Projecting Images on a Screen of Water
Water screens provide an exciting display medium for public celebrations, but the technology is more subtle than it appears.
by Valery Ivanov, Oleg Mikhailov, and Maxim Tomilin
MODERN SHOW TECHNOLOGIES are widely used for various festivals, celebrations, and holidays, and they utilize optical effects ranging from lighting fountains of water with colored light beams to decorating the skies with moving lasers.
For the celebration of St. Petersburg's 300th anniversary, it was decided to project video images of the historical and architectural sights of the city before a large audience. Because the demonstration was to take place between 10:30 p.m. and midnight on May 31, 2003, those who live in the middle latitudes – residents of Madrid, New York, Tokyo, and Seoul, for example – might expect the ambient lighting conditions to be dark. However, St. Petersburg is located at almost 60° north latitude, and late spring and summer is the time of the marvelous White Nights, when the sky is never dark, so these were the conditions under which the St. Petersburg show occurred (Fig. 1).
The video images were to be displayed by projecting them onto the wall of dense water streams. In order to optimize the video information being projected onto the water screens in rear-projection mode, it was necessary to determine the required level of effective screen illumination. We also had to determine the contrast ratio of the image, the influence of the parasitic light characteristic of the White Nights on the viewers' perception, and the influence of weather conditions on screen performance and illumination if a cloudless night sky turned to fog or rain.
In a general way, effects of the type we were going to attempt are well known around the world. For example, an image of the French national flag has been created with light in Paris close to the Arc de Triomphe, and a color image of the Russian flag has been produced in Moscow close to City Hall. These images are projected on particles in the air orwater droplets and observed in reflective mode.
But in our case, the formation of high-contrast video images on a screen essentially consisting of streams and droplets of water was much more complicated. It was necessary to form images having a contrast of not less than 6:1, despite the background of light skies, the parasitic reflections from the surface of the Neva River, and light from other aspects of the celebration (Fig. 2).
The minimum image luminance had to be not less than 10 cd/m2. The luminance of the various colors on the screen at Moscow City Hall was 3.8–6.0 cd/m2.
Specifying the System
The idea of a video show was very attractive, but only if comfortable viewing of the images could be obtained under any conditions. In the end, a successful show was realized, but only after light conditions were carefully analyzed and an appropriate projection system was specified and obtained. We calculated that the required illumination on a water screen measuring 10 x 12 m and 0.02 m in depth (water screens do have depth) could be obtained with a projector having an output of 12,000 ANSI lumens.
(a) (b) (c) Valery Ivanov
Fig. 1: The illumination conditions during the period of the water-screen video show were quite bright because the show was presented at the time of St. Petersburg's White Nights: (a) 10:00 p.m.; (b) midnight; and (c) 1:00 a.m.
Quite a bit of calculation concerning the optics of the situation went into determining the required output, including an analysis of the spatial image parameters and their dependence on the dispersed structure of water and fog screens, a determination of the effective screen illumination during the projection of the video information, and determination of the influence of parasitic light on ease of perception.
The useful illumination E obtained from the projector's luminous flux produced a luminous excitance M on the opposite side of the water-droplet screen, which determined the image luminance in the observer's direction (Fig. 3). The useful luminance L was equal to 50 cd/m2 at a projector-to-screen distance of 40 m. The distance from the water screen to the VIP platform was not taken into consideration because it did not influence on-screen luminance. In comparison, the usual CRT monitor has a luminance of up to 100 cd/m2, LCDs go up to about 500 cd/m2, and the brightest plasma panels are now breaking the 1000-cd/m2 barrier.
At the same time, the influence of parasitic light-sky radiation near the sunset zone, scattered radiation, and reflection from the water surface on screen luminance was analyzed. The coefficients of transparency and scattering of the atmosphere and water, their refractive indices, and the indicatrices directions – the directional dependency of optical variables in materials – were all taken into consideration in the calculations.
Fig. 2: It was necessary to form images with a contrast of not less than 6:1 despite the background of light skies (not evident in this photo because of the exposure settings), the parasitic reflections from the surface of the Neva River, and light from other aspects of the celebration.
Fig. 3: This schematic of the projection system with water screens indicates that the useful illumination E obtained from the projector's luminous flux produced a luminous excitance M on the opposite side of the water-droplet screen, which determined the image luminance in the observer's direction. The useful luminance L was equal to 50 cd/m2 at a projector-to-screen distance of 40 m.
Fig. 4: This overview shows the location of the water screens, placed close to the Petropavlovsky Bastion on the opposite side of the Neva River from the Winter Palace because there was less light pollution on that side of the river.
For the specified conditions, including a screen luminance of 50 cd/m2 in the direction of the observers, the contrast with a sunset background was 7; with a background of dark skies during the White Nights, 17; and with the background of the famous Petropavlovsky Bastion wall, 49. In the case of bad weather conditions (cloudy, raining), the contrast in this last case would decrease to 38.
The water screen is obviously a critical component of the projection system. The calculations were made for two positions: in front of the Winter Palace close to the Dvortzovaya embankment, with very complicated light conditions, and on the opposite side of the Neva River close to the Petropavlovsky Bastion (Fig. 4), where there was less light pollution. The screen position close to the Petropavlovsky Bastion was chosen for project realization. Two Sanyo PLC-XF45 projectors with an XGA (1024 x 768) pixel format, a total luminous output of 20,000 lm, and an on/off contrast ratio of 1100:1 produced the video images for the unusual screen. The projectors each incorporate three 1.8-in. poly-Si TFT-LCD imagers. Nozzles placed on a special deck along with the projection equipment formed the water streams and droplets (Fig. 5).
The Final Effect
The special challenge of a water screen was in forming a micro-dispersed scattering structure based on air bubbles and stream boundaries. It was calculated that, as the limiting angular resolution for the human eye under twilight conditions is equal to 50 angular seconds, the linear resolution of the object at a distance of 100 m is equal to 6 cm. Therefore, the water screen would not degrade the image quality because the scattering elements in the dispersed water droplets were less than 1 cm.
The presentation of color information differs from the presentation of black-and-white information because the luminance of primary colors is less than the luminance of white light.
As a result, the image contrast for blue and red colors decreases to 4 or 5, and to 6 for green. The color quality of the image does not change when image contrast decreases because it depends on the light source, the spectral characteristics of the water screen, and the relative sensitivity of the adapted eye. All of these parameters stay constant in conventional projection systems.
Fig. 5: A special deck was constructed for the water-screen nozzles and projectors.
Fig. 6: This portrait of a young woman was one of the images projected in rear-projection mode on the water screen. The photograph was taken from the projection side, i.e., the side opposite from the intended direction of observation, so it does not do justice to the actual projected image.
The analysis and calculations of the projection/water-screen system made it possible to produce a successful and striking show of video images just after 11:00 p.m. on May 31, 2003. The optical output of the two projectors was sufficient to obtain images of good quality (Fig. 6), which added to the festivities surrounding St. Petersburg's 300th anniversary. •