for each scanning line, and the reading direction is reversed in the next scanning.

  The modulation circuit 103 adjusts the output of the laser light source device 2 based on the image signal so that the intensity of the light L emitted from the laser light source device 2 changes with time according to the gradation for each pixel. Although the detailed configuration of the laser light source device 2 will be described later, the laser light source device 2 includes a plurality of semiconductor laser elements having different wavelengths of emitted light. The modulation circuit 103 adjusts the output for each of the plurality of semiconductor laser elements. Light emitted from the plurality of semiconductor laser elements is synthesized by a color synthesis element such as a dichroic mirror, and the synthesized light L is emitted from the laser light source device 2.

  The relay optical system 12 includes a collimating optical system 121 and a condensing optical system 122. The light L emitted from the laser light source device 2 is collimated by the collimating optical system 121 and then condensed by the condensing optical system 122. The relay optical system 12 can be appropriately changed according to the light distribution characteristics of the laser light source device 2 and the like. For example, when the parallelism of the light emitted from the light source device is extremely high, the relay optical system 12 can be simplified or omitted. In addition, the relay optical system 12 may be provided with a function (beam shaping function) for adjusting the cross-sectional shape and size of the light beam in a plane orthogonal to the optical axis of the light emitted from the light source.

  The scanning optical system 13 includes a first deflection mirror 131 that changes the optical axis of incident light in the main scanning direction on the projection surface, and a second deflection mirror 132 that changes the optical axis of incident light in the sub-scanning direction on the projection surface. Including. For example, the main scanning direction is a horizontal direction on the projection surface, and the sub-scanning direction is a vertical direction orthogonal to the horizontal direction on the projection surface. The first deflection mirror 131 is configured by a micro mechanical mirror formed by MEMS technology or the like, and the second deflection mirror 132 is configured by a galvano mirror or the like.

  The first deflection mirror 131 is driven by a mirror driving unit (not shown). The mirror drive unit receives the drive amount from the mirror drive circuit 105, and rotates the first deflection mirror 131 around the predetermined rotation axis at an angular velocity and amplitude corresponding to the drive amount. As a result, the normal direction of the surface on which light enters in the first deflection mirror 131 changes with respect to the optical axis of the incident light L, and the optical axis of light reflected by this surface changes based on the timing signal. . The second deflection mirror 132 is driven by the mirror driving unit in the same manner as the first deflection mirror 131.

  2A is an explanatory diagram illustrating an example of a scanning method, FIG. 2B is a plan view illustrating an example of an image displayed by the projector 1, and FIG. 2C is illustrated in FIG. It is a figure which shows the time change of the light quantity target value at the time of displaying an image, and the time change of a laser current target value.

  As shown in FIG. 2A, the light emitted from the scanning optical system 13 scans the screen along the main scanning direction while changing the gradation corresponding to the position of the pixel Pix, and performs the first scanning. The line SL1 is drawn. When the drawing of the first scanning line SL1 is finished, the light is shifted again to the position corresponding to the second scanning line SL2 in the sub-scanning direction, and then again passes through the second scanning line SL2 along the main scanning direction. draw. Similarly, an image is displayed by alternately repeating drawing of scanning lines in the main scanning direction and position shift in the sub-scanning direction. Here, an image is drawn in both the scanning toward one side in the main scanning direction and the scanning toward the other side.

  For example, when displaying an image Im as shown in FIG. 2B, the laser current target value is changed over time as shown in FIG. 2C, and the light amount target value is changed over time. The image Im in this example is a gradation-like image in which the outline is substantially rectangular and the brightness increases from one corner (upper left in the figure) toward the opposite corner (lower right in the figure). In order to draw the image Im, the light amount target value is gradually increased in the time for drawing the first scanning line SL1. The light amount target value is gradually lowered in the time for drawing the first scanning line SL1 and the second scanning line SL2 whose drawing direction is reversed. In order to gradually increase the light amount target value, the laser current target value is gradually increased. The laser current target value is set by, for example, adding a current (gradation current) that generates a light amount corresponding to the light amount target value to a minimum current (threshold current) that causes laser oscillation.

