Scanned-beam heads-up display and related systems and methods

A heads-up display that includes a scanner and a projection assembly. The scanner generates an image by sweeping a beam of electromagnetic energy, and the projection assembly directs the image into a predetermined viewing space having a region with a substantially uniform intensity profile. Such a heads-up display can often be made smaller than a conventional heads-up display, and can often generate an image having a higher quality than an image generated by a conventional display. Furthermore, one can often calibrate and recalibrate such a display without physically modifying or replacing a component of the display or of a vehicle incorporating the display.

CLAIM OF PRIORITY

This application claims priority to co-pending International Application number PCT/US2005/003730 filed on Feb. 4, 2005 which claims priority to U.S. Provisional Application Ser. No. 60/541,619 filed on Feb. 4, 2004, and U.S. Provisional Application Ser. No. 60/565,059 filed on Apr. 23, 2004, both of which are incorporated by reference.

BACKGROUND OF THE INVENTION

Referring toFIG. 1, a heads-up display10typically generates a virtual image of one or more gauges, e.g., a speedometer and an odometer (not shown), for viewing by an operator12of a vehicle such as an automobile14. The term “heads up” indicates that the operator12need not lower his gaze to a dashboard16to view the gauges. That is, the operator12can keep his head up and his “eyes on the road” while viewing the gauges. Therefore, the display10is an added convenience that also increases the level of safety with which the operator12can operate the vehicle14.

In operation, the heads-up display10generates and projects the virtual image of the one or more gauges (not shown) onto a windshield, i.e., a wind screen18, which reflects the image into the eyes20of the operator12such that the image appears within the operator's field of view (FOV), sometimes appearing to be at some apparent distance beyond the wind screen. For example, the display10may include a light-emitting-diode (LED), liquid crystal (LCD), vacuum fluorescent, or other display technology (not shown) for generating the image, and an optical train (not shown inFIG. 1) for projecting the image onto the wind screen18.

Typically, the operator12is best able to view the virtual image while his eyes20are within a three-dimensional viewing space22, which is sometimes called an eye box. Although the dimensions (e.g., height, width, and/or depth) of the viewing space22are typically fixed, they are typically sufficient to accommodate the anticipated ranges of up-and-down, side-to-side, and front-to-back movements of the operator's head20while the operator is operating the automobile14. Furthermore, because the range of operator heights is relatively large (e.g., 5 feet tall-7 feet tall), the display10may allow the operator12to adjust the vertical position of the viewing space22so as to align the viewing space with the operator's eyes20.

Unfortunately, the typical heads-up display10may have several shortcomings. For example, the display10may be relatively bulky, and may consume a relatively large amount of power. Furthermore, the quality of the virtual image within the viewing space22may be poor. Moreover, because each wind screen18, even for the same model car, may be slightly different than every other wind screen, the procedure for calibrating the display10for each new vehicle may be relatively complex, and may include physically modifying or replacing one or more parts of the display, or perhaps either replacing the entire heads-up display or wind screen. Furthermore, if one ever needs to repair or replace the windscreen, the need to recalibrate the display10by modifying or replacing a part or all of the display may significantly increase the time and expense required for an otherwise ordinary repair.

SUMMARY OF THE INVENTION

One aspect of the invention is a heads-up display that includes a scanner assembly and a projection assembly. The scanner assembly generates an image by sweeping a beam of electromagnetic energy, and the projection assembly directs the image into a predetermined viewing space having a region with a substantially uniform intensity profile.

Such a heads-up display can often be made smaller than a conventional heads-up display, and can often generate an image having a higher quality than an image generated by a conventional display. Furthermore, one can often calibrate and recalibrate such a display without physically modifying or replacing a component of the display or of a vehicle incorporating the display.

DETAILED DESCRIPTION

The following discussion is presented to enable a person skilled in the art to make and use the invention. The general principles described herein may be applied to embodiments and applications other than those detailed below without departing from the spirit and scope of the present invention. Therefore the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed or suggested herein.

The Viewing Space

FIG. 2is an isometric view of a viewing space30according to an embodiment of the invention. Generally, it is desired that a heads-up display, such as the scanned-beam heads-up display discussed below in conjunction withFIGS. 3-22, generate the viewing space30such that parameters of the viewing space are within respective predetermined ranges that allow a virtual image to have an acceptable and substantially uniform quality regardless of the position of the operator's eyes within the viewing space. Therefore, some of these viewing-space parameters, as well as desired values for these parameters, are discussed below in conjunction withFIG. 2. Following the discussion of the viewing space parameters is a discussion, in conjunction withFIGS. 3-22, of a heads-up display that can achieve desired parameter values.

Still referring toFIG. 2, the viewing space30has a height h (y dimension), a width w (x dimension), and a depth d (z dimension), and the virtual image perceived when the eye is within the viewing space30is formed by diffraction orders, i.e., beamlets32, which, at any point in time, each carry the same pixel of the virtual image—for clarity, only four beamlets32a-32dare shown. Therefore, as long as the operator's eyes (not shown inFIG. 2) are capturing the light energy of at least one of the beamlets32, the operator will perceive the pixel of the image. During the generation of the center pixel of the image, the beamlets32are positioned approximately parallel to the z axis as shown inFIG. 2. During the generation of other pixels, the beamlets32are canted (canting not shown inFIG. 2) at slight angles relative to the z axis. That is, as the heads-up display (not shown inFIG. 2) scans the image pixel by pixel and line by line, the beamlets32effectively “pivot” up/down and left/right, within the FOV, from the nominal beamlet angle of the center-pixel position; it is this “pivoting” that causes one or more of the beamlets32to be scanned onto the retinas (not shown) of the operator's eyes. The location at which the beamlets32“pivot” is a cross section34that is coplanar with the x-y plane and that is located in the middle (d/2) of the viewing space30in the z dimension. Therefore, as a result of this “pivoting,” the height h and the width w of the viewing space30(which includes beamlets from all pixels within the FOV) actually changes slightly as one moves out from the cross section34in either direction in the z dimension. But because this expansion is slight, describing the viewing space30as having constant values of h and w along the entire depth d is a good approximation.

Parameters of the viewing space30that affect the virtual image quality and uniformity include the aperture, the expanded-beam intensity profile, and the fill factor. The aperture of the viewing space30is of the same shape as that of the cross section34. The expanded-beam intensity profile, which is discussed below in conjunction with FIGS.3and7-12, is a plot of the relative expanded-beam intensity at each location within the viewing space, and the fill factor is a measure of the spacing S between adjacent beamlets32. Ideally, the aperture of the viewing space30should match the x-y area within which an operator is likely to move his head, the expanded beam-intensity profile should be uniform throughout the viewing space, and the fill factor should be such that there are no visible gaps between adjacent beamlets32.

The aperture of the viewing space30has an elongated hexagonal shape, which has been found to closely mimic the typical x-y area within which the operator may move his eyes while operating a vehicle (neither operator, eyes, nor vehicle shown inFIG. 2). The pattern of the beamlets32, which is further discussed below in conjunction withFIGS. 7-8, defines the aperture of the viewing space30. In this example, the beamlets32are in a hexagonal pattern having a compressed height h and an elongated width w as compared to a symmetrical hexagon—h is less than, i.e., is compressed relative to, w because for the exemplary application the horizontal range within which an operator might move his eyes while operating the vehicle is greater than the vertical range within which the operator typically moves his eyes. Consequently, the aperture of the viewing space30has a shape that accommodates relatively greater horizontal movement and relatively less vertical movement. In one example, the viewing space30has a height h≈150 millimeters, a width w≈200 millimeters, and a depth d≈400 millimeters. Other applications may be characterized by a range of operator eye positions that has a different shape. The aperture of the viewing space30may be similarly modified to accommodate such a different shape.

