Image display apparatus

An image display apparatus includes a light source having a plurality of light emitting devices, and a projection optical system capable of making lights from the light source scan in a main scanning direction and in a subscanning direction to display on a screen an image having a predetermined number of pixels. The scanning lines in the main scanning direction are formed by the lights emitted from each of the light emitting devices and controlled to be superposed one on another on the screen.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image display apparatus for displaying an image on a screen such as a television screen or computer screen.

2. Related Background Art

Projection-type laser image display apparatuses have been widely provided which display an image such as a television image by modulating laser beams of three colors: red, green, and blue, and by scanning the laser beams in a horizontal direction and in a vertical direction.

FIG. 1is a diagram schematically showing a configuration of a laser image display apparatus such as the one disclosed in Japanese Patent Application Laid-open No. 9-134135. InFIG. 1, there are illustrated an image information source1, an image controller2, laser oscillators11,14, and17which generate red laser light, laser oscillators12,15, and18which generate green laser light, laser oscillators13,16, and19which generate blue laser light, beams of laser light61to69generated by the laser oscillators11to19, modulators21to29which amplitude-modulate laser beams61to69, beam-combining optical systems31to33each of which combines three laser beams respectively having the three colors on one optical axis, laser beams71to73amplitude-modulated and three-color-combined, horizontal scanning devices41to43, collimator/condenser lenses111to113which collimate the horizontally scanning laser beams and condense the collimated beams on vertical scanning devices51to53, the vertical scanning devices51to53, projection lenses121to123, and a screen110. A vertical scanning signal is indicated by131and a horizontal scanning signal is indicated by132.

The first stage of the arrangement shown in the uppermost section ofFIG. 1will be described. Red, green and blue light beams are produced by the laser oscillators11to13, are amplitude-modulated by the modulators21to23, and are thereafter combined on one optical axis. The combined laser beam is made to scan two-dimensionally by the horizontal scanning device41and the vertical scanning device51to project an image on the screen110. The same operation is performed in each of the second and third stages.

In the above-described arrangement, an image is divided into three modulators (e.g., modulators21to23), and one horizontal scanning device (e.g., device41) is provided with respect to each divided image. Therefore, ⅓ of the ordinary frequency range and ⅓ of the ordinary scanning frequency suffice as the frequency range of these modulators (e.g., modulators21to23) and the scanning frequency of the horizontal scanning device (e.g., device41). This means that a high resolution image can be formed.

The conventional art presupposes use of a laser oscillator having a sufficiently high optical output level and lacks consideration of replacement of such a light source with a semiconductor laser or an LED of a lower optical output level. For example, light emitting diodes (LEDs) having an optical output of about several milliwatts may be used as light sources. Since the LEDs can be directly modulated, there is no need for external modulators (e.g., modulators21to23).

However, ten or more LEDs are required with respect to each of red, green and blue to avoid a deficiency of screen brightness. In the case of the arrangement shown inFIG. 1, ten or more groups of light sources and scanning devices are required if LEDs are used as the light sources. It is not realistic to use LEDs in the conventional art.

Also, gradation of a certain color, e.g., red, depends on the performance of an external modulator (e.g., modulator21). On the other hand, in a case where an LED is used as a light source, gradation depends on the modulation frequency at which the LED is directly modulated (by pulse-width modulation or amplitude modulation), and it is difficult to increase gradation steps since the LED is not suitable for high-speed modulation.

In a case where LEDs of one color, e.g., red, are substituted for the lasers11to13in the arrangement shown inFIG. 1, a loss of ⅓ is necessarily caused even if the beam-combining optical system31is suitably formed. In this case, use of an increased number of LEDs is not effective, and it is not possible to avoid a deficiency of screen brightness.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a multipurpose image display apparatus capable of displaying an image having a suitable number of gradation steps with sufficiently high screen brightness even if a light source such as a semiconductor laser or an LED of a comparatively low optical output level is used.