  The ratio (light emission efficiency) of light quantity actually obtained with respect to the laser current supplied to the semiconductor laser element and the threshold current vary depending on the use conditions (for example, temperature) of the semiconductor laser element and aging deterioration. The laser light source device 2 according to the present invention can correct changes in luminous efficiency and threshold current in real time during operation of the laser light source device 2. Thereby, the light quantity actually obtained can be matched with the light quantity target value with high accuracy, and the projector 1 can display a high-quality image. Hereinafter, a laser light source device 2 will be described as a second embodiment of the present invention.

[Second Embodiment]
FIG. 3 is a schematic diagram showing the configuration of the laser light source device 2 according to the second embodiment of the present invention. The actual laser light source device 2 includes three light source systems of red, green, and blue. All three light source systems have the same configuration, and here, one light source system will be representatively described. As shown in FIG. 3, one light source system constituting the laser light source device 2 includes a semiconductor laser element 20, a drive system 21, a light amount detection unit 22, and a light amount correction unit 3.

  The laser light source device 2 generally operates as follows. The drive system 21 receives the gradation value D from the modulation circuit 103. The drive system 21 supplies a laser current to the semiconductor laser element 20 based on the gradation value D, the gradation offset value (first variable) Doffset, the gain (first variable) A, and the threshold current value (second variable) Dset. To do. The semiconductor laser element 20 emits light L having a light amount corresponding to the supplied current.

  Most of the light L is output to the outside, and a part of the light L enters the light amount detection unit 22. The light amount detector 22 detects the amount of incident light. The light quantity detection value P of the light L is output to the light quantity correction unit 3. The light amount correction unit 3 corrects the gradation offset value Doffset, the gain A, and the threshold current value Dset so that the light amount detection value P approaches the light amount target value. Thereby, the light quantity output from the laser light source device 2 to the outside is corrected in real time. Hereinafter, the components of the laser light source device 2 will be described.

  The drive system 21 includes a first current source 211, a second current source 212, a Vds measuring device 218, and an A / D converter 219. The first current source 211 is in parallel with the second current source 212.

  The first current source 211 is a portion that mainly supplies gradation current. The first current source 211 includes a first FET 213, a gain amplifier 214, and a first D / A converter 215. The gradation value D input to the first current source 211 is added to the gradation offset value Doffset, and then the gain is adjusted by the gain amplifier 214. Data output from the gain amplifier 214 is converted into a voltage by the first D / A converter 215.

  The first FET 213 has a gate connected to the output terminal of the first D / A converter 215, a source connected to the ground line, and a drain connected to the input terminal of the semiconductor laser element 20. When the voltage output from the first D / A converter 215 is applied to the gate of the first FET 213, the first drain current Id corresponding to the gate voltage flows through the channel of the first FET 213, and the first The drain current Id is supplied to the semiconductor laser element 20.

  The second current source 212 is a part that mainly supplies a laser oscillation threshold current. The second current source 212 includes a second FET 216 and a second D / A converter. The threshold current value Dset input to the second current source 212 is converted into a voltage by the second D / A converter 217. The second FET 216 has a gate connected to the output terminal of the second D / A converter 217, a source connected to the ground line, and a drain connected to the input terminal of the semiconductor laser element 20. When the voltage output from the second D / A converter 217 is applied to the gate of the second FET 216, a second drain current Id2 corresponding to the gate voltage flows through the channel of the second FET 216.

  A Vds measuring device 218 is connected in common between the drain of the first FET 213 and the semiconductor laser element 20 and between the drain of the second FET 216 and the semiconductor laser element 20. Since the first FET 213 and the second FET 216 are both connected to the ground line, and the drains of the first FET 213 and the second FET 216 are connected to each other, the source-drain voltages substantially match each other. The Vds measuring device 218 measures the source-drain voltage, and the measurement result is converted into digital data by the A / D converter 219. The Vds measuring device 218 and the A / D converter 219 constitute a power supply voltage adjusting unit (described later). The digital data output from the A / D converter 219 is used to adjust the light emission voltage Vlaser applied to the semiconductor laser element 20.

