Multispectral Color Reproduction Using DLP

Abstract: The color gamut, i.e. the range of reproducible colors, is in most conventional display systems not sufficient for accurate color reproduction of highly saturated colors. Any conventional three-primary display suffers from a color gamut limited within the triangle spanned by the primary colors. Even by using purer primaries, enlarging the triangle, there will still be a problem to cover all the perceivable colors. By using a system with more than three primary colors, in printing denoted Hi-Fi color, the gamut will be expanded into a polygon, yielding a larger gamut and better color reproduction. Digital Light Processing (DLP) is a projection technology developed by Texas Instrument. It uses a chip with an array of thousands of individually controllable micromirrors, each representing a single pixel in the projected image. A lamp illuminates the micromirrors, and by controlling the amount of time each mirror reflect the light, using pulse width modulation, the projected image is created. Color reproduction is achieved by letting the light pass through color filters, corresponding to the three primaries, mounted in a filter wheel. In this diploma work, the DLP projector InFocus® LP™350 has been evaluated, using the Photo Research® PR®-705 Spectroradiometer. The colorimetric performance of the projector is found to be surprisingly poor, with a color gamut noticeably smaller then that of a CRT monitor using standardized phosphors. This is due to the broad banded filters used, yielding increased brightness at the expense of the pureness of the primaries. With the intention of evaluating the potential for the DLP technology in multi- primary systems, color filters are selected for additional primary colors. The filters are selected from a set of commercially available filters, the Kodak Wratten filters for science and technology. Used as performance criteria for filter selection is the volume of the gamut in the CIE 1976 (L'u'v') uniform color space. The selected filters are measured and evaluated in combination with the projector, verifying the theoretical results from the filter selection process. Colorimetric performance of the system is greatly improved, yielding an expansion of the color gamut in CIE 1976 (L'u'v') color space by 79%, relative the original three-primary system. These results indicate the potential for DLP in multiprimary display systems, with the capacity to greatly expand the color gamut, by using carefully selected filters for additional primary colors.