For the next 10 years a photograph of Mark S. Collins, convicted of aggravated rape in 1987, will hang on a bulletin board in the lobby of the Cambridge Police Department beneath the words "Sexual Offender Community Notification."

The public posting of serious convicted sex offenders' names, which began in August, is required under the Massachusetts Sexual Offender Registry law, passed in July 1996.

The law requires convicted sex offenders to register at the police departments in the communities in which they live and work.

The names of sex offenders who register are then made available to the public.

Implementation of the law did not begin until last October because of court cases on the constitutionality of such a law.

"Last October was when we really got up and running," said Elizabeth Dillon, domestic violence and sexual assault liaison at the Cambridge Police Department, who is also coordinator of the Sex Offender Registry for Cambridge.

"There had been a ban on releasing information until a class-action lawsuit had gone through the court process and the appeals court for the state of Massachusetts upheld the sex-offender registry law," Dillon said.

Dillon said that it is currently not the responsibility of police departments to seek out offenders who have not registered.

If an individual is stopped by police, a background check is run, and the person is identified as someone who has failed to register, the person will face additional charges in court, Dillon said.

Once a person registers, the state Criminal History Systems Board and its Sex Offender Registry Board classifies the individual as either a "Level One," "Level Two" or "Level Three."

The 5'9", 190-pound Collins is the only one of the 115 registered sex offenders living or working in Cambridge identified by the board as a "Level Three," namely a "high risk to re-offend," according to Dillon.

Level one offenders are identified as being at a low risk to commit a repeat offense, level two a moderate risk and level three a high risk.

But the police acknowledged that they do not themselves fully comprehend fully the criteria for the different categories. "At this point I do not completely understand the distinction," Dillon said.

"It is my understanding that anyone who has been convicted of a sex offense which involved children will automatically be identified as a level three," Dillon said.

The members of the Massachusetts Sex Offender Registry Board include psychiatrists, psychologists and child development specialists, Dillon said.