  The semiconductor laser element 20 is supplied with a first drain current Id and a second drain current Id2 as laser currents. The semiconductor laser element 20 emits light L having a light amount corresponding to a current value obtained by subtracting a threshold current Ith that is a minimum current that causes laser oscillation from the laser current. The light L is mainly extracted outside and used for image display and the like. A part of the light L is split by a half mirror or the like and enters the light amount detection unit 22.

  The light quantity detection unit 22 includes a light receiving element 221 and an IV converter 222. The light receiving element 221 is configured by a photodiode or the like, and generates a current corresponding to the amount of incident light. The current generated in the light receiving element 221 is read to the IV converter 222 and converted into a voltage value, and this voltage value is converted into digital data. Digital data indicating the light quantity of the light L is output to the light quantity correction unit 3.

  The light quantity correction unit 3 applies the light quantity correction method of the present invention and is constituted by a digital filter. Before describing the configuration of the light amount correction unit 3, an embodiment of the light amount correction method of the present invention will be described.

  The light quantity correction method of this embodiment is generally based on the following policy. Assuming that the light quantity target value corresponding to the gradation value D is T and the laser light quantity is P, the difference between P and T is represented by a light quantity error δ. The gradation offset value Doffset, gain A, and threshold current value Dset, which are circuit parameters, are set so as to minimize the light amount error δ. Further, by sequentially updating the circuit parameters while measuring the laser light amount P, the light amount target value T is converged to a desired value according to environmental changes such as temperature and device characteristics. The circuit parameter setting method will be specifically described below.

  Since the source of the first FET 213 is connected to the ground line, the gate-source voltage Vgs is the same as the gate voltage Vg. The gate voltage Vg is expressed by the following equation (1) using the gradation value D, gradation offset value Doffset, gain A, and conversion coefficient Dda of D / A conversion. The first drain current Id with respect to the gate-source voltage Vgs is expressed by the following equation (2) using the threshold voltage Vth and the coefficient β. When the laser light quantity is expressed by the output voltage of the light quantity detector 22, the laser light quantity (light quantity measurement value) P is expressed by the following equation (3) using the coefficient K.

Here, by setting the gray scale value D, when the procedure was repeated for measuring the quantity of laser light to k-th tone value in each step _{{D 1, D 2, ··} , D i, ··, D k for}, the light amount measurement value _{{P 1, P 2, ··} , P i, ··, P k}, the light amount target value _{{T 1, T 2, ··} , T i, ··, T k} And The evaluation function ε _k is expressed as the sum of squares of the light amount error δ _i as shown in the following equation (8). In each step, the variables a, b, and c that minimize the evaluation function ε _k are sequentially obtained by changing the variables a, b, and c in the direction in which the evaluation function ε _k decreases. Specifically, slopes, ie, differential coefficients (∂ε _k / ∂a, ∂ε _k / ∂b, ∂ε _k / ∂c) due to changes in the variables a, b, and c are obtained, and a , B, and c are used. For example, a, b, and c may be changed so that the absolute value of the differential coefficient approaches zero. Moreover, you may correct | amend in order from the thing with the largest absolute value of the related differential coefficient among a, b, and c. By repeating such correction, it is possible to obtain variables a, b, and c in which the inclination approaches 0 and the evaluation function ε _k is minimized. Thereby, _ak , _bk , _kk can be represented by the following formulas (9) to (11).

  The light quantity measurement value P is a quadratic expression of the gradation value D as can be seen from Expression (6). Therefore, if the light quantity target value T is defined as described above, the sensitivity of the gradation value D with respect to the light quantity target value T can be matched with the sensitivity of the gradation value D with respect to the light quantity measurement value P. Also, according to Weber-Fechner's law, human senses respond logarithmically to the intensity of stimulation. If the light quantity target value T is defined as described above, the step size of the light quantity target value T becomes smaller in the relatively dark gradation than in the bright gradation, so that the gradation characteristics can be matched to human perception.

