After six decades in which the venerable greenback never changed its look, the U.S. currency has undergone a slew of makeovers. The most amazing is yet to come.

A new security thread has been approved for the $100 bill, The Associated Press has learned, and the change will cause double-takes.

The new look is part of an effort to thwart counterfeiters who are armed with ever-more sophisticated computers, scanners and color copiers. The C-note, with features the likeness of Benjamin Franklin, is the most frequent target of counterfeiters operating outside the United States.

The operation of the new security thread looks like something straight out of the Hogwarts School of Witchcraft and Wizardry. This magic, however, relies on innovations produced from decades of development.

It combines micro-printing with tiny lenses — 650,000 for a single $100 bill. The lenses magnify the micro-printing in a truly remarkable way.

Move the bill side to side and the image appears to move up and down. Move the bill up and down and the image appears to move from side to side.

"It is a really complex optical structure on a microscopic scale. It makes for a very compelling high security device," said Douglas Crane, a vice president at Crane & Co. The Dalton, Mass-based company has a $46 million contract to produce the new security threads.

Larry Felix, director of the Bureau of Engraving and Printing, confirmed details about the security thread in an AP interview.

The redesign of the $100 is about one-third of the way complete. The bill is expected to go into circulation late next year.

Starting in 2003, splashes of color have spruced up the $20 bill and other currencies. Those changes followed the addition of a first round of security features in the mid-1990s.

Benjamin Franklin's latest makeover was delayed while the government searched for a high-tech security device that would provide extra protection on the bill.

The $100 bill represents more than 70 percent of the $776 billion in currency in circulation, two-thirds of which is held overseas.

Holograms, used extensively on credit cards, were considered for the $100. They were rejected because they did not offer the strong visual signal the government wanted.

"We were looking for features that had very distinctive types of actions so that we could tell the American public, you will know that it is authentic if you do this and the note does that," Felix said.

The new security thread is used on the Swedish 1,000 kroner note and has been selected by the government of Mexico for some higher denomination notes.

Felix said many other devices expected to be included in the $100 redesign will be similar to features added over the past four years to the $20, $50 and $10 bills. That means subtle pastel colors on the currency and patches of micro-printing that are difficult to duplicate, along with a touchup on Ben Franklin's portrait.

Originally there were no plans to redesign the $5 bill. That decision was reversed once counterfeiters started bleaching $5 bills and printing fake $100 bills over the bleached paper; certain security features were in the same location on both bills.

The new $5 design will be made public on Sept. 20 and will go into circulation early next year.

The bleached bills represent the latest skirmish in a battle with counterfeiters.

"Counterfeiting is becoming highly organized and highly efficient," Felix said. He said some clandestine printing plants in Latin America and Eastern Europe have been caught counterfeiting not only the U.S. currency but other countries' notes.

The government says $118.1 million in counterfeit U.S. currency was detected in 2006, an increase of 3.8 percent from 2005.

While that is a fraction of the currency in circulation, the Secret Service is concerned with the threat, especially the challenge posed by new digital technology. Digital copies account for about half of all counterfeit notes passed in the U.S., compared with less than 1 percent of all counterfeit bills detected in 1995.

"The quality of the counterfeit currency has gone down, but the ease by which people can make this currency and the access to the computer equipment has had an impact on the rising numbers," Secret Service spokesman Eric Zahren said.

To stay ahead of the counterfeiters, the Bureau of Engraving and Printing plans to redesign U.S. currency every seven years to 10 years. That is a far cry from the practice for most of the 20th century — from 1929 to the 1990s — when the currency stayed the same without any major changes.

"We had three generations of engravers who spent their entire careers at the bureau and never saw their designs hit the streets," Felix said. "Now since 1996, we have all of these changes."

All the new security devices have added to the complexity of making money. The government churns out 38 million notes each business day with a face value of $750 million at two facilities — one in Washington, D.C., and the newest one in Fort Worth, Texas.

By order of Congress, the $1 bill, which accounts for 45 percent of the notes printed each year, will not be redesigned. Lawmakers were concerned about the cost to business if low-end vending machines that only take coins and $1 bills had to be upgraded.

In addition to redesigning the money, the bureau is putting in new printing presses with more capabilities to handle the increasingly sophisticated security features.

The new presses can vary the size of the bills being printed. That is something the American Council for the Blind is urging the government to consider as a way of helping the visually impaired distinguish between different denominations of currency.

Felix says no decision has been made on such a change. The government is appealing a federal court ruling that could force such a redesign.

In its continuing effort to stay ahead of counterfeiters, the bureau is reviewing a wide range of new ideas such as adding a sense of depth to the designs.

"Currency is essentially a confidence situation," Felix said. "You have to always stay ahead in changes."