Counterfeiting currency is neither a new problem nor is it limited to any one country or currency. For example, since the euro was released in 2002, both the quality and quantity of counterfeiting has increased. With improved computers, scanners and printers at their disposal, counterfeiters are becoming adept at creating near-perfect reproductions. Detectors that scan bills have been helpful, but as the fakes look increasingly like the real thing, better optical devices are required.Many counterfeit currency detectors sort the good from the bad by scanning bills with a UV mechanism. Unfortunately, the accuracy of this technology is only about 20 to 40 percent. New detectors that scan IR frequencies are more accurate and need less time to spot a fake.Every currency has the same security characteristics or features, including magnetic inks and threads, precise dimensions that can be optically measured, and markings that are designed to be seen under IR and UV light. Newer-generation detectors analyze currency with an array of sensors that account for each of these characteristics. For example, many counterfeiters do not include IR markings in their forged currency, so including an IR sensor in the detector spots those bills immediately.Figure 1. A US $10 bill is fed into a currency authentication system, where it is scanned for its five security characteristics.LaseOptics Corp. of Amherst, N.Y., offers a currency authentication system with IR diodes of 880 to 915 nm operating at 20 to 120 mW. The system has embedded-microcontroller, IR, UV, magnetic-ink, magnetic-thread and optical-sensing mechanisms. The amount of laser light that passes through currency inserted into the mechanism is sensed by photodiodes, and the amount of UV radiation reflected by the currency is detected by UV sensors. Noncontact readers and detectors determine the presence of magnetic ink and metal thread. As a bill passes through the device, three optical sensors determine its exact length.Figure 2. A US $50 bill passes through the system, having been verified as legitimate.When a single bill is fed into the device (Figure 1), the data scanned is compared with stored data. When comparing the information scanned from the bill with programmed reference templates, it is easy to determine whether the bill is genuine or counterfeit. If a genuine bill passes through, a green LED lights up, and the liquid crystal display shows the denomination and country of origin (Figure 2). If a bill is counterfeit, it is ejected, and a red LED lights up (Figure 3). Each bill is processed at a speed of 0.8 s. Because the counterfeit problem is international, the system can detect 20 currencies.Figure 3. Caught red-handed: When a counterfeit is detected, the system flashes a red liquid crystal display, indicating that the bill has not passed inspection.The challenge in producing a counterfeit detector is in engineering the internal components to fit in the device. The LaseOptics unit, for example, uses 10 5.6-mm IR sources and 10 photodiodes, making its circuitry and geometry complex. This complexity also can make it difficult to get accurate measurements. Accuracy is important because the numerous security features built into the currency must match up with the device's components, and the result must be efficient as well as accurate.With the proper implementation of optical technologies, state-of-the art counterfeit detectors are finally beginning to keep pace with the advances in the printing and scanning industries that have helped create the near-perfect fakes.