Still referring toFIG. 2, the expanded-beam intensity (i.e., brightness) profile within the viewing space30is substantially uniform. That is, the brightness of the image is substantially the same regardless of the position of the operator's eyes (not shown inFIG. 2) within the viewing space30. A relatively uniform expanded-beam intensity profile allows the operator move his eyes within the viewing space30without perceiving, and thus being distracted by, a noticeable change in the image intensity. There are two primary factors that determine the expanded-beam intensity profile within the viewing space30: the intensity profile of the beam envelope (not shown inFIG. 2), and the intensity profile of the beamlets32. For many embodiments, the beamlets each have substantially the same intensity profile. As further discussed below in conjunction withFIGS. 9A-12, as long as the intensity profile of the beam envelope and the intensity profile of the beamlets32are both substantially uniform, then the expanded-beam intensity profile within the viewing space30is also substantially uniform. In one example, the brightest region of the expanded-beam intensity profile within the viewing space30is approximately 11% brighter than the dimmest region of the beam intensity profile within the viewing space. That is, (Ibrightest−Idimmest)/(Ibrightest+Idimmest)≦˜11%.

The fill factor of the viewing space30is high enough to insure that an operator's eye (not shown inFIG. 2), while within the viewing space, captures at least one beamlet32, and thus perceives the virtual image regardless of the eye's position within the viewing space. That is, there are no “holes” in the viewing space30where an operator can “lose” the image. The fill factor can range from 0% where the beamlets32do not exist, i.e., have a zero-diameter aperture, to 100% where the beamlets32are contiguous at all points of their respective peripheries such that there is no “empty” space between the beamlets, to greater than 100% where the beamlets32overlap. Although it is possible for the beamlets to overlap, it is assumed for purposes of this discussion that no such overlapping occurs. The aperture of the beamlets32controls the maximum fill factor of the viewing space30. For example, with beamlets having a circular aperture, the maximum fill factor when the beamlets are packed with their diameters just tangent to one another is less than 100% because of the interstitial spaces between the circles. If beamlets32have a hexagonal shape,the beamlets may be closely packed within the viewing space30and have a fill factor of 100% without overlapping. Similarly, flattened hexagonal, square, rectangular, parallelogram, etc. beamlet shapes may be closely packed without overlapping. To ensure that an operator's eye receives at least one beamlet32, the spacing S between the sides of adjacent beamlets is less than the smallest diameter of the eye's pupil (not shown inFIG. 2). In bright ambient conditions, the pupil's diameter may be as small as about 2 millimeters (mm). Consequently, when the spacing S is less than ˜2 mm, the fill factor is high enough to ensure that there are no “holes” in the viewing space30.

Still referring toFIG. 2, alternate embodiments of the viewing space30are contemplated. For example, the viewing space30and the beamlets32may have apertures other than hexagonal. Furthermore, the aperture of the beamlets32need not be a scaled-down version of the aperture of the viewing space30. For example, the viewing space30may have an elongated hexagonal aperture and the beamlets32may have a symmetrical hexagonal aperture as shown inFIG. 2. Moreover, the expanded-beam intensity profile within the viewing space30may not be substantially uniform, yet may still yield an image of acceptable quality in at least some locations within the viewing space. In addition, the cross section34may be curved instead of planar. For example, the cross section34may have a curvature corresponding to the arc swept by the operator's eyes in one or both of the x and y dimensions. Furthermore, although only one cross section34is shown inFIG. 2, it is understood that the viewing space30includes an infinite number of cross sections that are parallel to the cross section34. Moreover, the cross section where the beamlets32“pivot” may be located other than in the middle (d/2) of the viewing space30. In addition, although described as comprising visible light, the beamlets32may include electromagnetic energy outside of the visible spectrum. For example, the beamlets32may include infrared energy such that the operator (not shown inFIG. 2) wears infrared goggles (not shown) to view the virtual image within the viewing space30.

Heads-Up Display for Generating the Viewing Space of FIG.2

FIG. 3is a diagram of a scanned-beam heads-up display40, which is operable to generate the viewing space30ofFIG. 2according to an embodiment of the invention. Also shown is a wind screen42which the display uses as a reflector to direct the virtual image into the viewing space30. By using a scanned beam to generate the virtual image, the display40may be smaller, have higher performance (eg, superior contrast, sharpness and brightness), be more energy efficient, and/or be easier to calibrate than conventional heads-up displays. Furthermore, where the display40includes substantially no scattering surfaces and the scaned beam comprises coherent electromagnetic energy such as laser light, visible speckle in the virtual image may be reduced or eliminated. Moreover, using the display40to generate a virtual image of vehicle information may allow one to simplify the physical vehicle information displays within the vehicle. For example, if the display40generates a virtual image of the fuel gauge, then one can omit a physical fuel gauge from the vehicle's dashboard.

Still referring toFIG. 3, the heads-up display40includes a beam generator44, a scanner46, a scanned-beam-conditioning assembly48, which includes a lens50, an exit-pupil expander52, and a beam-projection assembly54, which includes a lens56and a reflector58or a combined lens-reflector. Both the beam-conditioning assembly48and the beam-projection assembly54may be or include respective optical trains.

The beam generator44generates a color output beam60, and includes image electronics62, red (R), green (G), and blue (B) beam sources64,66, and68for respectively generating R, G, and B beams70,72, and74, and a beam-combiner-and-conditioner assembly76. Some heads-up displays may alternatively use a single color or a subset of R, G and B. The beam generator may also include other wavelengths of light, for example to further improve color gamut, brightness, etc.

The electronics62modulate the R, G, and B beam sources64,66, and68, and thus the beams70,72, and74, such that the output beam60has the desired intensity and color content for a pixel of the image that the electronics is currently generating. The electronics62may modulate the beam sources using time modulation, where a beam is full “on” for a predetermined portion of the pixel-generation time and full “off” for another portion of the pixel-generation time, or using intensity modulation, where the intensity of a beam is modulated to a predetermined level for an entire pixel-generation time.

The beam sources64,66, and68may be conventional R, G, and B lasers or light-emitting diodes (LEDs), and the beams70,72, and74may propagate to the beam-combiner-and-conditioner assembly76through the air (or other medium) or via an optical fiber (or other optical path). For example, coupling a laser beam from one of the beam sources64,66, or68to the assembly76via a single-mode optical fiber may improve the quality of the beam at the assembly as compared to a beam that propagates to the assembly through a medium such as air. Furthermore, coupling a laser beam to the assembly76via an optical fiber allows the corresponding beam source to be located remotely from the rest of the display40, such as in the trunk of an automobile (not shown inFIG. 3). Remotely locating a beam source64,66, or68may be advantageous where the beam source is a gas laser or otherwise is relatively large or dissipates30a relatively large amount of heat. Alternatively, if a beam source64,66, or68is a semiconductor laser or LED, then it may be located near the assembly76. Moreover, some of the beam sources64,66, and68may be disposed in locations remote from the assembly76, and others of the beam sources may be disposed in locations local to the assembly.

The beam-combiner-and-conditioner assembly76, which is further discussed below in conjunction withFIGS. 4-6, combines the R, G, and B beams70,72, and74into the output beam60, and imparts to the output beam a top-hat intensity profile and the same hexagonal shape (or scaled version thereof) as the beamlets32ofFIG. 2.

The scanner46sweeps the output beam60in two dimensions (e.g. vertical and horizontal) to generate the virtual image viewed by the operator (not shown inFIG. 3) in the viewing space30—two positions of the swept output beam are shown emanating from the scanner in dashed line. The scanner46may be a conventional microelectromechanical system (MEMS) scanner, or other type of scanner. A MEMS scanner typically sweeps the output beam60sinusoidally, i.e., resonantly, in the horizontal dimension, and linearly in the vertical dimension, although a MEMS scanner may sweep the beam resonantly in the vertical dimension as well. The resonant operation allows the MEMS scanner to consume little power, and the relatively small size of the MEMS scanner may allow the heads-up display40to be more compact and lighter than conventional heads-up displays. Examples of a MEMS scanner suitable for use as the scanner46is further discussed in U.S. patent application Ser. No. 10/984,327, entitled MEMS DEVICE HAVING SIMPLIFIED DRIVE, invented by Randall B. Sprague et al., filed Nov. 9, 2004, which is hereby incorporated by reference.

