Movie director Roman Polanski, who had pleaded guilty to unlawful sexual intercourse with a 13-year-old girl, yesterday was ordered imprisoned for a 90-day psychiatric study to help the judge decide his sentence.

However, the 44-year-old Polanski, director of the films "Chinatown" and "Rosemary's Baby," won a three-month delay of his incarceration in order to finish a movie he is making in Tahiti.

Superior Court Judge Lawrence Rittenband, noting the teen-age girl "looks older than her years" and was sexually experienced, nevertheless scolded the director for taking advantage of her.

As Polanski stood calm and silent beside his attorney, the judge described for the first time the young model whose identity had been kept secret since Polanski was first charged last March with rape and drug abuse.

"She has a 17-year-old boyfriend with whom she had sexual intercourse at least twice prior to the offense involved." He said that perhaps the boyfriend should also be charged with a crime.

Rittenband said the girl also "was not unfamiliar with the drug Quaalude, she having experimented with it as early as her 10th or 11th year. This was the same drug furnished to her by the defendant."

But the judge said none of this excused Polanski, who had contracted to photograph the girl for a French fashion magazine. Prosecutors said he took her to the home of actor Jack Nicholson while Nicholson was away, fed her champagne and Quaaludes, then committed numerous sex acts with her.

The probation report indicated that she consented. The judge said it made no difference.

The technical effect of the judge's decision will be for Polanski to spend some time in prison without having the record of a prison sentence against him unless he is eventually placed behind bars under a formal sentence.

Among the problems Polanski faces is possible deportation. However, the law provides automatic deportation only for those convicted of crimes of moral turpitude who are sentenced to one year or more in prison.