IT IS HARD to believe that the justices of the Supreme Court had in mind a case like that tried Wednesday in Westminster, Md., when they gave a green light to secrecy in some criminal proceedings. They were worried - or so it seemed from their various opinions - that publicity about such proceedings might prejudice a jury and deprive a defendant of a fair trial. But in Westminster the trial conducted in secret lasted less than two hours. There was not - could not have been - any publicity, prejudicial or otherwise, before the jury returned its verdict.

This trial was held in secret simply because the defendant wanted it that way. His lawyer explained the defendant was "nervous" about the case and did not wish to see complaining witness, a woman he had raped, embarrassed by having to testify "before a lot of people." There was not one word in the request for secrecy about prejudicial publicity, the touchstone of the Supreme Court's decision.

The prosecutor, whome the Supreme Court had said would protect the public's interest in open trials by objecting when secrecy was unjustified, did not object. "Until we get a ruling as to whether or not a defendant has a right to a private trial," he said, "I don't see how I can oppose motions to close trials." He said he feared the conviction might be reversed if he insisted on a public trial.

The judge, whom the Supreme Court had said would protect the public's interest if the prosecutor did not, said he had no choice. Once both side agree to a secret trial, he explained, the judge must go along.

So much, then, for what the author of the Supreme Court's opinion"the general desirability of open judicial proceedings" and the duty of prosecutors and judges "to protect the societal interest in an open trial." The Westminster case, and a half dozen others in nearby states during the last month, indicate that many lower court judges are ready to forget those words anytime a defendant is "nervous" - and what defendants aren't? - and asks for secrecy.

There is a respectable, though not wholly persuasive, argument to be made for conducting secret criminal proceedings in order to protect a defendant's right to a fair trial. But no such argument exists for conducting secret trials to gratify the whims of a defendant. The high regard in which this country's judicial system is held rests in large part on the openness with which it has conducted its business. Those judges who are now endorsing secrecy have forgotten the old lesson that "justice must not only be done but must manifestly be seen to be done."