The scanned-beam conditioning assembly48conditions the swept output beam60for input to the exit-pupil expander52. For example, the lens50is a conventional telecentric lens that causes the swept output beam60to enter the exit-pupil expander52normal to the focal plane (not shown) of the expander regardless of the beam's angular position. Furthermore, as discussed in more detail below, the assembly48may include other conventional components such that at the input plane of the expander52, the intensity profile of the beam60is the spatial Fourier Transform in both phase and amplitude of the beam's intensity profile at the output of the beam generator44.

The exit-pupil expander52, which is discussed further below in conjunction withFIGS. 7-18, converts the swept beam60at the expander's input plane into an expanded beam78, which includes the beamlets32(FIG. 2) within a beam envelope79—two positions of the beam envelope are shown in dashed line. The expanded beam78is a larger, i.e., expanded, version of the beam60, and eventually forms the viewing space30. That is, the expanded beam78effectively “sweeps out” the viewing space30. More specifically, as the scanner46sweeps the beam60, the beamlets32within the beam envelope79generate the image in the viewing space30by sweeping across the retinas of the operator's eyes (not shown inFIG. 3). As further discussed below, the expander52may be a diffractive, refractive, reflective, or combination optical element.

The beam-projection assembly54conditions the expanded beam78from the exit-pupil expander52for direction into the viewing space30by the wind screen42. For example, the lens56is a conventional focusing lens, and the reflector58, which is discussed further below in conjunction withFIG. 20, is an aspheric mirror having a curvature designed to optically “match” the curvature of the wind screen42. The assembly54may also include a negative-power lens (not shown inFIG. 3) at the output of the exit-pupil expander52to further expand the beam78before it propagates to the lens56. Furthermore, as discussed in more detail below, the assembly54may include other conventional components such that at the cross section34(FIG. 2) of the viewing space30, the intensity profile of the beam envelope79is the spatial Fourier Transform in both phase and amplitude of the intensity profile of the beam envelope at the output plane (not shown inFIG. 3) of the exit-pupil expander52, and the intensity profile of the beamlets32is the spatial Fourier Transform in both phase and amplitude of the intensity profile of the scanned output beam60at the input plane (not shown inFIG. 3) of the exit-pupil expander.

Although the wind screen42need not be part of the heads-up display40, it is typically the final optical component that directs the expanded beam78from the reflector58to the viewing space30. For example, a region80of the wind screen42that directs the expanded beam78is usually located in the lower third of the wind screen, although other parts of the wind screen may be used. This region may optionally be treated with one or more optical coatings to enhance the wind screen's reflection of the expanded beam. For example, the region80may be treated with narrow-band optical coatings that reflect the R, G, and B wavelengths that compose the expanded beam78but that pass other wavelengths. It has been found that such coatings have a negligible affect on the level of the wind screen's transparency, and, therefore, have little or no adverse affect on the operator's ability to see through the region80of the wind screen42.

Still referring toFIG. 3, the operation of the heads-up display40is discussed according to an embodiment of the invention.

The image electronics62modulates the beams70,72, and74to generate the pixels of the image to be scanned. A range of image aspect ratios may be used. In some examples, the image has an aspect ratio between approximately 2:1 and 3:1, for example, 600 pixels wide (horizontal (x) dimension) by 200 pixels high (vertical (y) dimension). In a typical automotive application where the heads-up display angular field of vehicle is 6×2 degrees, this translates into a resolution in the viewing space30of approximately 100 pixels per degree, which is greater than the approximately 60 pixels-per-degree resolution of the human eye. Such a high number of pixels-per-degree may be used to make text and graphics appear to the operator to have high definition and sharpness, compared to lower resolution displays. In some applications, a relatively high number of nominal pixels-per-degree can be used to adjust or compensate the shape of the image (for example, to compensate for wind screen curvature tolerances) without substantial loss of effective resolution visible to the eye.

The beam generator44generates from the modulated beams70,72, and74the output beam60having a substantially top-hat (i.e., substantially uniform) intensity profile and a hexagonal aperture that is substantially the same shape as the hexagonal aperture of the beamlets32(FIG. 2).

The scanner46vertically and horizontally sweeps the beam60, which propagates to the exit-pupil expander52via the scanned-beam conditioning assembly48.

The optical components of the beam-combining-and-conditioning assembly76and the scanned-beam conditioning assembly48together transform the top-hat intensity profile and hexagonal aperture of the beam60at the input plane of the exit-pupil expander52into the spatial Fourier Transform, in both phase and amplitude of the top-hat profile and hexagonal aperture at the input of the exit-pupil expander52. Because the spatial Fourier Transform of a top-hat profile is a spatial sinc (sin(x)/x) function, the beam60has a two-dimensional sinc-like intensity profile at the input plane of the exit-pupil expander52.

The exit-pupil expander52effectively converts the sinc-like intensity profile of the beam60at its input plane into the beam envelope79, which also has a sinc-like intensity profile at the output plane of the expander. Furthermore, as discussed below in conjunction withFIG. 7, overlap of neighboring sinc-like outputs in the near-field of the expander52form beamlets32in the substantially uniform far field of the expander, each of which also has an intensity profile similar to the profile out of the aperture. Because the expanded beam profile is a result of diffraction, the beam envelope79is sometimes called a diffraction envelope. The beamlets32are introduced within this diffraction envelope from the interference of diffraction envelopes from neighboring pixels.

The expanded-beam projection assembly54projects the expanded beam78onto the wind screen42, which directs the expanded beam into the viewing space30.

The wind screen42and the optical components of the beam-projection assembly54together transform the sinc-like intensity profiles of the beam60and the beam envelope78into the respective spatial Fourier Transforms, in both phase and amplitude, of these sinc-like intensity profiles in the viewing space30. Because the spatial Fourier Transform of a sinc function is a top-hat profile, the beam envelope79has, in the viewing space30, a substantially uniform intensity profile and the hexagonal aperture shown inFIG. 2. As discussed above in conjunction withFIG. 2, the beam envelope79defines the x-y dimensions of the viewing space30, and thus has the same aperture as the cross section34of the viewing space. Moreover, as discussed further below in conjunction withFIG. 8, the hexagonal aperture of the beam envelope79depends only on the structure of the exit-pupil expander52, and is, therefore, independent of the aperture of the beam60. In addition, because the spatial Fourier Transform of the beamlet sinc functions is a top-hat profile with a hexagonal aperture, each beamlet32(FIG. 2) has a substantially uniform intensity profile and an aperture having substantially the same hexagonal shape as the aperture of the beam60does.

Furthermore, an operator (not shown inFIG. 3) may rotate the reflector58about an axis82, which is normal to the page ofFIG. 3, to adjust the position of the viewing space30in the vertical (y) dimension.

Still referring toFIG. 3, alternate embodiments of the heads-up display40are contemplated. For example, the display40may generate a monochrome image instead of a color image, and thus two of the three beam sources64,66, and68may be omitted from the beam generator44, and the beam-combining function may be omitted from the assembly76. Furthermore, the telecentric lens50may be omitted from the scanned-beam conditioning assembly48as discussed further below in conjunction withFIGS. 13 and 17. Moreover, although described as generating a virtual image for viewing by an operator of a vehicle, the display40may be constructed and disposed for generating an image for viewing by another occupant of the vehicle. For example, the display40may play a movie for one or more passengers. The display40may reflect the movie off of a portion of the wind screen42remote from the operator, or from a surface within the vehicle other than the wind screen, and this surface may be transparent, opaque, or partially transparent. Because such a movie comprises a stream of virtual images, a passenger watching the movie is less likely to experience motion sickness than if he were watching the movie on a conventional movie player. In addition, the display40may be used in systems other than a vehicle. Furthermore, the telecentric lens50, the exit-pupil expander52, and a negative power lens (not shown) at the output of the expander for further expanding the beam78may be integrated into a single unit. Moreover, one may replace the telecentric lens50with a graded-index-of-refraction (GRIN) lens, which can be integrated with the exit-pupil expander52into a single unit. In addition, the aperture of the beam envelope79may be annular such that the beam envelope has a “hole” in it. Furthermore, the display40may generate other types of images, such as a map or a telephone directory.

