More Footcandles On The Pavement
By Karl Ruling
A police aerial surveillance searchlight is an unlikely inspiration for a creative lighting tool, but that's what provoked the development of the Obie Company's Xescan. "Marc Brickman happened to see the bottom of a police helicopter and thought, 'That'd be really cool!'" says Rick Romano, Obie's product development manager. "We said we'd take a look at it." The resulting Xescan is a high-output, high-precision moving light suitable for extremely long throws from the back of an arena. "We said, 'Let's sacrifice the effects and have a bit dirtier beam, and just get as many footcandles down to the pavement as we can,'" explains Romano.
The Xescan is a lens-less instrument that uses a custom 2kW xenon lamp in a rhodium-plated, electro-formed, parabolic reflector. A 2kW xenon lamp puts out only about 1/3 the total lumen output of a 2.5kW HMI lamp, but the lamp's compact luminous plasma ball in the parabolic reflector makes an extremely efficient optical system. "This has about a 70% collection efficiency on the lamp," says Romano. "Almost all of the light that gets collected --minus maybe another 20% -- gets stuffed out the front end. With a standard ellipsoidal reflector through a lens train the collection efficiency is not nearly that good." The Xescan puts out about 300fc at a throw of 250' and lights an area roughly 6.5' in diameter. The lamp position is motorized, the lamp can be moved off the reflector's focal point to flood the beam to a maximum of 20 degrees. At 250' and full flood the Xescan will light an area 87' in diameter to 80fc. The short arc of the custom lamp is stable without using any stabilizing magnets, and the instrument can be operated in any position. The xenon lamp also puts out an almost perfectly flat spectrum with no color variation from lamp to lamp or over the life of a lamp.
The original intention was to mount the xenon head in a moving yoke, "but banging a 2k lamp around at the speeds [Brickman] wanted didn't make sense," says Romano. "The past work I've done with Disney mounting xenon lamps on roller-coasters and things [has shown that] vibration tends to extinguish the lamp and it has to re-strike. The last thing you want is a lamp having to re-strike all the time with a microprocessor about a foot away from it."
The solution was to use a Telescan moving mirror to direct the beam through 270 degrees of pan and 90 degrees of tilt. "The head itself -- the mirror pan and tilt mechanism -- is made by Cameleon in France," says Romano. "We actually purchase that and then drive it with our own digital front end." The digital front end is 8051-based and provides control of pan and tilt, color, douser, lamp focus, lamp strike and kill, and re-calibration of the servo loops. The Xescan can be controlled via Telescan Mark II protocol, 10 channels of plainvanilla DXM512, or a special channel mapping tailored to the Compulite Animator. Pan, tilt, and douser are all 12-bit parameters mapped into two channels each of DMX512. Twelve bits is used on the douser for smooth fades, but smoothing algorithms are also use to get the best compromise between smooth fades and a high flash rate when the douser channels are assigned to the grand master of a Wholehog control desk.
The Xescan dispenses with conventional gobos and prism effects in the interest of high output, but the almost parallel rays of light coming off the reflector make it possible to project soft-edged shapes by simply inserting patterns into the light beam between the lamp and the mirror. The patterns on the Xescan are called "Gelbos": laser-cut images in the gel string on the modified Wybron Coloram scroller. The Coloram holds both a 24-frame gel string and the douser mechanism. "We searched the market for a scroller we liked, and we liked the Coloram," says Romano. "We liked the direct drive on it; we liked the digital protocol. We had a bunch of them on Janet Jackson and we couldn't make them fail. And Keny [Whitright, president of Wybron] was willing to work with us and build to our specifications in a timely manner."
There are about 36 Xescans in existence right now, but more will be built as the market dictates. The Obie Company's plan is to lease them for concert tours, and sell them to clients in the architectural /permanent installation markets.
Pink Floyd World Tour 1994 (U.S. Leg)