EXHIBIT 5
                           Thermedics Detection Inc.
                                81 Wyman Street
                       Waltham, Massachusetts 02254-9046

                                 July 23, 1997



Thermedics Detection Inc.
220 Mill Road
Chelmsford, Massachusetts 01824

            Re: Registration Statement on Form S-1 Relating to 643,500 Shares of
                the Common Stock, $.10 par value, of Thermedics Detection Inc.

Dear Sirs

     I  am  General  Counsel  to  Thermedics  Detection  Inc.,  a  Massachusetts
corporation  (the  "Company"),  and have acted as counsel in connection with the
registration  under the  Securities  Act of 1933,  as amended,  on Form S-1 (the
"Registration Statement"), of 643,500 shares of the Company's Common Stock, $.10
par  value  per  share  (the  "Shares"),  which may from time to time be sold by
certain shareholders of the Company.

     I or a member of my staff have reviewed the corporate  proceedings taken by
the Company with respect to the  authorization  of the issuance of the Shares. I
or a member of my staff have also examined and relied upon  originals or copies,
certified  or  otherwise  authenticated  to my  satisfaction,  of all  corporate
records,  documents,  agreements or other  instruments of the Company,  and have
made investigations of law and have discussed with the Company's representatives
questions  of fact  that I or a member  of my staff  have  deemed  necessary  or
appropriate.

     Based  upon and  subject to the  foregoing,  I am of the  opinion  that the
Shares have been duly  authorized by the Company and are validly  issued,  fully
paid and non-assessable.

     I  hereby  consent  to the  filing  of this  opinion  as  Exhibit  5 to the
Registration  Statement,  including any amendments thereto, and to the use of my
name under the  caption "Legal  Opinion" in the  prospectus constituting  a part
thereof.



                                       Sincerely,         
                                       
                                       
                                       
                                       
                                       Seth H. Hoogasian
                                       General Counsel