Projection and Display Options

Lumio has developed technology to provide projected interfaces for situations where printed interfaces or conventional, flat panel LCD screens etc are insufficient or inappropriate. Our micro-projector modules can project large, clearly visible, crisp images onto almost any surface from a low height, at strongly oblique angles and minimal throw distances.

For static templates, micro diffractive modules provide the ultimate in miniaturization and power efficiency. For dynamic projected interfaces, mini LCD modules incorporating customized, segmented LCDs can respond to human interaction and provide dynamically updateable output.

Static, diffractive projectors

From an optical efficiency standpoint, the most efficient method of projecting a fixed monochromatic image is to employ a diffractive optical pattern generator. This is because the diffractive element simply redirects the incident light to form the diffractive pattern rather than masking significant portions of the light as with an ordinary slide projection technique. Some light is lost to undesired higher diffraction or zero orders; however with proper design of the element these losses are minimized. Diffractive pattern generators containing thousands or tens of thousands of lit diffraction orders can be fabricated wherein a majority of the incident light is directed into the projected image [2]. The residual zero order and higher diffraction orders can then be masked, if desired, in the far field of the element.

2. Projection geometry for a virtual keyboard

Another advantage of the diffractive pattern generator is that light can also be diffracted at relatively large angles in order to generate large images at short distances. The overall efficiency of the diffraction is reduced somewhat as the diffraction angles increase but still far exceeds that of a classical lens based image projector. For the virtual keyboard accessory, as shown in [2], in order to project a full sized keyboard from a height of approximately 70mm the maximum diffraction angles exceed 100 degrees.

Construction of a diffractive micro-projector module

A diffractive pattern generator is typically comprised of a coherent light source such as a diode laser, a

3. Schematic illustration of the diffractive micro-projector module assembly.

collimating lens and a phase hologram pattern generator as shown in [3]. In order to miniaturize the assembly and simplify the alignment the collimating lens (1) also serves as the mechanical mount for the diode laser (2) and the diffractive optical element (DOE) (3). This structure is molded from a single piece of plastic to which the diode laser is actively aligned and fixed in place. The DOE is then snapped into the assembly from the other side without requiring any additional alignment. A heat sink (4) is then attached to the laser diode to complete the assembly.

A picture of the fully assembled micro-projector module is shown in [4]. The entire projector assembly measures less than one cubic centimeter.

Provided that the projection geometry is the same, the module can accept different DOEs to project different templates. An example of a Japanese / English projected keyboard layout is shown in [5].

4. Micro projector module with heat sink and laser driver

5. Virtual keyboard accessory with static diffractive module projecting a Japanese/English keyboard layout

Dynamic projectors and modules

When it is necessary to modify the projected template in response to user input diffractive elements are not applicable. While variable diffractive projectors are currently being developed (for example real time calculation for a reflectiveLCOS matrix) they do not yet provide sufficient quality to project highquality image templates. Relatively small LEDbased projectors using DLP matrixes are also being developed by a number ofcompanies but nothing that is available today meets the form factor, price and power consumption requirement for a projected dynamic interface.

LCD micro-projectors

Design constraints

As mentioned above, virtual interface often must be projected from a very low height onto a nearby surface at a strongly oblique angle. This results in considerable variation of the incident angle over the image and a severe keystone distortion that can not be corrected by the optics of the projection system. The actual image focus can be maintained by tilting the LCD in the appropriate Sheimpflug condition. The keystone effect, however, must be corrected for in the layout of the LCD itself. Thus, for a given projection geometry, the keystone is calculated and the segments on the LCD are pre-distorted in order to obtain the correct projection geometry. An added benefit is that since the LCD is being pre-distorted additional distortions in the projection lens can be corrected for as well allowing for simplifications in the projection lens design. This can be particularly significant what attempting to project at large angles from the optical axis. Restricting the illumination LED to a single colour allows for monochromatic lens designs, further simplifying the projection system.

The oblique projection and distorted LCD requires considerable modification to the illumination system as well in order to maintain a fairly uniform intensity in the projected image. The physical segments on the LCD that are projected to the far side of the image can be several times smaller than those nearer to the projector.

Projector construction and results

A schematic illustration of the micro-projector we have developed is shown in [6] below. A single 160 lumen luxeon LED from Lumileds along with a reflective collimator are housed in an aluminium body that also serves as a heat sink. A customized aspheric fresnel lens is designed to redistribute the incident light on the LCD in order to concentrate more light on the smaller segments. The LCD assembly is tilted to adjust the focus and an F/2 projection lens generates the image.

The complete projection engine, shown in [7], is 1 inch in diameter and approximately 2 inches long.

6. Schematic illustration of the dynamic micro-projector

7. Photograph of the LCD projection module

An example of a dynamically re-configurable projected interface that we have developed is a variable numeric keypad. In this application, developed for Visa, a standard numeric keypad for entering a personal identification number (PIN code) is projected onto the surface.Both the projector and sensing modules are integrated into a single housing. The layout of the pre-distorted segmented LCD and actual component are shown in [8]. As a security feature, each time the device is used the numbers are projected in a random order as shown in [9]. An additional projection layout used for access control is shown in [10].

8. A pre-distorted segmented LCD layout and actual implementation for projecting a variable numeric keypad.

9. Two views of the dynamic numeric keypad projected by the micro-projector using the LCD of [8] showing different keypad layouts.

10.Keypad interface for access control

The micro-projectors described here are capable of projectingbright images up to approximately 30 cm across at extreme geometries. When coupled with appropriatesensing modules they can be used to convert almost any surface into an interactive interface. For fixed templates, the diffractive modules we have developed provide the ultimate performance with respect to size and overall power efficiency. For dynamic templates, we have developed LED modules that are small and flexible enough to make them appropriate for integration into a wide variety of projected interface applications.