Robotically Controlled Super Prism Displays

Scientists at the Swiss Federal Institute of Technology in Zurich have developed a new experimental advanced display technology that, to say the least, differs significantly from currently used technologies.

As regular readers of this column know, most advanced display technologies use the same fundamental approach that their predecessor, the cathode ray tube, uses to create a color image-red, green, and blue light are combined in various proportions and levels to reproduce the colors in the image being displayed.

There are a few exceptions to this rule. Some advanced displays employ auxiliary subpixels in addition to red, green, and blue subpixels. Some LCD displays add gray subpixels to enhance the display's ability to render the darker components of an image. A handful of experimental units also have subpixels of other colors, i.e., cyan, magenta, and yellow, to enhance the display's ability to render colors that fall outside the red, green, and blue subpixel gamut.

We may prove that it is theoretically possible to generate all the colors we can perceive using a mixture of just three primary colors. Red, green, and blue lights are typically mixed in a light-generating or transmissive device such as a video display; cyan, magenta, and yellow inks are usually mixed in a reflective device such as printing on a sheet of paper.

We know that nothing is perfect in this world, however, and there are limits to the purities and exact hues of the colors we use in either transmissive or reflective color mixing. Deviations from the ideal in the filters used in an RGB display generate red, green, and blue lights that are not entirely perfect in hue and purity. The result is that a given device has a practical gamut of colors that does not include all possible colors discernible by the human visual system.


In the case of a CRT, the red, green, and blue phosphors determine the color gamut. This is why it is important in professional video operations to use monitors that have CRTs with standardized phosphors, so when material is exchanged, all parties see the same colors when they look at the same images. In the United States, these professional CRT phosphors are standardized by SMPTE.

Similarly, in the case of advanced displays such as LCDs and plasma panels, the red, green, and blue filters, or phosphors, determine the display's color gamut.

Likewise, the colors and purities of the cyan, magenta, yellow, and black inks used in four-color printing, along with the "whiteness" of the paper, determine the color gamut in a particular situation.

Typically, the color gamut of a video display is considerably larger than that of ink on paper. This is in large part because the primary colors produced by the inks used in printing are not as pure in hue as the red, green, and blue lights of a video display, and because paper is typically not nearly as white as RGB light.

Those who use a computer to print pictures on paper are well aware of the differences in color gamut between displays and prints, and that it is nigh impossible to perfectly match a printed photograph with its on-screen counterpart. Interestingly, because cyan is the least pure hue of printing ink, the rendition of blues on the printed page is significantly inferior to the blues that may be achieved on an RGB display.

However, because yellow is the printing ink with the purest hue, and because yellow must be created in an RGB display by mixing red and green lights, the rendition of yellow on a video display is significantly inferior to its rendition on the printed page, particularly on very high-quality white paper.


The experimental display engine generated at the Swiss Federal Institute of Technology does not use red, green, and blue filters. It uses a diffraction grating, which may be visualized as a tiny set of Venetian blinds: a surface that contains a row of narrow slits or striations.

When light from a white LED is shined on a striation in the grating, it acts as a prism, spreading the light into its component colors, in just the same way that Isaac Newton's prism spread light into a spectrum.

Depending on the angle to which the striation is twisted, any color in the diffracted spectrum can be viewed or projected through a tiny hole in a light barrier placed in front of the grating.

The diffraction grating used in the Swiss experiments is not rigid, but is made of a flexible polymer that is typically used to make artificial muscles for robots.

When a voltage is applied to the polymer, it contracts proportionally to the applied voltage. In this way, a given color from the diffracted spectrum may be viewed or projected as required, and an almost infinite number of colors may be so displayed, as opposed to the limited gamut possible using red, green, and blue light only.

The experimental array consists of 400 such gratings side by side. This is too small to be used for a practical display, but it proves the concept. Initially, it required several thousand volts to flex the polymer adequately, but the research team has reduced the voltage requirement to about 300.

This "super prism" array controlled by artificial muscles is very much a laboratory experiment at the moment, but it would seem to harbor some interesting potential for future advanced video display technologies.

Randy Hoffner