To attaint the above-described object, according to an aspect of the present invention, there is provided an image display apparatus having a light source having a plurality of light emitting devices, and making lights from the light source scan in a main scanning direction and in a subscanning direction to display on a screen an image having a predetermined number of pixels, wherein scanning lines in the main scanning direction formed by the lights emitted from each of the light emitting devices are controlled to be superposed one on another on the screen.

In the above image display apparatus, the lights from the light emitting devices may be irradiated on the same point on the screen simultaneously or at different times with a certain time lag.

In the above image display apparatus, the light emitting devices may be arranged unidimensionally or two-dimensionally.

In the above image display apparatus, the light emitting devices may be arranged in a direction corresponding to main scanning.

In the above image display apparatus, the light emitting devices may be arranged in a direction corresponding to subscanning while being spaced apart from each other by a distance determined on the basis of a pixel pitch in the subscanning direction.

In the above image display apparatus, the light emitting devices may be arranged in a direction not parallel to each of a direction corresponding to main scanning and a direction corresponding to subscanning, and the distance between the light emitting devices in the direction corresponding to subscanning may be equal to a distance determined on the basis of a pixel pitch in the subscanning direction.

In the above image display apparatus, each of the light emitting devices may be designed so as to have an optical output of any of multivalue intensities.

In the above image display apparatus, each of the light emitting devices may output different quantities of light by modulating at least one of a pulse width and an amplitude.

In the above image display apparatus, the plurality of light emitting devices in the light source may be separated into a certain number of light emitting device groups respectively outputting lights with which different image areas on the screen are irradiated. The light emitting device groups may be arranged in a direction corresponding to subscanning or in a direction corresponding to main scanning while being spaced apart from each other by a distance determined on the basis of a pixel pitch in the main scanning direction or the subscanning direction. Further, the light emitting devices in each of the light emitting device groups may be arranged in a direction corresponding to subscanning while being spaced apart from each other by a distance determined on the basis of a pixel pitch in the subscanning direction.

In the above image display apparatus, the light source may have light emitting devices capable of emitting red light, green light, and blue light.

In the above image display apparatus, the lights from the emitting devices may be substantially uniform in color.

The above image display apparatus may further comprise scanning means in which a scanning frequency and a scanning angle in the main scanning direction or the subscanning direction are controlled in such a manner that the light emitting devices in the light source are arranged in predetermined array while being spaced apart from each other by a distance determined on the basis of a pixel pitch in the main scanning direction or the subscanning direction. The scanning means may comprise a galvanometer mirror or a rotating polygon mirror.

In the above image display apparatus, each of the light emitting devices may be a laser, a light emitting diode or a super-luminescent diode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In this specification, scanning at a higher speed in one direction is defined as main scanning, while scanning at a lower speed in another direction is defined as subscanning. Since in ordinary cases horizontal scanning is performed at higher speed, main scanning and subscanning are assumed to be a scanning in a horizontal direction and vertical scanning, respectively, in the following description. Needless to say, image display can be performed even if the relationship between main scanning and subscanning is reversed with respect to the scanning direction.

FIG. 2Ais a diagram schematically showing a configuration of an image display apparatus which represents a first embodiment of the present invention.FIG. 2Bis an enlarged diagram of a screen shown inFIG. 2A.

For example, three red semiconductor lasers201ato201chaving an optical output of about 30 mW are arranged in the light source201at intervals of 100 μm in a direction221corresponding to horizontal scanning.

The direction corresponding to horizontal scanning is a direction enabling projection of beams from the light source201such that when the beams are made to scan in a horizontal directions, scanning lines213ato213cformed on the screen211are superposed one on another.

The size of an image area215on the screen is assumed to be 14 inches (284 mm in width and 213 mm in height). To display an image, for example, in Video Graphic Array (VGA) format (having a horizontal resolution of 640 pixels and a vertical resolution of 480 pixels) in the image area215, the number of mirror faces and the rotating speed of the rotating polygon mirror205are set such that the scanning frequency is about 30 kHz. InFIG. 2A, an arrow240indicates a direction of rotation of the mirror205.

