Laser scanning techniques allow the creation of high quality micro projection devices for insertion into mobile devices such as mobile phones, smartphones, laptops, etc. The main merits of laser scanning projection methods are a small size of light sources, a good color gamut and a small size of the spatial light modulator, without needed imager matrices like liquid crystal matrices or DLP (digital light processing). One of the main difficulties of laser scanning is the exact combining of red, green and blue laser beams into one beam in order to create a point of the image in designed coordinates on the screen.
Laser scanning projection techniques use two or more dichroic mirrors for a combination of three light beams of different colours, e.g. red, blue and green or any other colour, into one light beam of mixed colour such as white with similar intensity. Generally dichroic and polarizing mirrors 1, 2, 3, 4 may be used for combining three laser beams as can be seen from FIG. 1 and in U.S. Pat. No. 7,445,339. FIG. 1 illustrates the combining of red, green and blue laser beams into the one red, green, blue (RGB) color controlled light beam 5. Dispersive optical elements are suggested for the beam-combining. A dispersive optical element is an element that spreads out light beams with output directions depending on their wavelengths. If incoming light beams impinge on a dispersive element with specific directions, a mixed light beam may arise from the dispersive optical element.
FIG. 2 illustrates the general structure of a laser projection system 200 with a dispersive light beam combiner element 3 in form of a prism. The general structure of a laser projection system is also shown in U.S. Pat. No. 7,891,817. The red, green and blue light beams of three (red, green and blue) light sources 1A, 1B and 1C pass respective optical units 2A, 2B, 2C that include beam-shaping optical elements 21A, 21B, 21C and beam-converting optical elements 22A, 22B, 22C before the light beams arrive at the prism 3 that combines the light beams to a single light beam projected onto a screen 6. A scanning unit 4 including reflecting mirrors 41A, 42A and scan driving portions 41B, 42B scans the light and controls projection via a light-guiding mirror 5 onto the screen 6.
FIG. 3 illustrates the principle of transmittance diffractive grating working with angles calculations, which is also further described in U.S. Pat. No. 7,891,817. The illustration of FIG. 3 shows light beam propagation through light-transmittance diffractive grating for calculation of the incident beam angle θ0. The incident light beam with incident beam angle θ0 with respect to normal direction is refracted at the diffractive grating surface and passes through the diffractive grating with an emergent beam angle θ1 with respect to normal that is greater than the incident beam angle.
Optical projection systems as described above use general dispersive elements or arrangements of multiple mirrors and scan driving portions. General dispersive optical elements are prisms and transmittance diffractive gratings (DGs). Prisms and transmittance diffraction gratings are volumetric elements, therefore they do not allow a sufficient decreasing of the total size of the RGB beam combiner. Analogously, arrangements of multiple mirrors and scan driving portions consume a lot of space inside the projection system. Pico-projection systems have strict size requirements, for example their size should not be more than a few cm3. Therefore, there is a need to reduce the size of such devices.