TIFTON, Ga. (AP) _ U.S. Department of Agriculture scientists have developed an infrared device that takes the guesswork out of buying watermelons, cantaloupes and honeydews.

After 16 years of research, Gerald Dull, a USDA chemist in Athens, has unveiled a meter that measures the sugar content of individual melons.

''The No. 1 parameter of quality is sweetness,'' Dull said in a recent interview. ''If you haven't got sweetness, you haven't got a good melon.''

Growers and consumers currently have to rely on appearance in judging ripeness and quality, Dull noted, but similar-looking melons can vary from 5 percent to 10 percent in sweetness.

''There's no one who can look at the outside and tell you whether that melon is good,'' he said.

The melon meter shines infrared light into the top of the fruit and then measures the amount of light that comes out the side. The computerized device determines the amount of soluble solids, based on the amount of light that is absorbed by the fruit.

Dull said soluble solids are predominantly sugars.

Doyle Smittle, a horticulturist at the University of Georgia's Coastal Plain Experiment Station, said ripeness is critical because melons can add a quarter of a pound of sugar in just a few days. Sugar is produced during the photosynthesis process.

Melons need a sugar content of at least 9 percent to taste good, and the best contain 12 percent to 14 percent sugar.

''If you get one that is 7 or 8 percent, it's an unpleasant experience,'' Dull said. ''It's kind of like eating a raw cucumber.''

The melon meter is 85 percent accurate in the laboratory, and the chemist is testing a portable version with hundreds of melons grown under Smittle's supervision near Tifton.

Dull developed the meter with help from USDA engineer Gerald Birth, who recently retired. Another USDA scientist, physicist Dick Leffler of Athens, is perfecting the computer program that will analyze signals from the light detector and give a sweetness reading.

Several companies have already expressed an interest in producing melon meters, Dull said, and they probably will be installed initially in packing sheds, where the fruit is graded, packed and shipped.

He believes they will appeal to suppliers who want to provide consumers with the highest-quality melons.

''I think we're just opening the door'' on analyzing fresh produce, he said. ''With computers and high-speed equipment, we might be able to get down to (measuring) the vitamins.''