The galvanometer mirror209is driven with a sawtooth wave at a frequency of about 60 Hz. The galvanometer used in this embodiment is operated at a comparatively low speed and may be selected from ordinary ones on the market using a machine-wound drive coil unlike one formed by a semiconductor process and used in a third embodiment described below.

The power of the entire projection optical system including the projection lens209is assumed to be 10.

If the above-mentioned scanning frequency is set, the number of scanning lines formed from one semiconductor laser device is 500 per frame, and 480 lines among them are used for actual image formation.

The screen211is not limited to a particular type. A specially designed screen may be used or images may be projected onto a wall or a ceiling.

When the light beams are made to scan horizontally by using the image display apparatus shown inFIG. 2A, scanning lines213ato213care formed on the screen211by the light beams from the red semiconductor lasers201ato201c.

Since the red semiconductor lasers201ato201care arranged in the direction corresponding to horizontal scanning, the scanning lines213ato213care superposed on a straight line on the screen211. However, the scanning lines213ato213care shifted one from another in the horizontal direction by 2 mm corresponding to the product of the power and the interval between the red semiconductor lasers201ato201c.

Therefore, for compensation for this shift, signals are applied to the red semiconductor lasers201ato201cwith relative time lags set by considering the horizontal shifts of the scanning lines to effect multiple projection to predetermined points (pixels), thus enabling control of the luminance of pixels.

In this embodiment, each of the red semiconductor lasers201ato201cis pulse-width-modulated or amplitude-modulated, for example.

FIG. 3is a diagram showing pulse waveforms when, for example, three light emitting devices (light sources a to c, corresponding to the semiconductor lasers201ato201cinFIG. 2A) are pulse-width-modulated. The ordinate represents light power. In actuality, the light beams pass the position of one pixel at different times since the light beams from the three light emitting devices are projected to different positions on the screen. However, the relationship between the beams is expressed by ignoring the time differences for ease of description. The pulse width is modulated on every pixel, as shown in the diagram, thus achieving expression of multivalue gradation for a high-resolution image. Pulse-width modulation needs to be performed at a high frequency and is therefore suitable for use with a semiconductor laser.

FIG. 4is a diagram showing pulse waveforms when, for example, three light emitting devices (light sources a to c) are modulated by binary (ON/OFF) switching. The ordinate represents light power. Also in this case the waveforms are shown by ignoring the time differences. In this case, the number of gradations steps is the number of light sources+1 (including zero luminance). The number of gradation steps is reduced in comparison with that in the case of modulation such as shown inFIG. 3, but a comparatively low modulation frequency suffices. Therefore, this method is suitable for use with LEDs.

Further, the optical outputs of, for example, three light emitting devices (light sources a to c) may be set to different levels in advance, as shown inFIG. 5. Also inFIG. 5, the waveforms are shown by ignoring the time differences. The optical outputs of the light emitting elements set to three levels “1”, “2”, and “4”, for example. If the outputs are set to such levels that the optical output proportions correspond to the factorial of 2 as in this case, the number of gradations expressible is maximized and the number of gradations in this case is the number determined by multiplying 2 by itself the number of times corresponding to the number of light sources (including zero luminance). Thus, the number of gradations can be increased in comparison with the method shown inFIG. 4.

One of the methods shown inFIGS. 3,4, and5may be selected according to one's use.

Thus, signals are modulated with respect to the pulse width or amplitude to enable gradational expression and to thereby display a high-resolution image.

And, as shown inFIG. 2B, vertical scanning with the scanning lines213ato213cwith a pixel pitch (213 mm/480=0.44 mm) set in the vertical direction is performed to form an image. InFIGS. 2A, and2B, the three scanning lines213ato213care shown in a state of being slightly shifted one from another in a vertical scanning direction232for ease of explanation. However, the scanning lines coincide with each other in the vertical scanning direction.

