A spate of "funny money" passed at shops in downtown Fairfield has prompted police to warn local merchants to be on alert.

There have been five instances of counterfeit $100 bills used in the last few weeks to pay for a small amount of merchandise, according to police Sgt. Suzanne Lussier. In each case, the person received close to $90 in change.

The most recent incident was reported Thursday by the owner of Liquid Lunch in the Brickwalk shopping center. A woman, described as white and in her 30s, used the bill to buy lunch. While counting money from the register, the owner noticed the $100 bill, which according to the report had a picture of Abraham Lincoln on the lower right corner.

The image of Benjamin Franklin is the centerpiece on the $100; Lincoln's portrait appears on the $5 bill.

Lussier said the description of the person passing the bills has been different each time, but investigators believe that all the incidents are related.

Fake $100 bills have also been used recently at the Fairfield University Bookstore, Archie Moore's restaurant, Sweet Rexie's candy store and 16 Handles frozen yogurt.

There are security features on the bills that merchants can check as a way of detecting a counterfeit, according to the U.S. Bureau of Engraving and Printing. Since 1996, the $100 bills have a watermark that is visible when held up to the light, and use color-shifting inks used in the numeral on the lower-right corner on the face of the note. The ink appears green when viewed directly and changes to black when the note is tilted. They also have larger, off-center portraits.

Lussier said if a merchant gets a suspected counterfeit bill, the merchant should try to delay the customer and contact police immediately.