The Reagan administration yesterday announced the first redesign of U.S. paper currency in more than 50 years, but the changes -- a transparent stripe on one side of the dollar bill and small print around the bills' portrait -- will not be noticeable.

The changes, which result from six years and $7.5 million in research, are aimed at thwarting professional counterfeiters and a new class of "casual" counterfeiters whom the U.S. Secret Service fears may begin photocopying money for busfare, groceries and vacations with the advent of color copying machines.

A clear, polyester thread will be woven into the paper and run vertically on the left side of the Federal Reserve seal on all bills except the $1 bill, where it will be located between the seal and George Washington's portrait.

The other major change will be the repeated printing of "United States of America" in words so small around each president's portrait that copiers will be unable to reproduce the letters, the Treasury Department said.

There will be no recall of old currency when the new bills go into circulation within 15 to 18 months, said Treasury Secretary James A. Baker III.

"The development of advanced copying machines that permit high-resolution color reproduction, even by unskilled operators, is rapidly increasing," the Treasury said in a statement. "The future widespread availability of such copiers threatens to create a new kind of problem involving so-called 'casual counterfeiters' with access to such equipment. This 'crime of opportunity' involving small amounts of counterfeit notes in widely dispersed locations could seriously hamper the Secret Service's enforcement efforts."

But the fact that the changes will go largely unnoticed by most Americans makes the whole exercise moot, critics said. Counterfeiters generally take money to places where large amounts of bills change hands, such as supermarkets and race tracks.

Since the transparent stripe can only be seen when held to the light, shopkeepers and others who handle cash payments are not likely to take the time to detect counterfeits, critics said.

Rep. Frank Annunzio (D-Ill.), chairman of the House Banking consumer affairs and coinage subcommittee, which has studied the issue, said that in addition to getting new bills, each American should be issued a magnifying glass to determine whether their bills are real.

Annunzio's committee has suggested that the money be printed so that the word "void" appears when it is photocopied, as it does on federal checks, a committee spokesman said. Or, ink could be used on certain parts of the bill that would disappear when photocopied.

"The average American won't notice the changes," the spokesman said. "That's the whole problem with the thing."

However, a Treasury Department spokesman said that most other methods to foil counterfeiters would involve altering the appearance of the currency, which would upset the American public. The Treasury rejected suggestions that the new currency be painted pink or other colors.

Last year, the Secret Service confiscated $61 million in counterfeit bills before they were passed to the public, and $6.4 million in phony currency was discovered in circulation out of $209.02 billion in notes outstanding, a Treasury spokesman said.

"Both the new currency and existing currency will be legal tender and will circulate side by side," the Treasury Department said in a statement. "Old currency will be removed from circulation in the normal course of currency processing at the Federal Reserve Banks and branches. It will remain legal tender as long as it is in circulation."