Take the Green line Westbound B train and exit at BU Central (beyond the Kenmore Square stop), which is at the corner of St. Mary’s Street. Walk up St. Mary’s street away from Marsh Plaza (and the Charles River) one half block. The Photonics Center will be on your left.
Exit bus #57 at St. Mary’s Street. It runs along Commonwealth Avenue from Kenmore square, Brighton Avenue through Union Square, and Cambridge Street through Brighton Center.