FIG. 4is a diagram of the beam-combining-and-conditioning assembly76ofFIG. 3according to an embodiment of the invention.

The assembly76includes three optical trains90,92, and94, each for conditioning a respective one of the R beam70, the G beam72, and the B beam74, and also includes a conventional beam-combining X cube96, which combines the conditioned beams from the three optical trains into the output beam60.

The optical train90includes a collimating lens98, a top-hat converter100, a hexagonal clipping aperture102, and an optional focusing lens104, which are all designed for the wavelength of the R beam70. In one example, the wavelength of the R beam70is between 635 and 660 nanometers (nm).

Similarly, the optical train92includes a collimating lens106, a top-hat converter108, a hexagonal clipping aperture110, and an optional focusing lens112, which are all designed for the wavelength of the G beam72, and the optical train94includes a collimating lens114, a top-hat converter116, a hexagonal clipping aperture118, and an optional focusing lens120, which are all designed for the wavelength of the B beam74. In one example, the wavelengths of the G beam72and the B beam74are between 440 and 490 nm and between 500 and 550 nm, respectively.

The X-cube96includes a first inner surface122, which is treated with an optical coating that passes the conditioned G and B beams but that reflects the conditioned R beam, and includes a second inner surface124, which is treated with an optical coating that passes the conditioned R and G beams but that reflects the conditioned B beam.

The operation of the beam-combining-and-conditioning assembly76according to an embodiment of the invention is discussed below in conjunction withFIGS. 4 and 5. But first,FIG. 5is discussed.

FIG. 5is a plot of a Gaussian intensity profile126of an input beam, such as one of the beams70,72, and74ofFIG. 4, laid over a plot of a top-hat intensity profile128of an output beam, which is generated by passing the input beam through a top-hat converter, such as one of the converters100,108, and116ofFIG. 4, according to an embodiment of the invention. Although this plot is two dimensional, because the input and output beams are symmetrical, the plot represents any “slice” of these beams that is coplanar with the beams' center axis130. That is, one can form a three-dimensional plot of the input and output beams' intensity profiles by spinning this two-dimensional plot about the beams' center axis130.

The input beam (the beam having the intensity profile126) has a maximum relative intensity of approximately 1 at its center axis130, and the intensity tails off symmetrically according to a Gaussian function as one moves away from the center axis. Theoretically, a Gaussian function approaches, but never equals, zero; consequently, the input beam theoretically has an infinite aperture. Therefore, to aid discussion, one defines the full-width at half maximum (FWHM) of the beam as the width of the beam at the point where it has ½ of its maximum intensity. In this example, at the point where the input beam has an intensity=½×1=50%, it has a relative width of 1; therefore, the input beam has a relative FWHM of 1.

In contrast, the output beam (the beam having the intensity profile128) has a uniform (flat) intensity of approximately 35% across virtually its entire width of approximately 2 times the FWHM width of the input Gaussian. Consequently, the top-hat converter through which the input beam having the Gaussian intensity profile126propagates redistributes the intensity of the input beam to form an output beam having the top-hat intensity profile128. Of course, one can adjust the dimensions and other properties of the top-hat converter to vary the width and intensity of the output beam. Thus, in general, for an input beam having a Gaussian intensity profile, the smaller the width of the top-hat output beam, the higher the intensity of the output beam, and vice versa.

Referring again toFIG. 4, the operation of the beam-combining-and-conditioning assembly76is discussed where each of the beams70,72, and74has a Gaussian intensity profile such as the Gaussian intensity profile126ofFIG. 5. Furthermore, for clarity, only the operation of the optical train90is discussed in detail, it being understood that the optical trains92and94operate in a similar manner.

First, the collimating lens98collimates the aperture of the R beam70.

Next, the top-hat converter100converts the collimated R beam70having a Gaussian intensity profile (FIG. 5) into an R beam having a top-hat intensity profile (FIG. 5) in a conventional manner.

Then, the aperture102clips the beam output from the top-hat converter100to impart a hexagonal aperture to the beam, the hexagonal aperture being approximately the same shape as the aperture of the beamlets32ofFIG. 2.

Next, the focusing lens104passes the R beam70from the aperture102to the X-cube surface122, which reflects the beam to form the R component of the output beam60. One function of the focusing lens104is to condition the R component of the output beam60such that at the input plane (not shown inFIG. 4) of the exit-pupil expander52(FIG. 3), the intensity profile of the output beam is the spatial Fourier Transform, in both phase and amplitude, of the intensity profile of the output beam as it exits the X cube96. Consequently, the focusing lens104(and the focusing lens112and120) may be omitted where other techniques are used to obtain this Fourier Transform relationship. For example, the focusing lens104(and the focusing lens112and120) may be omitted when the telecentric lens50is omitted from the scanned-beam conditioning assembly48(FIG. 3).

The optical trains92and94each operate in a similar manner on the G and B beams, respectively, and the X-cube96passes the G beam and the surface124reflects the B beam to form the color output beam60by “overlapping” the reflected R and B beams and the passed G beam. The output beam60has a top-hat intensity profile with an intensity level substantially equal to the sum of the intensities of the R, G, and B beams, and has substantially the same hexagonal aperture as each of the R, G, and B beams.

FIG. 6is a diagram of the beam-combining-and-conditioning assembly76according to another embodiment of the invention. One difference between the assemblies76ofFIGS. 4 and 6is that the assembly ofFIG. 6includes a reduced number of components, and thus may be smaller, easier to manufacture, and less expensive than the assembly ofFIG. 4.

The assembly76includes three collimating lens140,142, and144, each for collimating a respective one of the R beam70, the G beam72, and the B beam74, the beam-combining X cube96, which combines the collimated beams from the three optical trains into an intermediate beam146, and an optical train148, which converts the intermediate beam into the output beam60.

The optical train148includes an achromatic (i.e., multi-wavelength) top-hat converter150, an achromatic hexagonal aperture152, and an optional achromatic focusing lens154, which are designed for the wavelengths of the R, G, and B beams70,72, and74in a conventional manner. In one example, the R, G, and B beams have the same respective wavelengths as discussed above in conjunction withFIG. 4.

Still referring toFIG. 6, the operation of the beam-combining-and-conditioning assembly76is discussed according to an embodiment of the invention where each of the input R, G, and B beams70,72, and74has substantially the same Gaussian intensity profile such as the profile126ofFIG. 5.

Next, the X-cube96passes the G beam, and the surfaces122and124respectively reflect the R and B beams such that the reflected R and B beams “overlap” the G beam to form the color intermediate beam146having substantially the same Gaussian intensity profile as the input R, G, and B beams70,72, and74.

Then, the top-hat converter150conventionally converts the intermediate beam146into a beam having a top-hat intensity profile such as the profile128ofFIG. 5.

Next, the aperture152clips the beam from the top-hat converter150to impart a hexagonal aperture to the beam, the hexagonal aperture being approximately the same shape as the apertures of the beamlets32(FIG. 2).

Then, the focusing lens154receives the beam from the aperture152and generates the output beam60, which has the top-hat intensity profile and the hexagonal aperture. For reasons similar to those discussed above in conjunction withFIGS. 4 and 5, the focusing lens154may be omitted.

Referring again toFIGS. 4-6, alternate embodiments of the beam-combining-and-conditioning assembly76are contemplated. For example, although shown as respective single optical components, one or more of the top-hat converters100,108,116, and150may each include multiple optical components. Furthermore, one can replace the X-cube96with a slab beam combiner such as the one disclosed in U.S. patent application Ser. No. 10/828,876, filed Apr. 23, 2004, and which is incorporated by reference. In addition, one can omit one or more of the top-hat converters and design the one or more of the corresponding clipping apertures102,110,118, and152to have a clipping width equal or approximately equal to the FWHM width of the corresponding R, G, or B beams and the intermediate beam146. For example, where a beam has the Gaussian intensity profile126(FIG. 5) this clipping technique will impart to the beam60the top portion of the profile126, i.e., the profile126with the all portions of the beam blocked beyond a relative width of +/−0.5 from the center axis130. This to portion of the Gaussian profile126roughly approximates a top-hat profile. But although such an embodiment reduces the number of optical components in the assembly76, it is less efficient because it discards more beam energy than embodiments, such as the embodiments ofFIGS. 4 and 6, which use a top-hat converter. Furthermore, because this embodiment does not impart a true top-hat intensity profile to the output beam60, the beamlets32(FIG. 2), and thus the expanded beam78, may have a non-uniform intensity profile within the viewing space30.