  In order to minimize the light amount error δ, as can be seen from the equation (6), the variables a to c may be adjusted. In the present embodiment, the variables a to c are sequentially adjusted based on the light quantity measurement value P in a drawing period in which an image is drawn in the main scanning direction and a blanking period in which the scanning line is shifted in the sub-scanning direction.

Here, by setting the gray scale value D, when the procedure was repeated for measuring the quantity of laser light to k-th tone value in each step _{{D 1, D 2, ··} , D i, ··, D k for}, the light amount measurement value _{{P 1, P 2, ··} , P i, ··, P k}, the light amount target value _{{T 1, T 2, ··} , T i, ··, T k} And The evaluation function ε _k is expressed as the sum of squares of the light amount error δ _i as shown in the following equation (8). In each step, the variables a, b, and c that minimize the evaluation function ε _k are sequentially obtained by changing the variables a, b, and c in the direction in which the evaluation function ε _k decreases. Specifically, the slopes due to changes in the variables a, b, and c, that is, the derivative (∂ε _k / ∂a, ∂ε _k / ∂b, ∂ε _k / ∂c) are obtained so that the slope approaches zero. The steepest descent method for correcting a, b, and c is used. Thereby, _ak , _bk , _kk can be represented by the following formulas (9) to (11).

In the equations (9) to (11), the coefficients μ _a , μ _b , and μ _c are values that define the step sizes for changing the variables a, b, and c. Here, the coefficients μ _a , μ _b , and μ _c are set to constant values in steps 0 to k. In addition, a method may be used in which the coefficients μ _a , μ _b , and μ _c are made variable as a function of the slope (∂ε _k / ∂a, ∂ε _k / ∂b, ∂ε _k / ∂c), for example. . Specifically, the following methods can be considered.

As the evaluation function ε _k approaches the minimum value, the absolute value of the slope ∂ε _k / ∂a approaches zero. If the coefficient mu _a so coefficients as the absolute value of the slope ∂ε _{k /} ∂a decreases mu _a decrease in the changing step by step, the variable a in the vicinity of the evaluation function epsilon _k is the minimum value The step size becomes fine, and the evaluation function ε _k can be minimized with high accuracy and the oscillation of the convergence value can be avoided. In addition, since the step size is relatively large in a range away from the minimum value, convergence can be accelerated.

  The following formulas (12) to (14) are obtained when the differential parts of the formulas (9) to (11) are obtained using the formula (6).

  Comparing Expressions (6) and (7), it is considered that the variable a gradually approaches the coefficient M and the variables b and c gradually approach 0 as the light quantity measurement value P approaches the light quantity target value T. Using this assumption, the equations (12) to (14) can be approximated and arranged as the following equations (15) to (17).

When formulas (15) to (17) are written in a form that can be sequentially calculated, the following formulas (18) to (21) are obtained. In Expressions (18) to (21), values other than a _k , b _k , and _ck are nominal values and the like, and are known. Therefore, the evaluation function ε _k is minimized by Expressions (18) to (21). Such variables a, b, and c are obtained sequentially. Even if the values other than a _k , b _k , and c _k have an error from the nominal value, the evaluation function ε _k remains the same, and the variables a _k , b _k , c taking into account the error remain unchanged. _k is obtained.

By the way, in order to actually drive the semiconductor laser element 20, it is necessary to convert the variables a, b, and c into circuit parameters (A, Doffset, Dset). As the variable conversion method, the following two methods are conceivable. The first method is a method of obtaining circuit parameters after obtaining variables a, b, and c that minimize the evaluation function ε _k . The second method is to obtain circuit parameters sequentially.

  First, the first method will be described. In the second FET 216, the second drain current Id2 with respect to the gate-source voltage Vg2 is expressed by the following equation (22) using the threshold voltage Vth2 and the coefficient β2. The gate voltage Vg2 is expressed by the following equation (23) using the threshold current value Dset and the conversion coefficient Dda of D / A conversion. Using the equations (5), (22), and (23), the gain A, the gradation offset value Doffset, and the threshold current value Dset are expressed by the following equations (24) to (26).