Some area where no image can be displayed exists on the screen211due to the shift of the scanning lines213ato213cin the horizontal scanning direction231. In this embodiment, when an image is actually displayed on the screen211within the image displayable area215, the screen brightness is about 100 cd/m2. Thus, an image in VGA format can be displayed so as to be easily seen in an ordinarily lighted room.

In this embodiment, the number of red semiconductor lasers201ato201cmay be increased and the distance therebetween may be changed. In particular, if the distance is excessively reduced, interference between the semiconductor lasers may influence modulation or a high degree of manufacturing processing accuracy may be required. If the distance is excessively large, the number of product laser devices per laser wafer area is reduced to cause a reduction in yield. The distance between the lasers may be optimized by considering these conditions.

A light emitting diode (LED) or super-luminescent diode (SLD) may be used instead of the red semiconductor laser. Further, the power of the optical system and the lens configuration may be changed.

FIG. 6Ais a diagram schematically showing the configuration of an image display apparatus which represents a second embodiment of the present invention. This image display apparatus uses a light source251in which semiconductor lasers251ato251care arranged in a direction222corresponding to vertical scanning (a direction perpendicular to the direction corresponding to horizontal scanning in Embodiment 1) unlike those shown inFIG. 2A. InFIG. 6A, components corresponding to those shown inFIG. 2Aare indicated by the same reference characters.FIG. 6Bis an enlarged diagram of the screen shown inFIG. 6A.

The size and resolution of an image formed on the screen and the modulation frequency at each scanning device are the same as those in Embodiment 1.

The distance between the semiconductor lasers251ato251cis assumed to be an integer multiple of a value obtained by dividing the pixel pitch (213 mm/480=0.44 mm) in the vertical direction of an image formed on the screen211by the power (e.g., 10) of the projection optical system. In this embodiment, the distance is set to 132 μm obtained by multiplying the result of this division by 3.

Because the semiconductor lasers251ato251care arranged in this manner, the i−th one from the top in scanning lines253a,the (i−3)th one from the top in scanning lines253b,and the (i−6)th one from the top in scanning lines253ccoincide with each other when vertical scanning with each of the scanning lines with pixel pitch in the vertical-direction is performed. The 480 scanning lines each formed by three of the above-described scanning lines coinciding with each other are used to enable display of an image in VGA format in an image area255. A modulation method used in this embodiment may be selected from various methods such as those described above with respect to Embodiment 1. InFIG. 6B, the group of scanning lines consisting of scanning lines253aand253bis denoted by261; the group of scanning lines consisting of scanning lines253a,253band253cis denoted by262; and the group of scanning lines consisting of scanning lines253band253cis denoted by263.

In this embodiment, distance between the semiconductor lasers may be any of possible values determined as integer multiples of the value obtained by dividing the pixel pitch by the power of the projection optical system. It may be selected by considering interference between the semiconductor lasers, the manufacturing process, yield, etc.

Further, a light source301such as shown inFIG. 7may be used in which semiconductor lasers301ato301nare arranged in a direction inclined at a predetermined angle from the horizontal scanning direction. Also in this case, the semiconductor lasers301a,301band so on may be arranged while being inclined so that distance d3thereof in the direction corresponding to vertical scanning is an integer multiple of the value obtained by dividing the pixel pitch by the power of the projection optical system. Thus, the distance between the semiconductor lasers can be freely selected with a high degree of design freedom.

Further, light emitting devices such as semiconductor lasers or LEDs may be arranged in a two-dimensional array, and horizontal scanning and vertical scanning may be performed so that all the scanning lines therefrom are superposed with suitable time lags.