FIG. 7is a side view of the exit-pupil expander52ofFIG. 3according to an embodiment of the invention. In this embodiment, the expander52is a dual-microlens array (DMLA), the structure and operation of which are thoroughly discussed in “A Novel Approach for Exit Pupil Expansion in Wearable Displays”, Karlton Powell, et al., “Exit Pupil Expander: Image Quality Performance Enhancements and Environmental Testing Results”, Karlton Powell, et al., U.S. provisional application Ser. No. 60/541,619 filed on Feb. 4, 2004, which are incorporated by reference. Therefore, for brevity, only an overview of the DMLA is presented below.

The DMLA exit-pupil expander52includes first and second microlens arrays (MLAs)160and162, which are made from a transparent optical material such as plastic or glass and which include a number of lenslets164and166, respectively. The MLA160has focal plane168and a focal length f from the focal plane; likewise, the MLA162has a focal plane170, and the same focal length f from the focal plane170. The MLAs160and162are positioned such that their focal planes168and170are separated by the focal length f, and the gap between the MLAs is filled with air. Each lenslet164and166has a width D, which is the pitch of the MLAs160and162, and each lenslet164is aligned with a corresponding lenslet166.

FIG. 8is a plan view of the MLA160ofFIG. 7according to an embodiment of the invention, it being understood that the MLA162is similar. The lenslets164have a hexagonal footprint, are arranged in a “honeycomb” pattern, and are contiguous such that there are no spaces between adjacent lenslets where the lenslets join the backplane of the MLA160.

The operation of the DMLA exit-pupil expander52is discussed in conjunction withFIGS. 7-8according to an embodiment of the invention where the width of the scanned beam60as it exits the beam generator44(FIG. 3) is approximately D, the footprint of each of the lenslets164and166has the same hexagonal shape as the output beam's aperture, and the output beam's aperture has the same orientation as the footprints of the lenslets.

As discussed above in conjunction withFIG. 3, the scanned-beam conditioning assembly48causes the intensity profile of the scanned beam60to be sinc-like at the input of the DMLA exit-pupil expander52.

When the beam60is centered on a lenslet164a,the corresponding lenslet166aoutputs an expanded center beam172also having a sinc-like intensity profile.

As discussed above in conjunction withFIG. 3, the expanded-beam projection assembly54performs a spatial Fourier Transform in phase and amplitude on the expanded center beam172within the viewing space30.

Consequently, ignoring side beams (discussed below) for the moment, in the viewing space30the center beam172is bounded by the hexagonal beam envelope79and has a uniform intensity profile and no beamlets32. That is, the envelope79effectively bounds a single beamlet, the center beam172, which has a top-hat intensity profile within the viewing space30(FIG. 2). Furthermore, the hexagonal aperture of the beam envelope79and the center beam172is due to the honeycomb layout of the MLAs160(FIG. 8) and 162and the hexagonal aperture of the lenslets164and166, but is independent from the aperture of the beam60. That is, the beam envelope79and the center beam172would still have a hexagonal aperture even if the aperture of the beam60was other than hexagonal. Moreover, because the center beam172is generated by refraction, the beam envelope79is independent of wavelength. That is, the beam envelope79has the same aperture regardless of the wavelength or wavelengths of light that compose the scanned beam60.

But because the intensity profile of the beam60is sinc-like at the input of the DMLA exit-pupil expander52(also the case for Gaussian or truncated Gaussian beams), some of the beam energy is incident on the adjacent lenslets164band164c,thus causing the corresponding lenslets166band166cto output side beams174and176—becauseFIG. 7is a two-dimensional side view, other side beams outside of the plane ofFIG. 7may exist. Furthermore, even if none of the beam energy is incident on the adjacent lenslets164band164cwhen the beam60is centered on the lenslet164a,the beam60still overlaps multiple lenslets as the scanner46(FIG. 3) sweeps the beam from one lenslet to another, and this overlapping gives rise to output side beams174and176.

Still referring toFIGS. 7-8, the side beams174and176interfere with the expanded center beam172in the far field, and this diffraction phenomenon yields the expanded beam78having the beamlets32, which are sometimes called diffraction orders. As discussed above in conjunction withFIG. 3, the hexagonal aperture and top-hat intensity profile of the beamlets32within the viewing space30are the same as the aperture and the intensity profile of the output beam60as it exits the beam generator44; but the aperture and intensity profile of the beamlets are independent of the layout pattern and aperture of the lenslets164and166. Furthermore, because the beamlets32are generated by interference of overlapping diffraction envelopes, the size of the beamlet aperture is dependent on wavelength. Consequently, one may design the DMLA exit-pupil expander52for the center wavelength, for example the wavelength of the G component, of the scanned beam60.

In summary, the intensity profile of the beam envelope79depends only on the optical properties of the DMLA exit-pupil expander52, the aperture of the beam envelope79depends only on the layout pattern and aperture of the lenslets164and166, the intensity profile of the beamlets32depends only on the intensity profile of the scanned beam60, and the aperture of the beamlets depends only on the aperture of the scanned beam.

Still referring toFIGS. 7 and 8, alternate embodiments of the exit-pupil expander52are contemplated. For example, the gap between the MLAs160and162may be filled with a transmission medium other than air. For example, the fill medium may be a solid to resist gap compression or expansion that may alter the optical properties of the DMLA expander52, and one can alter the properties (e.g., pitch, lenslet curvature, index of refraction) of the MLAs160and162in a conventional manner to compensate for the different index of refraction (relative to the index of refraction of air) of the filler medium. Additional embodiments are discussed below in conjunction withFIGS. 13-18.

FIGS. 9A-11Billustrate the dependence of the intensity profile of the expanded beam78(FIGS. 3 and 7) within the viewing space30on the intensity profiles of the beam envelope79(FIGS. 2 and 7) and the beamlets32(FIG. 2), and artifacts caused by the expanded beam having a non-uniform intensity profile. As discussed below, artifacts such as “banding” may be more severe when an operator's eye is outside of the cross section34of the viewing space30.

FIG. 9Ashows the beam envelope79having a uniform (flat) intensity profile, andFIG. 9Bshows that the beamlets32, which each have a non-uniform, Gaussian-like intensity profile, combine to yield a non-uniform beamlet intensity profile within the beam envelope79.

FIG. 9Cshows the non-uniform intensity profile of the expanded beam78within the viewing space30resulting from the beam-envelope and combined-beamlet intensity profiles ofFIGS. 9A and 9B. Each sinc-like beam output172,174, and176at the exit-pupil expander52transforms into a top-hat intensity profile in the far field. Interference due to overlap of these beam outputs172,174, and176forms beamlets32at a spacing defined by the lenslet array pitch and layout. The envelope of the input beam60in phase and amplitude transforms to form the intensity profile of each beamlet32in the far-field within the expanded beam78. Thus it can be considered that the resulting intensity profile of the expanded beam78is a convolution, in phase and amplitude of the expanded beam-envelope ofFIG. 9A, the beamlet envelope, and the beamlet spacing and layout ofFIG. 9B. Consequently, even though the beam-envelope intensity profile (FIG. 9A) is uniform, the non-uniformity of the combined-beamlet intensity profile (FIG. 9B) causes the intensity profile of the expanded beam78to be non-uniform. And although not shown, a uniform combined-beamlet intensity profile combined with a non-uniform beam-envelope intensity profile also yields a non-uniform resulting intensity profile.

