[ATTACHMENT A TO PROXY STATEMENT]



                                  FORM OF PROXY

                            THERMOSPECTRA CORPORATION

      PROXY FOR SPECIAL MEETING OF STOCKHOLDERS TO BE HELD            , 1999

           THIS PROXY IS SOLICITED ON BEHALF OF THE BOARD OF DIRECTORS

         The undersigned hereby appoints Barry S. Howe, Theo Melas-Kyriazi
and Kenneth J. Apicerno, or any one of them in the absence of the others, as
attorneys and proxies of the undersigned, with full power of substitution,
for and in the name of the undersigned, to represent the undersigned at the
Special Meeting of the stockholders of ThermoSpectra Corporation, a Delaware
corporation (the "Company"), to be held on         ,           , 1999, at
10:00 a.m., at the offices of Thermo Electron Corporation, 81 Wyman Street,
Waltham, Massachusetts 02454-9046, and at any adjournment or adjournments
thereof, and to vote all shares of common stock of the Company standing in
the name of the undersigned on            , 1999, with all of the powers the
undersigned would possess if personally present at such meeting.

            (IMPORTANT - TO BE SIGNED AND DATED ON THE REVERSE SIDE.)






                         SPECIAL MEETING OF STOCKHOLDERS
                            THERMOSPECTRA CORPORATION
                                           , 1999

         1. To consider and vote on a proposal to approve an Agreement and Plan
of Merger dated as of May 21, 1999 (the "Merger Agreement") pursuant to which TS
Acquisition Corporation, a newly-formed company and wholly owned subsidiary of
Thermo Instrument Systems Inc., will be merged (the "Merger") with and into the
Company and each stockholder of the Company (other than stockholders who are
entitled to and have perfected their dissenters' rights, shares held by the
Company in treasury and shares held by Thermo Instrument Systems Inc. and Thermo
Electron Corporation) will become entitled to receive $16.00 in cash, without
interest, for each outstanding share of common stock, $.01 par value, of the
Company owned by such stockholder immediately prior to the effective time of the
Merger. A copy of the Merger Agreement is attached as Appendix A to and is
described in the accompanying Proxy Statement.

         [  ]  For                  [  ]  Against             [  ]  Abstain

         2. To consider and act in their discretion upon such other matters as
may properly come before the Special Meeting or any adjournment or adjournments
thereof.

         [  ]  For                  [  ]  Against             [  ]  Abstain

         THE SHARES REPRESENTED BY THIS PROXY WILL BE VOTED "FOR" THE PROPOSALS
SET FORTH ABOVE IF NO INSTRUCTION TO THE CONTRARY IS INDICATED OR IF NO
INSTRUCTION IS GIVEN.

         Copies of the Notice of Special Meeting and of the Proxy Statement have
been received by the undersigned.

                           PLEASE DATE, SIGN AND PROMPTLY RETURN THIS
                           PROXY IN THE ENCLOSED ENVELOPE.

                           Signature(s)
                                       --------------------------------

                           Date
                                ---------------------------------------

                           Note: This proxy should be dated, signed by the
                           stockholder(s) exactly as his or her name appears
                           hereon, and returned promptly in the enclosed
                           envelope. Persons signing in a fiduciary capacity
                           should so indicate. If shares are held by joint
                           tenants or as community property, both should sign.

PLEASE DATE, SIGN AND MAIL YOUR PROXY CARD BACK AS SOON AS POSSIBLE!