FIG. 8is a diagram schematically showing a configuration of an image display apparatus which represents a third embodiment of the present invention. This image display apparatus uses as a horizontal scanning device a galvanometer mirror271having micromirrors formed by a semiconductor process or the like, which is different fromFIG. 2A.

Such a micromirror is described, for example, in a publication “Silicon Microopitcal Scanner” pp 13–17, No. 3, Vol. 14, Microoptics group organ, The Japan Society of Applied Physics.

Such a micromirror is suitable for reducing the overall size, weight, and power consumption of the display apparatus, and is capable of high-speed oscillation at several ten kilohertz.

In this embodiment, the size, weight, price, and power consumption of the image display apparatus can be reduced in comparison with that using the rotating polygon mirror205shown inFIG. 2A, or the like.

FIG. 9Ais a diagram schematically showing a configuration of an image display apparatus which represents a fourth embodiment of the present invention.FIG. 9Bis a schematic diagram showing a relationship between a light source shown inFIG. 9Aand a screen. A light source400in which surface-type LEDs, surface emitting lasers, or the like are two-dimensionally arranged is shown as a light source in this embodiment.

The light source400has LED groups401to403consisting of n number of LEDs401ato401n, n number of LEDs402ato402n, and n number of LEDs403ato403n, respectively. In this embodiment, an ON/OFF method such as that shown inFIG. 4is used as a modulation method for the light source400. If such LEDs are used, the need for external optical modulators is eliminated.

Divisional images1to3shown inFIG. 9Bare respectively displayed by the LED groups401to403.

The distance d4between the LED groups401,402, and403is set to 7.1 mm, i.e., ⅓ of the value obtained by dividing the entire pixel area width (e.g., 213 mm) in the vertical scanning direction on the screen by the power of the projection optical system (e.g., 10).

The distance between the LEDs401a,401b, and so on in the LED groups401to403is set to 50 μm, for example. The optical output of the LEDs401aand so on is set to about 3 mW.

The arrangement of the optical devices other than the light source, etc., are the same as those shown inFIG. 2A, and the plurality of scanning lines from the LEDs for each divisional image are superposed in the same manner as those in Embodiment 1.

Vertical scanning with the plurality of scanning lines with the pixel pitch in the vertical direction is performed by using the light source400shown inFIG. 9Ato form an image. The display apparatus of this embodiment has the following advantages in comparison with those shown inFIG. 2Aand so on.(1) Since ⅓ of the ordinary modulation frequency suffices as the modulation frequency of the light source400, the LEDs not easy to modulate at a high speed can be easily modulated. Further, since the time assigned to each LED with respect to each of pixels forming an image is increased, the optical output for obtaining substantially the same screen brightness can be reduced.(2) The scanning frequency of the horizontal scanning device can be reduced to ⅓.(3) The scanning angle of the vertical scanning device can be reduced to ⅓. This means that a galvanometer mirror having a smaller scanning angle may be used or a galvanometer mirror having the same scanning angle as that shown inFIG. 2Abut unsatisfactory in time-angle linearity when driven with a sawtooth wave may be used with respect to a time zone with higher linearity, thus enabling use of a galvanometer mirror advantageous in terms of cost.

The number of LEDs401aand so on and the number of LED groups401and so on are not limited to those in the above-described example. These numbers may be freely set provided that, with respect to the number N corresponding to the number of divisional images and the number of LED groups, the distance d4is 1/N of the value obtained by dividing the image area width in the vertical scanning direction on the screen by the power of the projection optical system. The modulation frequencies and the scanning angles of the horizontal scanning device and the vertical scanning device may be suitably set according to the number of LEDs and the number of LED groups.

FIG. 10Ais a diagram schematically showing a configuration of an image display apparatus which represents a fifth embodiment of the present invention.FIG. 10Bis a schematic diagram showing a relationship between a light source shown inFIG. 10Aand a screen.

