EXHIBIT 5
 
                           Thermedics Detection Inc.
                                 220 Mill Road
                     Chelmsford, Massachusetts  01824-4178


                               February 5, 1997


Securities and Exchange Commission
Judiciary Plaza
450 Fifth Street, N.W.
Washington, DC  20549

Re:     Registration Statement on Form S-1 (Registration No. 333-19199) 
        Relating to Shares of  Common Stock, Par Value $.10 Per Share, of 
        Thermedics Detection Inc. and Subscription Rights to Purchase Such
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        Shares
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Ladies and Gentlemen:

        I am General Counsel to Thermedics Detection Inc. (the "Company") and
have acted as counsel in connection with the registration under the Securities
Act of 1933, as amended (the "Act"), on Form S-1 (Registration No. 333-19199)
(the "Registration Statement") of (i) shares of the Company's Common Stock, par
value $.10 per share (the "Common Stock"), with a proposed maximum aggregate
offering price of $34,488,000 (such shares, together with any shares of Common
Stock registered under a registration statement related to the offering
contemplated by the Registration Statement that is to be effective upon filing
pursuant to Rule 462(b) under the Act (a "462(b) Registration Statement"), are
collectively referred to herein as the "Shares") and (ii) transferable
subscription rights (the "Rights") to purchase the Shares.

        I or a member of my legal staff have reviewed the corporate proceedings
taken by the Company with respect to the authorization of the issuance of the
Shares and the Rights.  I or a member of my legal 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 all investigations of law and have
discussed with the Company's representatives all questions of fact that I have
deemed necessary or appropriate.

        Based upon and subject to the foregoing, I am of opinion that the Shares
and the Rights have been duly authorized by the Company, and, when issued as
contemplated by the Registration Statement, or by a 462(b) Registration
Statement, will be validly issued, fully paid and non-assessable.

 
        Pursuant to the requirements of the Act, I hereby consent to the filing
of this opinion as Exhibit 5 to the Registration Statement and to any 462(b)
Registration Statement, including any amendments thereto, and to the use of my
name under the caption "Legal Opinions" in any prospectus constituting a part
thereof.

                                               Very truly yours,
       
       
       
                                               Seth H. Hoogasian
                                               General Counsel

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