Next, the second method will be described. When the equation (5) is used in the equations (16) to (17) and written in a form that can be sequentially calculated, the following equations (27) to (28) are obtained. Using the equations (27) to (28), the gain A _k , the gradation offset value Doffset _k , and the threshold current value Dset _k can be calculated sequentially, and the convergence value may be used as a circuit parameter.

  The circuit parameter can be obtained by either the first method or the second method as described above, and the light amount can be corrected by adjusting the circuit parameter. The light quantity correction as described above can be executed by software, or can be executed by hardware such as a digital filter. Hereinafter, a specific example of a digital filter (light amount correction unit 3) that performs light amount correction will be described.

  FIG. 4 is a block diagram illustrating a configuration of the light amount correction unit 3. The light quantity configuration unit 3 includes multipliers 301, 302, 308, adders 303, 310, 317, subtractors 306, 313, 316, 320, amplifiers 305, 309, 312, 315, 319, flip-flops 304, 307, 311. 314, 318, 321 are included.

The amplifier 315 outputs a value (M · D _i = T _i ) obtained by amplifying the gradation value D _i with the gain M to the subtractor 316. The subtractor 316 outputs a value obtained by subtracting T _i from the light amount measurement value P _i (light amount error δ _i ). The multiplier 301 outputs a square value of the gradation values _{D i} to the multiplier 302. The multiplier 302 outputs a value (δ _i · D _i ² ) obtained by multiplying the square value of the gradation value D _{i by} the light amount error δ _i to the adder 303. Adder 303 outputs the addition value of the output value of multiplier 302 and the output value of flip-flop 304 to the D input terminal of flip-flop 304. The flip-flop 304 is a D-type flip-flop, and reflects the input value of the D input terminal as an output value in synchronization with the synchronization signal. That is, the adder 303 and the flip-flop 304 constitute an integration circuit that integrates the product of the square value of the gradation value D _i and the light amount error δ _i . Flip-flop 304 outputs the integrated value _{Sa i (= δ 0 · D} 0 2 + σ 1 · D 1 2 + ·· + δ i · D i 2) the amplifier 305. Amplifier 305 outputs the amplified the integrated value Sa _i gain mu _A value in the subtracter 306. The subtractor 306 outputs a value obtained by subtracting the output value of the amplifier 305 from the output value of the flip-flop 307 to the D input terminal of the flip-flop 307. The flip-flop 311 is a D-type flip-flop, and reflects the input value of the D input terminal as an output value in synchronization with the synchronization signal. The subtractor 306 and the flip-flop 307, the output value of the _{A k = A k-1 -μ} A · Sa k to calculate a correction circuit is configured, flip-flop 307 of the formula (27) _{A k} It becomes.

The multiplier 308 outputs the multiplication value output value ([delta] _i) of the subtractor 316 to the gradation value _{D i} a _{(D i} · δ _i) to the amplifier 309. The amplifier 309 outputs a value (2M · D _i · δ _i ) obtained by amplifying the output value of the multiplier 308 with a gain (2M) to the adder 310. The adder 310 and the flip-flop 311 integrate the output values of the multiplier 308 in the same manner as the adder 303 and the flip-flop 304. The flip-flop 311 outputs the integrated value Sb _i (= 2M · D ₀ · δ ₀ + 2M · D ₁ · δ ₁ + ·· + 2M · D _i · δ _i ) to the amplifier 312. The amplifier 312 outputs a value obtained by amplifying the integrated value Sb _i with a gain μ _b to the subtractor 313. The subtractor 313 outputs a value obtained by subtracting the output value of the amplifier 312 from the output value of the flip-flop 314 to the D input terminal of the flip-flop 314. The flip-flop 314 is a D-type flip-flop, and reflects the input value of the D input terminal as an output value in synchronization with the synchronization signal. The subtractor 313 and the flip-flop 314 constitute a correction circuit for calculating Doffset _k = Doffset _k−1 −μ _b · Sb _i represented by the equation (28). The output value of the flip-flop 314 is Doffset _k. It becomes.