Referring toFIG. 10A, a light source500is formed in such a manner that a plurality of unidimensional laser arrays301each having a semiconductor lasers301aand so on such as those shown inFIG. 7are combined to form a two-dimensional array, and LEDs or surface emitting laser arrays or the like are substituted for the semiconductor lasers301aand so on.

FIG. 10Ashows an example of the light source500having 10 LED groups500ato500j. For example, the LED group500aconsists of three LEDs501a,502a, and503a. A pulse-width modulation is used as a modulation method for the light source500.

As shown inFIG. 10A, the LED groups500ato500jrespectively form divisional images a to j, which are combined to form a synthesized image.

The distance d51between each adjacent pair of the LED groups in the direction corresponding to vertical scanning is set to 44 μm, i.e., the value obtained by dividing the pixel pitch (e.g., 0.44 mm) in the vertical direction on the screen by the power of the projection optical system (e.g., 10). The distance between each adjacent pair of the LED groups in the horizontal direction corresponding to horizontal scanning401is set to 50 μm, for example.

Also, the distance d52between the LEDs in each LED group is set to 440 μm ten times larger than the value obtained by dividing the pixel pitch (e.g., 0.44 mm) in the vertical direction on the screen by the power of the projection optical system (e.g., 10).

In this embodiment, the optical output of the LEDs501aand so on is set to about 3 mW. Further, the scanning frequency in the horizontal scanning direction is 1/10 of that in the arrangement shown inFIG. 6A. Vertical scanning with the plurality of scanning lines with a pitch ten times larger than the pixel pitch in the vertical direction is performed, so that the scanning lines formed by the plurality of LEDs in one of the LED groups (e.g., LEDs501ato503a) are superposed.

If the light source500shown inFIG. 10Ais used, the modulation frequency of the light source500and the scanning frequency of the horizontal scanning device can be reduced to 1/10 of those in the arrangement shown inFIG. 6A. Accordingly, direct modulation of the LEDs501aand so on can be easily performed and pulse width modulation can also be performed. Also, the optical output of the LED can also be limited.

In this embodiment, the number of LEDs501aand so on and the number of LED groups500aand so on are not limited to the above-mentioned examples. Preferably, with respect to the number N of divisional images, the distance d51is set to the value obtained by dividing the pixel pitch in the vertical direction on the screen by the power of the projection optical system, or aN+b times larger than this value (a: an integer equal to or larger than 1; b: an integer equal to or larger than 1 and smaller than N), and the distance d52is set N times larger than the value obtained by dividing the pixel pitch in the vertical direction on the screen by the power of the projection optical system, or to an integer multiple of this value. In this case, the scanning frequency of the horizontal scanning device may be reduced to 1/N of those in the arrangement shown inFIG. 6A.

FIG. 11Ais a diagram schematically showing a configuration of an image display apparatus which represents a sixth embodiment of the present invention.FIG. 11Bis a schematic diagram showing a relationship between a light source shown inFIG. 11Aand a screen.

Referring toFIG. 11A, a light source600is formed in such a manner that a plurality of unidimensional laser arrays301each having a semiconductor lasers301aand so on such as those shown inFIG. 7are combined to form a two-dimensional array, and LEDs or surface emitting laser arrays or the like are substituted for the semiconductor lasers301aand so on.

The light source600has LED groups600ato600ceach having n LEDs, e.g., LEDs601a,602a,603a, and so on. A pulse-width modulation is used as a modulation method for the light source600. Each of the LED groups600ato600chas about 10 LEDs.

As shown inFIG. 11B, the LED groups600ato600crespectively form divisional images a, b, and c.

The distance d62between the LED601c,602cand so on is set to 88 μm twice as large as the value obtained by dividing the pixel pitch (e.g., 0.44 mm) in the vertical direction on the screen by the power of the projection optical system (e.g., 10).

The distance d61between the LED groups,600a,600b, and600cis set to 9.5 mm, i.e. ,⅓ of the value obtained by dividing the entire pixel area width (e.g., 284 mm) in the horizontal scanning direction on the screen by the power of the projection optical system (e.g., 10). The optical output of the LEDs601aand so on is set to about 3 mW.