FIGS. 10A and 10Billustrate intensity patterns perceived by an operator (not shown inFIGS. 10A and 10B) for different eye positions within the viewing space30for a non-uniform expanded-beam intensity profile similar to that ofFIG. 9C. And discussed below are artifacts that can arise when the expanded-beam intensity profile is not uniform throughout the viewing space.

FIG. 10Aillustrates a first expanded beam78athat generates a first pixel at the top of a virtual image, and a second expanded beam78bthat generates a second pixel at the bottom of the image, each expanded beam having the same non-uniform intensity profile. Although the first and second pixels, and thus the first and second expanded beams78aand78b,are generated at different times, both are shown inFIG. 10Afor purposes of illustration. Furthermore, other expanded beams are generated between the expanded beams78aand78bto generate corresponding pixels between the first and second pixels, but these other expanded beams are omitted for clarity.

For a single expanded beam78, any cross section within the viewing space30has the expanded beam's intensity profile. For example, at the cross sections180and182, the same non-uniform intensity profile exists within each expanded beam78aand78b.

At the cross section34of the viewing space30, the expanded beams78aand78boverlap at all points such that at each point along the cross section34, the expanded-beam intensity profile has the same level for each pixel. For example, at a location184or186, the intensity-profile level is respectively the same for each expanded beam78, and is thus the same for each pixel of the image.

But at locations outside of the cross section34, the expanded beams78aand78bdo not overlap at all points, and thus the intensity-profile level may change from pixel to pixel. For example, at a location188(FIG. 10B), the intensity-profile level changes from pixel to pixel as further explained below.

Therefore, if an operator's eye pupil (not shown inFIGS. 10A and 10B) is aligned with the cross section34, then the operator typically perceives a uniform image intensity if he does not move his eye, but may perceive a non-uniform intensity if and while he moves his eye from one location of the cross section34to another location of the cross section34. For example, assume that the eye pupil is at the location184. Because at the location184the level of the expanded-beam intensity profile is the same for each pixel, the operator perceives a uniform intensity profile for the image as long as he does not move his eye from the location184. That is, although the absolute intensity at different locations of the image may change with the image content, the operator does not perceive “banding,” where one region of the image has a higher average brightness than another region. But if the operator moves his eye from the location184to the location186, then he may notice that the image brightness changes while his eye effectively traverses the portion of the non-uniform expanded-beam intensity profile between the locations184and186.

But if the operator's eye (not shown inFIGS. 10A and 10B) is outside of the cross section34, then he may perceive the image having a non-uniform intensity profile even if he does not move his eye. For example, assume that the eye pupil is at the location188. Because at the location188the expanded beams78aand78bdo not overlap at all points, the level of the expanded-beam intensity profile is different for each pixel. For the pixel carried by the expanded beam78a,the operator perceives the intensity-profile end at a point190on the expanded-beam intensity profile, and for the pixel carried by the beam78b,the operator perceives the intensity-profile level at a point192on the expanded-beam intensity profile. That is, as the scanner46(FIG. 3) sweeps the beam60(FIG. 3), the expanded beam78effectively “pivots” from the position of the beam78ato the position of the beam78b.During this “pivoting,” the portion of the expanded-beam intensity profile between the points190and192effectively moves past the location188, and thus past the operator's eye. Therefore, even without moving his eye, the operator perceives the image as having an intensity profile equal to the portion of expanded-beam intensity profile between the points190and192. Because this portion of the expanded-beam intensity profile is not uniform, the operator may, depending on the magnitude of the non-uniformity, perceive intensity “banding” even when he is not moving his eye, and may perceive shifting of the “banding” pattern as he moves his eye in any direction. Of course, this “banding” pattern may disappear if the operator moves his eye into alignment with a location of the cross section34of the viewing space30.

FIGS. 11A-12Billustrate how the heads-up display40reduces the severity of, or altogether eliminates, the artifacts discussed above in conjunction ofFIGS. 10A and 10Baccording to an embodiment of the invention. Specifically, the display40generates the expanded beam78having a uniform intensity profile as discussed above in conjunction withFIGS. 4-8.

FIG. 9Ashows expanded-beam envelope78having a uniform intensity profile as discussed above, andFIG. 11Ashows that the beamlets32also each having a uniform intensity profile.

FIG. 11Bshows the uniform intensity profile of the expanded beam78resulting from the uniform beam-envelope and combined-beamlet intensity profiles ofFIGS. 9A and 11B. Specifically, the expanded-beam intensity profile is the combination of the beam-envelope intensity profile ofFIG. 9Awith the combined-beamlet intensity profile ofFIG. 11B. That is, the convolution of uniform beam-envelope and combined-beamlet intensity profiles yields a uniform expanded-beam intensity profile.

FIG. 12illustrates intensity patterns perceived by an operator (now shown inFIG. 12) for different eye positions within the viewing space30for the uniform expanded-beam intensity profile ofFIG. 11Baccording to an embodiment of the invention.

More specifically,FIG. 12illustrates a first expanded beam78cthat generates a first pixel at the top of the virtual image, and illustrates a second expanded beam78dthat generates a second pixel at the bottom of the image.

For a single expanded beam78, any cross section within the viewing space30has the expanded-beam intensity profile ofFIG. 11B. For example, at the cross sections200and202, the same uniform expanded-beam intensity profile exists within each expanded beam78cand78d.

At the cross section34of the viewing space30, the expanded beams78cand78doverlap at all points such that at any point along the cross section34, the expanded-beam intensity profile is the same for each pixel. Therefore, an operator (not shown inFIG. 12) will sense little or no change in the image brightness as he moves his eye from one location of the cross section34to another location of the cross section34.

Furthermore, at locations outside of the cross section34, even though the expanded beams78cand78ddo not overlap at all points, the expanded-beam intensity-profile level is the same for each pixel within the image perceived by an operator. For example, at a location204, the intensity-profile level does not change from pixel to pixel because the portion of the intensity profile between points206and208(same point inFIG. 12) is flat. That is, as the expanded beam78pivots from the position of the beam78cto the position of the beam78d,the level of the expanded-beam intensity profile does not change at the point204. Therefore, the operator does not perceive “banding” in the image when his eye is outside of the cross section34.

Consequently, an operator perceives a substantially uniform image intensity regardless of the location of his eye's pupil within the viewing space30and regardless of whether he is moving his eye. That is, because the expanded-beam intensity profile (FIG. 11B) is substantially flat, the operator perceives the image having substantially the same uniform intensity profile at any location within the viewing space30.

Referring toFIGS. 11A-12, alternate embodiments of the invention are contemplated. For example, instead of generating the uniform expanded-beam intensity profile (FIG. 11B) by generating uniform intensity profiles for both the beam envelope79and the beamlets32, the heads-up display40(FIG. 3) may generate the beam envelope79and the beamlets32having non-uniform intensity profiles such that the convolution of these non-uniform intensity profiles yields a uniform expanded-beam intensity profile such as shown inFIG. 11B.

FIG. 13is a plan view of an MLA210belonging to a DMLA exit-pupil expander52(FIG. 3) according to another embodiment of the invention, it being understood that the alternative DMLA includes a second MLA that is similar to the MLA210but that is omitted fromFIG. 13for clarity. The MLA210differs from the MLA160ofFIG. 8in that lenslets212are not fully contiguous with one another at the MLA backplane, and thus the MLA210has spaces214between adjacent lenslets212. The spaces214are often the result of a conventional manufacturing process that does not or cannot form fully contiguous lenslets212. But by treating the spaces214with an opaque coating, the spaces have little or no adverse affect on the operation of the alternative DMLA. Consequently, when including a DMLA having the MLA210and a similar second MLA, the heads-up display40(FIG. 3) can generate the viewing space30(FIG. 2) having substantially the same properties (e.g., uniform expanded-beam intensity profile, hexagonal aperture) as discussed above in conjunction withFIGS. 3-12. In a related embodiment, the spaces214on only one of the MLAs, such as the MLA and the input side of the DMLA expander52, are treated with an opaque coating.