The adder 317 and the flip-flop 318 integrate the output value (δ _i ) of the subtractor 316 in the same manner as the adder 303 and the flip-flop 304. The flip-flop 318 outputs the integrated value Sc _i (= δ ₀ + δ ₁ + ·· + δ _i ) to the amplifier 319. Amplifier 319 outputs the amplified the integrated value Sc _i gain _{mu Dset} value to the subtractor 320. The subtractor 320 outputs a value obtained by subtracting the output value of the amplifier 319 from the output value of the flip-flop 321 to the D input terminal of the flip-flop 321. The flip-flop 321 is a D-type flip-flop, and reflects the input value of the D input terminal as an output value in synchronization with the synchronization signal. The subtractor 320 and the flip-flop 321 is configured the correction circuit for calculating a _{Dset k = Dset k-1 -μ} Dset · Sc i represented by the formula (29), the output value of the flip-flop 321 is Dset _k It becomes.

As described above, _{A k} is output from the light quantity correction unit 3 as the gain A, Doffset _k as the gradation offset value Doffset is, Dset _k is output as the threshold current value Dset. The output gain A and gradation offset value Doffset are input to the gain amplifier 214 shown in FIG. The output threshold current value Dset is input to the second D / A converter 217. By driving the semiconductor laser element 20 based on the input circuit parameters, the difference between the light quantity target value T and the light quantity measurement value P, that is, the actual light quantity error is minimized.

  The laser light source device 2 of the present embodiment includes a power supply voltage adjusting unit that adjusts the laser power supply voltage Vlaser so that the laser power supply voltage Vlaser is minimized within a range in which the light amount can be corrected. Hereinafter, the power supply voltage adjustment unit will be described.

FIG. 5A is an explanatory diagram showing the VI characteristics of the FET, and FIG. 5B is an explanatory diagram showing the relationship of the laser light quantity P with respect to the laser power supply voltage Vlaser. The above description of the operation of the laser light source device 2 assumes that the laser power supply voltage Vlaser is sufficiently large and that the first and second FETs 213 and 216 are operating in a saturated state. Specifically, as shown in FIG. 5A, as the source-drain voltage Vds becomes lower (V ₀ → V ₁ → V ₂ ) from a state where the source-drain voltage Vds is sufficiently high (Vds = V ₀ ). The first drain current Id decreases (I ₀ → I ₁ → I ₂ ). When the source-drain voltage Vds becomes a certain level or less, the FET leaves the saturation state. Then, the first drain current Id cannot be determined only by the gate-source voltage Vgs, and the first drain current Id decreases. In such a state, it becomes difficult to satisfactorily control the first drain current Id, and the light amount correction accuracy is lowered.

  It is possible to set the laser power supply voltage Vlaser high to ensure that the FET is saturated. Further, the condition for the FET to become saturated is expressed by the following equation (30). According to the equation (30), it seems that the laser power supply voltage Vlaser can be minimized. However, since the FET threshold voltage Vth is likely to vary due to manufacturing tolerances and the like, it is actually necessary to provide a certain margin to the laser power supply voltage Vlaser. If the laser power supply voltage Vlaser is set high with a margin, there may be inconveniences such as a decrease in power efficiency due to a voltage drop in the FET and an overheating of the FET. The following two methods are conceivable as a method for obtaining the minimum value of the laser power supply voltage Vlaser within the range in which the light amount correction is possible.

  The first method is a method of adjusting the laser power supply voltage Vlaser so as to satisfy the saturation condition after estimating the threshold voltage Vth of the FET. In a state where the light amount correction loop is stabilized, the following equation (31) is established from equations (5) and (16). When equation (31) is solved, the following equation (32) is obtained. Using Expressions (30) and (32), the minimum value of the source-drain voltage Vds within the range satisfying the saturation condition can be obtained. By adjusting the laser power supply voltage Vlaser so that the source-drain voltage Vds becomes the minimum value, the laser power supply voltage Vlaser can be minimized while ensuring the accuracy of light amount correction. The source-drain voltage Vds is measured by the Vds measuring device 218, and the laser power supply voltage Vlaser is adjusted so as to coincide with the minimum value of the source-drain voltage Vds within the range satisfying the saturation condition obtained above.