Vertical scanning with the plurality of scanning lines with the pixel pitch in the vertical direction can be performed by using the light source600shown inFIG. 11Ato form an image. The modulation frequency of the light source600and the scanning angle of the horizontal scanning device can be reduced to ⅓ of those in the arrangement shown inFIG. 6A.

In this embodiment, the number of LEDs601aand so on and the number of LED groups600ato600care not limited to the above-mentioned examples. Preferably, with respect to the number N corresponding to the number of divisional images and the number of LED groups, the distance d61between the LED groups600a,600b, and600cis set to 1/N of the value obtained by dividing the image area width in the horizontal scanning direction on the screen by the power of the projection optical system, and the distance d62between the LEDs601c,602cand so on is set to the value obtained by dividing the pixel pitch in the vertical direction on the screen by the power of the projection optical system, or to an integer multiple of this value.

FIG. 12is a diagram schematically showing a configuration of an image display apparatus which represents a seventh embodiment of the present invention. InFIG. 12, there are illustrated a red LED array701having red LEDs701ato701c, a green LED array703having green LEDs703ato703c, a blue LED array705having blue LEDs705ato705c, a combining optical system707constituted by a dichroic mirror or the like, a collimator lens709, and a horizontal scanning device711. The direction of rotation of the horizontal scanning device711is indicated by740.

Light from the horizontal scanning device711travels via a vertical scanning device and a projection optical system to reach a screen, as does that in the arrangement shown inFIG. 2A.

In this embodiments the LED arrays701,703, and705for display in three colors are used to form a multicolor image. The color light source has arrayed elements each having a limited optical output level, such that even if LEDs having an optical output of several mW or less, sufficiently high screen brightness can be obtained.

The operation of the optical system shown inFIG. 12will be briefly described. Light beams emitted from the LEDs701aand so on in the LED arrays701,703, and705are color-mixed by the combining optical system707, travel through the collimator lens709, and strike the horizontal scanning device711. At this time, the light beams from the LEDs701a,703a, and705aare color-mixed to form a color-mixed light beam713a. Similarly, color-mixed light beams713band713care formed.

The color-mixed light beams713a,713b, and713care made to scan by the horizontal scanning device711to form scanning beams715a,715b, and715c,which travel through the projection optical system to reach the screen. On the screen, three scanning lines are formed by the color-mixed light beams713a,713b, and713c. The LED arrays in the light sources701,703, and705are placed so that the scanning lines strike the same point at different times suitably shifted. The placement of the LED arrays is performed in the same manner as that in the Embodiment 1 or 2.

The display apparatus is thus arranged to enable a color image in VGA format having sufficiently high screen brightness even if LEDs having a lower optical output level are used as light sources.

While various embodiments of the present invention have been described, an amplitude modulation method other than the modulation methods in the described embodiments may used for the light source. A method in which some of the above-described modulation methods are combined may also be used.

The placement of the light emitting devices is not limited to those described above. The light emitting devices may be placed in any other way if the scanning lines can be superposed by horizontal scanning and vertical scanning.

The light emitting device is not limited to the semiconductor light emitting device. A gas laser or a semiconductor laser-excited solid-state layer may also be used. If such a device is used, modulation may be performed by using an external modulator such as an acoustooptical modulator.

The size of images to be displayed may be selected as desired according to one's use. For example, a size of 10 to 15 inches may suffice for display on a computer display or a personal television display. In the case of display for a conference in which a large number of people participate, the screen size may be set to a comparatively large size, e.g., a size of 50 inches or larger. The power of the projection optical system, the optical output of the light source, and the number of light source arrays may be set according to the screen size and brightness.

According to the present invention, as described above, the scanning lines are superposed on the screen, so that the screen brightness can be improved even if a light source lower in optical output is used.