Just before adjourning this week, the U.S. Supreme Court handed down what is easily the most important decision of its 1988 term.

It's a decision that helps make sure a bright light gets beamed into some dark corners. And that's usually what it takes to keep government honest and responsible.In a landmark verdict, the high court upheld the Watergate-inspired law letting independent counsels investigate wrong-doing by top government officials.

Though the ruling represents a sharp rebuff for the Reagan administration, the decision amounts to a major victory for the public interest because it should help inspire confidence in the criminal justice system.

With only one dissenting vote, the justices removed an obstacle that could have impaired independent counsel Lawrence E. Walsh's investigation of the Iran-Contra scandal.

The verdict also leaves intact the perjury conviction of former White House aide Michael K. Deaver and the illegal lobbying conviction of former White House political director Lyn Nofziger. Both cases were probed by independent investigators.

Adopted after the Watergate scandal, the independent counsel law lets a special court panel select an independent counsel once the Justice Department signals the need for such a move. The law has been extended twice, including just last year, even though the Reagan administration challenged it.

The issue went to the Supreme Court after a lower court ruled that the law authorizing the appointment of special counsel violates the constitutional doctrine of separation of powers by taking prosecution powers from the president and giving them to someone not appointed by the attorney general or subject to his control.

Rejecting this argument, the Supreme Court wisely agreed with Congress that the law is needed because political leaders can't always be trusted to investigate scandals involving their own associates and still do a vigorous, objective job.

By upholding this law, the Supreme Court is simply helping to assure that allegations of wrongdoing by high officials are subject to a credible, impartial investigation. Now let's make sure this law is invoked only in major scandals and not for every passing political controversy that comes along.