The second method is a method in which the laser power supply voltage Vlaser is changed to obtain a range in which light amount correction can actually be performed appropriately. Specifically, as shown in FIG. 5B, the light quantity measurement value P is obtained under two or more conditions with different laser power supply voltages Vlaser. Here, the gradation value is set to the maximum gradation or the maximum gradation that guarantees the light amount correction. Then, the light quantity measurement value P is obtained under two conditions (Vlaser = V ₃ , V ₄ ) where the laser power supply voltage Vlaser is lower than the condition that reliably satisfies the saturation state (Vlaser = V ₀ ). The difference ΔP between the light quantity measurement values is obtained. A minimum value of the laser power supply voltage Vlaser where the difference ΔP is equal to or less than a preset value is obtained, and the laser power supply voltage Vlaser is adjusted to this minimum value.

According to the laser light source device 2 configured as described above, the circuit parameters calculated sequentially based on the gradation value D and the light quantity measurement value P are fed back to the first and second FETs 213 and 216. The amount of light can be corrected in real time. Since the light quantity target value T is defined by the square characteristic of the gradation value D, the sensitivity of the gradation value D with respect to the light quantity target value T can be matched with the sensitivity of the gradation value D with respect to the light quantity measurement value P. Can be matched with the light quantity target value T with high accuracy. In the supply path for supplying the laser current to the semiconductor laser element 20, there is almost no need to extract the laser current for light quantity correction, so that circuit components for extracting the laser current can be simplified or omitted, and the laser light source device 2 Can be made simple. In addition, the laser power supply voltage can be minimized by using the light amount detection unit 22 and the light amount correction unit 23 used for light amount correction, and the laser light source device 2 can have low power consumption.
As described above, the laser light source device of the present invention has a simple structure and a good one that can obtain a highly accurate light amount.

[Third Embodiment]
Next, a laser light source device according to a third embodiment of the present invention will be described. The third embodiment differs from the second embodiment in that the light amount is corrected by minimizing the relative error of the light amount error with respect to the gradation value. Hereinafter, after describing the light amount measurement method in the third embodiment, the configuration of the light amount correction unit in the third embodiment will be described.

In the present embodiment, in place of the evaluation function epsilon _k shown in equation (8), to the light amount correction by minimizing the evaluation function .epsilon.2 _k shown in the following equation (33). Similar to the first embodiment, when the steepest descent method is used and the assumption that the variable a is a coefficient M and the variables b and c are asymptotic to 0, the following equations (34) to (36) are obtained.

  FIG. 6 is a block diagram illustrating a configuration of the light amount correction unit 4 in the laser light source device of the third embodiment. The light quantity correction unit 4 is a digital filter that executes the sequential calculation of Expressions (34) to (36), and can sequentially calculate circuit parameters as in the second embodiment. The light amount correction unit 4 is different from the light amount correction unit 3 in that the multiplication units 301 and 308 are not provided and the division unit 401 is provided. In the light amount correction unit 4, the configuration downstream of the addition units 303, 310, and 317 is the same as that of the light amount correction unit 3.

In the light amount correction unit 4, the multiplication unit 302 outputs a value obtained by multiplying the gradation value D _i by the light amount error σ _i to the addition unit 303. The amplifying unit 309 amplifies the value (σ _i ) output from the subtracting unit 316 with a gain of 2M and outputs the amplified value to the adding unit 310. The light quantity error σ _i is input from the subtractor 316 to the first input In1 of the divider 401. A second input In2 of the division unit 401, the gradation value _{D i} is inputted. The division unit 401 outputs a value (σ _i / D _i ) obtained by dividing the first input In1 by the second input In2 to the addition unit 317. Hereinafter, A _k , Doffset _k , and Dset _k are calculated by the circuit portions downstream from the adding units 303, 310, and 317, respectively. The output gain A and gradation offset value Doffset are input to the gain amplifier 214 shown in FIG. The output threshold current value Dset is input to the second D / A converter 217. By driving the semiconductor laser element 20 based on the input circuit parameters, the light quantity error of the light quantity measurement value (actual output light quantity) with respect to the light quantity target value T is minimized.