FIG. 14is a side view of a DMLA exit-pupil expander52according to another embodiment of the invention. The DMLA ofFIG. 14differs from the DMLA ofFIGS. 7-8in that it includes curved MLAs220and222, which respectively include lenslets224and226. Corresponding pairs of lenslets224and226are aligned such that the scanned beam60(FIG. 3) follows radial paths228that are simultaneously coincident with the center axes of both lenslets of these respective lenslet pairs. Consequently, the curved DMLA allows one to omit the telecentric lens50from the scanned-beam conditioner assembly48(FIG. 3). The MLAs220and222each have the same focal length f in the radial dimension, and have respective focal curves230and232, which are spaced apart by f in the radial dimension. Furthermore, the lenslets224and226are arranged in the honeycomb pattern ofFIGS. 8 and 13, and the MLAs220and222may include opaquely coated spaces between the lenslets as discussed above in conjunction withFIG. 13. Consequently, when including the DMLA ofFIG. 14, the heads-up display40(FIG. 3) can generate the viewing space30(FIG. 2) having substantially the same properties (e.g., uniform expanded-beam intensity profile, hexagonal aperture) as discussed above in conjunction withFIGS. 3-12.

FIG. 15is an isometric view of a slab DMLA exit-pupil expander52according to another embodiment of the invention. This DMLA includes two MLAs240and242, which are each formed from a respective pair of slabs244and246, and248and250, of semi-cylindrical lenses each having the same pitch. The slab244is orthogonal to the slab246, and the slab248is orthogonal to the slab250. The MLAs240and242each have the same focal length f, and have respective focal planes252and254, which are spaced apart by f. The resulting lenslets of each MLA240and242are effectively arranged in a square pattern, not a hexagonal pattern; consequently, the resulting lenslets cause the beam envelope79(FIG. 3) to have a square shape. Therefore, to improve the fill factor, one can modify the beam generator44(FIG. 3) to impart a square aperture to the output beam60, and thus to the beamlets32(FIG. 2).

Still referring toFIG. 15, alternative embodiments of the slab DMLA exit-pupil expander52are contemplated. For example, one slab of each MLA240and242can have a different pitch than the other slab so that the resulting lenslets have a rectangular shape. To improve the fill factor for such an embodiment, one can modify the beam generator44(FIG. 3) to impart a rectangular aperture to the output beam60, and thus to the beamlets32(FIG. 2). Furthermore, although shown with the horizontally aligned slabs246and248facing one another, one can contract the slab DMLA expander52with the two vertically aligned slabs244and250facing one and other, or one vertically aligned slab facing a horizontally aligned slab. Moreover, if there are any spaces between the lenses at the slab backplane, one can treat them with an opaque coating as discussed above in conjunction withFIG. 13.

FIG. 16is an isometric view of a DMLA exit-pupil expander52according to another embodiment of the invention. This DMLA expander52includes two lenslet arrays256and258. The array256includes cylindrical lenses260having the same pitch and mounted between two slabs262and264of optical material such as glass or plastic. For example, the cylindrical lenses260may be optical fibers. The array258is similar to the array256, and includes cylindrical lenses265having the same pitch and mounted between two slabs266and267of optical material such as glass or plastic such that the lenses265are orthogonal to the lenses260. The resulting lenslets are arranged in a square pattern and cause the expanded-beam envelope79(FIG. 3) to have a square shape. Therefore, one can modify the beam generator44(FIG. 3) to impart a square aperture to the output beam60, and thus to the beamlets32(FIG. 2). Moreover, to mask spaces between the lenses260and265, one can treat corresponding regions of the slabs262,264,266, and267with an opaque coating as discussed above in conjunction withFIG. 13.

FIG. 17is a side view of a reflective DMLA exit-pupil expander52according to another embodiment of the invention. Like the DMLA expander52ofFIGS. 7-8, the reflective DMLA expander includes the MLA160. But the reflective DMLA expander52differs from the DMLA expander ofFIGS. 7-8in that it includes a reflector270instead of the MLA162. The reflector270includes a planar reflecting surface272that is located f/2 from the focal plane168of the MLA160. Consequently, the expanded beam78propagates to the viewing space30(FIG. 2.) from a front side274(i.e., the same side into which the scanned beam60propagates) of the MLA160. Therefore, when incorporating the reflective DMLA expander52ofFIG. 17, the heads-up display40(FIG. 3) can generate the viewing space30(FIG. 2) having substantially the same properties (e.g., uniform expanded-beam intensity profile, hexagonal aperture) as discussed above in conjunction withFIGS. 3-12. Furthermore, the reflective DMLA expander52may be easier to manufacture than the DMLA expander52ofFIGS. 7-8because it does not require alignment of two MLAs.

FIG. 18is a side view of a multi-pitch DMLA exit-pupil expander52according to another embodiment of the invention. Like the DMLA expander52ofFIGS. 7-8, the multi-pitch DMLA expander includes the MLA160having the lenslet pitch D. But the multi-pitch DMLA expander52differs from the DMLA expander ofFIGS. 7-8in that it includes a second MLA280having a different lenslet pitch D′>D, which allows one to omit the telecentric lens50from the scanned-beam conditioning assembly48(FIG. 3). The MLA280has a focal length f, a focal plane282that is located a distance f from the focal plane168of the MLA160, and lenslets284that are arranged in a honeycomb pattern similar to the pattern of the MLA160as shown inFIG. 8. Consequently, when incorporating the multi-pitch DMLA expander52, the heads-up display40(FIG. 3) can generate the viewing space30(FIG. 2) having substantially the same properties (e.g., uniform expanded-beam intensity profile, hexagonal aperture) as discussed above in conjunction withFIGS. 3-12. In an alternative embodiment of the multi-pitch DMLA expander52, D′ is less than D.

FIG. 19is an isometric view of a single-microlens-array (SMLA) exit-pupil expander52according to another embodiment of the invention. As its name implies, the SMLA expander52includes a single MLA290. Like the lenslets164of the MLA160(FIGS. 7 and 8), lenslets292of the MLA290are arranged in a honeycomb pattern perFIG. 8. But unlike the MLA160ofFIGS. 7-8, the MLA290includes lenslets292having different properties, e.g., height, radius of curvature, pitch, and index of refraction. Furthermore, the arrangement of the different lenslets290may be pseudo-random or ordered such that a group of lenslets may share the same properties. Although the SMLA expander52may be easier and less expensive to manufacture than the DMLA expander52ofFIGS. 7-8, the SMLA may be unable to generate the parameters (e.g., expanded-beam intensity profile,) of the viewing space30as close to the desired levels as the DMLA expander52ofFIGS. 7-8can generate these parameters.

Referring again toFIGS. 7-8and13-19, additional embodiments of the exit-pupil expander52(FIG. 3) are contemplated. For example, one may design an exit-pupil expander52by combining selected features shown in any of theFIGS. 7-8and13-19. In one such example, one can modify the curved DMLA expander52ofFIG. 14by replacing the curved MLA222with a curved reflector in a manner similar to that shown inFIG. 17. Furthermore, one can use an ordered or random-ordered diffuser, such as a diffraction grating, for the exit-pupil expander52. An example of a suitable diffraction grating is discussed in U.S. patent application Ser. No. 10/205,858 filed Jul. 26, 2002, U.S. patent application Ser. No. 10/889,963 filed Jul. 12, 2004, U.S. patent application Ser. No. 10/890,501, filed Jul. 12, 2004, and U.S. Pat. No. 6,768,588, issued on Jul. 27, 2004, which are incorporated by reference.

FIG. 20is an isometric view of a portion of the expanded-beam projection assembly54(FIG. 3) and a dashboard shield300according to an embodiment of the invention. In addition to the aspheric mirror58, the assembly54includes a mirror302, mirror304, and a cylindrical mirror306.

The operation of the illustrated portion of the expanded-beam projection assembly54is discussed according to an embodiment of the invention. The mirror302directs the expanded beam78(which includes the beamlets32(FIG. 2) within the beam envelope79(FIG. 3)) onto the mirror302, which directs the expanded beam78onto the mirror306. The mirror306, which is curved away from the mirror58, further expands the expanded beam78, and directs the expanded beam onto the aspheric mirror58, which directs the expanded beam through the shield300and onto the wind screen42(FIG. 3).

