One simple and effective method to protect a document against forgery is to alter its integral substance rather than add to it. Forging documents is much easier when you can remove or replace information, such as photographic prints or an inked signature. Maurer Electronics GmbH manufactures high-security systems that create identification documents -- in particular, documents that employ photographs, including passports, drivers' licenses and identification cards. Antiforgery, by default, is part of its business, and so are lasers, which carry several advantages. For its M550 Laser Engraver unit, Maurer selected the StarMark Nd:YAG laser from Baasel Lasertechnik, now part of Rofin-Sinar Laser GmbH in Hamburg, Germany. "When you're personalizing an identification document with a laser, you don't use ink, so you don't add something," said Thomas Maurer, managing director for Maurer Electronics. "That means you can't falsify it by removing something." By altering the substance from which personal identification documents are made, lasers can help prevent forgery. One high-security equipment provider identified laser stability as the crucial attribute for its marking system. Courtesy of Maurer Electronics GmbH. The M550 works by selectively altering the document's substrate material -- usually polycarbonate polyvinyl chloride, paper or a paper/polymer combination. When applied to layered substrates, the system can create congruent optical effects in each stratum to create pictures against printed information, such as watermarks or antiforgery patterns. The result is documents with high-resolution, forgery-resistant images and unique surface textures. Photographs for identification documents are often of questionable quality. This makes stability an important attribute of the laser. "The human eye distinguishes 1.5 percent in terms of halftone differences," Maurer said. "In order to realize photos, you need high stability or you see the instability of the laser as noise in the photograph." Maurer's control software tracks each pulse of the laser and compares it against an optimal power level to correct and optimize the burning of the next pixel in the image. The software guarantees little pulse-to-pulse variance in the laser's output and allows the M550 to capitalize on the company's image enhancement software package. The laser marks images in black and white, which only adds to the functionality of identification documents. Color doesn't play a part in recognition and can even disturb recognition. Studies show that people recognize quickly flashed facial images more often when color is absent, said Maurer, who added, "You learn from British spy books that the perfect disguise is a very colorful jacket."