  By the way, it is considered that the absolute value of the light amount generated by the light source increases as the gradation value increases, and the absolute value of the difference between the light amount measurement value and the light amount target value increases. In the laser light source device of the third embodiment, since the evaluation function is defined by multiplying the light amount error by σ and the inverse weight of the gradation value D, the absolute value of the light amount generated by the light source is the evaluation function. The influence on the absolute value can be reduced. Therefore, the dependency of the correction accuracy on the gradation value D can be reduced, and the light amount can be corrected with high accuracy over a wide range of the gradation value D.

DESCRIPTION OF SYMBOLS 1 ... Projector, 2 ... Laser light source device (light source device), 3 ... Light quantity correction | amendment part, 13 ... Scanning optical system, 103 ... Modulation circuit (light modulation device), 213 ... First FET (first field effect transistor), 216... Second FET (second field effect transistor), Doffset... Gradation offset data (first variable), A. (First variable), Dset ... threshold data (second variable)

Claims (9)

A light source;
A first field effect transistor that supplies the light source with a gradation current that generates light corresponding to a gradation value in the light source;
A second field effect transistor that supplies the light source with a threshold current that is minimally required to cause the light source to generate light;
A light amount measuring unit for measuring the amount of light generated by the light source;
The gate voltage of the first field effect transistor and the second electric field so that the light quantity measurement value measured by the light quantity measurement unit approaches a light quantity target value represented by a nonlinear relational expression with respect to the gradation value. A light source device comprising: a light amount correction unit that corrects the gate voltage of the effect transistor.   The light source device according to claim 1, wherein the nonlinear relational expression is a quadratic expression of the gradation value. The light amount correction unit is based on a derivative of an evaluation function defined by a difference between a variable light amount measurement value and the light amount target value with respect to a first variable that defines a gate voltage of the first field effect transistor. While adjusting the first variable,
3. The light source device according to claim 1, wherein the second variable is adjusted based on a derivative of the evaluation function with respect to a second variable that defines a gate voltage of the second field effect transistor.   The light amount correction unit sequentially changes the first variable in a direction to decrease the absolute value of the derivative of the evaluation function with respect to the first variable, and sets the absolute value of the derivative of the evaluation function with respect to the second variable. The light source device according to claim 3, wherein the second variable is sequentially changed in a decreasing direction.   The evaluation function is a function in which a function defined by a difference between the light quantity measurement value and the light quantity target value is normalized using the gradation value. Light source device. A power source for applying a voltage for light emission to the light source;
While decreasing the voltage, a change amount of the light quantity measurement value before and after the voltage decrease is detected, and the light emission voltage is set to a minimum value within a range where the change amount is not more than a predetermined set value. The light source device according to claim 1, further comprising: a power supply voltage adjusting unit. A power source for applying a voltage for light emission to the light source;
A threshold voltage of the first field effect transistor and the second field effect transistor is calculated using a correction result of the light amount correction unit, and the first field effect transistor and the second field effect transistor are saturated. And a power supply voltage adjustment unit configured to calculate a condition to be in a state based on the threshold voltage, and to set the light emission voltage to a minimum value under the condition to be in the saturation state. The light source device according to any one of 1 to 6. Applying a first gate voltage to a first field-effect transistor that supplies a gradation current that generates light according to a gradation value to the light source, and at least the minimum required to generate light from the light source Applying a second gate voltage to a second field effect transistor for supplying a threshold current to the light source;
Measuring the amount of light generated by a light source supplied with the gradation current and the threshold current;
The gate voltage of the first field effect transistor and the second field effect transistor are adjusted so that the light quantity measurement value measured by the light quantity measurement unit approaches a light quantity target value represented by a quadratic expression of the gradation value. And a step of correcting the gate voltage. The light source device according to any one of claims 1 to 7,
A light modulation device for modulating light emitted from the light source device;
And a scanning optical system that scans the light modulated by the light modulation device on a surface to be scanned.

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