As discussed above in conjunction withFIG. 3, an operator (not shown inFIG. 20) may adjust the position of the viewing space30(FIG. 2) for his particular height by rotating the mirror58or the entire heads-up display assembly about an axis308to move the viewing space30in the vertical (y) dimension. Alternatively, an automatic system (not shown inFIG. 20) can adjust the vertical position of the viewing space30by so rotating the mirror58or the entire heads-up display assembly. Also, as discussed above in conjunction withFIG. 3, the curvature of the mirror58is designed to optically compensate the curvature of the wind screen42so that the expanded beam78and displayed image does not stretch, distort or change position significantly as the operator moves their eyes inside the viewing space.

Still referring toFIG. 20, the shield300, which may optionally be formed to reduce glare, is mounted within an opening formed in the vehicle dashboard (not shown inFIG. 20) or on top of the head-up display package to protect the mirror58and the other components of the heads-up display40(FIG. 3) and yet allow the expanded beam78(FIG. 3) to propagate from the mirror58to the wind screen42(FIG. 3). The shield may also be used to minimize glare from the sun or other external illumination sources that might otherwise be reflected off the heads-up display and into the operators eyes. Both the exposed (top) and unexposed (bottom) surfaces310and312of the shield300are treated with an antireflective coating, and the exposed surface is also treated with an anti-fingerprint coating such as Teflon®. Furthermore, the shield300may be made from any transparent material such as glass or plastic.

Alternative embodiments of the expanded-beam projection assembly54are contemplated. For example, the assembly54may include more or fewer mirrors than shown inFIG. 20, and may also include refractive, catadioptric and/or diffractive optical elements in addition to or in place of the lens56(FIG. 3).

FIGS. 21 and 22illustrate how one can electronically calibrate the heads-up display40(FIG. 3) to compensate for image distortion caused by the wind screen42or other factors according to an embodiment of the invention. Because the calibration is done electronically, it is often less complex and less expensive than a calibration technique that requires a modification to or a replacement of a component of the heads-up display40such as the aspheric mirror58(FIGS. 3 and 20).

FIG. 21is an overlay of the available scanning area320of the exit-pupil expander52(FIG. 3), an image322that the beam60(FIG. 3) scans onto the area320, and a distorted image324as perceived by an operator (not shown inFIG. 20) whose eyes are within the viewing space30(FIG. 2) according to an embodiment of the invention—the area320and the images322and324may not be drawn to scale. The image322is rectangular, has a width p, and is centered within the scanning area320of the exit-pupil expander52.

With no distortion, the perceived image324would typically have the same shape, orientation, and relative location as the scanned image322. Note that the apertures of the expanded beam78and the beamlets32(FIG. 2) have no affect on the shape of the scanned image322; consequently, the heads-up display40can simultaneously generate the expanded beam and beamlets having hexagonal apertures and the image322having a rectangular shape. But the shape of the perceived image324is affected by distortion to the apertures of the expanded beam78and beamlets32as discussed below.

In this example, however, distortion causes the perceived image324to differ from the scanned image322in shape (trapezoidal vs. rectangular caused by a taper of β in the width p), orientation (rotated counterclockwise by α=45° relative to the scanned image), and relative location (the center of the perceived image is shifted left a relative distance q from the center of the scanned image). More specifically, this distortion introduces an undesired taperβ to the apertures of the expanded beam78(this component of the distortion is often called “key stoning”), rotates the aperture of the expanded beam counterclockwise 45° from its desired orientation, and shifts the aperture of the expanded beam left from its desired location by a relative distance q. Such distortion is often caused by an optical mismatch between the wind screen42(FIG. 3) and the aspheric mirror58(FIGS. 3 and 20), although this distortion may have other causes. Typically, such distortion may exist when the vehicle (not shown inFIG. 21) incorporating the windscreen42(FIG. 3) is first assembled, or when the wind screen is replaced.

Still referring toFIG. 21, where the distortion in the perceived image324is caused by an optical mismatch between the wind screen42(FIG. 3) and aspheric mirror58(FIGS. 3 and 20), a conventional solution is to replace either the wind screen or the aspheric mirror or to modify the curvature of the mirror to obtain a better optical match with the windscreen. But because this solution requires replacement or modification of components and the calculation of the curvature of at least one of the wind screen42and mirror58, this solution is often relatively time consuming and expensive.

But as discussed below, one can electronically calibrate or recalibrate the heads-up display40(FIG. 3) to compensate for such distortion according to an embodiment of the invention. Therefore, one can reduce or eliminate the distortion without replacing or modifying components or otherwise changing the physical structure of the display40.

FIG. 22is an overlay of the available scanning area320of the exit-pupil expander52(FIG. 3), an image330that the beam60(FIG. 3) scans onto the area320and that is operable to counteract distortion, and a distortion-compensated image332that is perceived as being substantially distortion free by an operator (not shown inFIG. 22) whose eyes are within the viewing space30(FIG. 2) according to an embodiment of the invention—the area320and the images330and332may not be drawn to scale. As discussed further below, the shape, orientation, and taper of the distortion-counteracting image330are the respective inverses of the shape, orientation, and taper of the distorted image324ofFIG. 21.

Referring toFIGS. 21 and 22, one can calibrate the heads-up display40(FIG. 3) to correct the distorted image324by programming the display to scan the distortion-counteracting image330onto the exit-pupil expander52(FIG. 3) according to an embodiment of the invention. More specifically, one programs the beam generator44(FIG. 3) of the display40to effectively generate the distortion-counteracting image332from the original scanned image322by shifting the original image a distance q to the right (i.e., a distance −q to the left), rotating the shifted original image by an angle −α, and tapering the sides of the shifted and rotated original image by an angle −β. The programmed image electronics62(FIG. 3) perform this shifting, rotating, and tapering by modulating the beam sources64,66, and68(FIG. 3) in an appropriate manner. To allow this shifting, rotating, and tapering, the area320of the exit-pupil expander52is larger, for example 15% larger, than the area of the original image322.

Consequently, the same distortion that transformed the original scanned image322into the distorted image324now transforms the distortion-counteracting image330into the distortion-compensated image332, which the operator perceives as being undistorted, i.e., as having the same relative, location, orientation, and shape as the original scanned image322.

Because one can calibrate the heads-up display40by programming instead of physical modification, he can often perform such calibration more quickly and cheaply than he can perform a conventional calibration. Furthermore, one can automate the calibration by programming a test/calibration machine (not shown inFIGS. 21 and 22) to capture the distorted image324, to calculate the parameters (e.g., shape, orientation, relative location) of the distortion-counteracting image330from the captured image, and to program the beam generator44for modulating the output beam60so that the scanner44scans the distortion-counteracting image onto the available area320of the exit-pupil expander52(beam generator, output beam, scanner, and exit-pupil expander shown inFIG. 3).

Still referring toFIGS. 21 and 22, other embodiments of the above-described distortion-calibration technique are contemplated. For example, one can program the beam generator44(FIG. 3) to correct distortions other than shift, rotational, and tapering distortions. For example, one can program the generator44to correct non-taper distortions to the shape of the image. Furthermore, one can program the beam generator44to modulate the intensity of the output beam60to correct distortions in the intensity profile of the expanded beam78.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, referring toFIGS. 3,21, and22, the heads-up display40may scan an image onto the exit-pupil expander52using a tiling technique. That is, the display40can generate multiple beams60such that the scanner46sweeps multiple beams, each beam scanning a different portion of the image onto the exit-pupil expander57. Tiling is discussed in U.S. patent application Ser. No. 09/858,287 filed May 15, 2001, U.S. Patent application Ser. No. 09/858,688 filed May 15, 2001, U.S. Pat. No. 6,755,536 issued on Jun. 29, 2004, and U.S. Pat. No. 6,639,719 issued on Oct. 28, 2003, which are incorporated by reference.