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                                                                          , 1999

8 EAST FORGE PARKWAY
FRANKLIN, MASSACHUSETTS 02038

Dear Stockholder:

    I am pleased to invite you to a Special Meeting of the stockholders of
ThermoSpectra Corporation ("ThermoSpectra") at which you will be asked to
approve an Agreement and Plan of Merger dated as of May 21, 1999 (the "Merger
Agreement") by and among ThermoSpectra, Thermo Instrument Systems Inc., the
parent company of ThermoSpectra ("Thermo Instrument"), and TS Acquisition
Corporation, a newly-formed subsidiary of Thermo Instrument (the "Merger Sub").
The Special Meeting will take place at 10:00 a.m. on             ,             ,
1999 at the offices of Thermo Electron Corporation, 81 Wyman Street, Waltham,
Massachusetts 02454.

    Under the terms of the Merger Agreement, the Merger Sub would merge with and
into ThermoSpectra, with ThermoSpectra being the surviving corporation (the
"Merger"). Each issued and outstanding share of ThermoSpectra common stock
(other than shares held by Thermo Instrument, Thermo Electron Corporation and
stockholders who are entitled to and who have perfected their dissenters' rights
under Delaware law) would be converted into the right to receive $16.00 in cash,
without interest. ThermoSpectra would become a private company. The Merger is
more fully described in the Merger Agreement, which is attached as Appendix A to
the enclosed Proxy Statement. If you choose to dissent from approval of the
Merger Agreement and wish to seek appraisal of the fair value of your shares of
ThermoSpectra common stock, please refer to the sections of the Proxy Statement
regarding the rights of dissenting stockholders under Delaware law.

    A special committee of the ThermoSpectra Board of Directors (the "Special
Committee"), acting in the interests of the stockholders of ThermoSpectra other
than Thermo Instrument, Thermo Electron Corporation and the directors and
officers of ThermoSpectra, Thermo Instrument and Thermo Electron Corporation
(the "Minority Stockholders"), evaluated the merits of, and negotiated the terms
of, the Merger. The Special Committee received an opinion from its financial
advisor, Tucker Anthony Cleary Gull, as to the fairness of the Merger from a
financial point of view to the Minority Stockholders. Please read carefully the
written opinion of Tucker Anthony Cleary Gull, dated May 21, 1999, which is
attached as Appendix B to the enclosed Proxy Statement. The Special Committee
recommended that ThermoSpectra's Board of Directors approve the Merger
Agreement.

    ThermoSpectra's Board of Directors and the Special Committee believe that
the proposed Merger is both substantively and procedurally fair to the Minority
Stockholders of ThermoSpectra, and the Board of Directors unanimously recommends
that stockholders vote "FOR" approval of the Merger Agreement. In considering
the recommendation of the Board of Directors with respect to the Merger
Agreement, stockholders should be aware that five of the seven members of the
ThermoSpectra Board of Directors are either directors of Thermo Instrument or
Thermo Electron Corporation, or employees of Thermo Electron Corporation or its
affiliates, and thus have interests that are in addition to, or different from,
your interests as stockholders of ThermoSpectra.

    Delaware law requires that a majority of the outstanding shares of
ThermoSpectra common stock entitled to vote at the Special Meeting vote in favor
of the Merger Agreement for the Merger Agreement to be approved. Thermo
Instrument, which owns approximately 82% of ThermoSpectra's outstanding common
stock, and Thermo Electron Corporation, which owns approximately 10% of
ThermoSpectra's outstanding common stock, intend to vote their shares in favor
of the Merger Agreement, thus assuring that the Merger Agreement will be
approved. Only stockholders of record at the close of business on       , 1999
will receive notice of and be able to vote at the Special Meeting or any
adjournment or adjournments thereof.

    The accompanying Proxy Statement provides you with a summary of the proposed
Merger and additional information about the parties involved and their
interests. Please give all this information your careful attention. You can also
obtain other information about ThermoSpectra, Thermo Instrument and Thermo
Electron Corporation from documents filed with the Securities and Exchange
Commission.

    WHETHER OR NOT YOU PLAN TO ATTEND THE SPECIAL MEETING, PLEASE TAKE THE TIME
TO VOTE BY COMPLETING AND MAILING THE ENCLOSED PROXY CARD TO US TODAY. IF YOU
DATE, SIGN AND MAIL YOUR PROXY CARD WITHOUT INDICATING HOW YOU WISH TO VOTE,
YOUR PROXY WILL BE COUNTED AS A VOTE IN FAVOR OF APPROVAL OF THE MERGER
AGREEMENT. YOUR VOTE IS VERY IMPORTANT, REGARDLESS OF THE NUMBER OF SHARES THAT
YOU OWN.

    Your Board of Directors believes that the transaction with Thermo Instrument
is in the best interests of ThermoSpectra and its stockholders, including its
Minority Stockholders, and has unanimously approved it. Your Board of Directors
unanimously recommends that stockholders vote for the approval of the Merger
Agreement. On behalf of the Board of Directors, I urge you to sign, date and
return the enclosed Proxy Card today.

    Please do not send any stock certificates to us now. Assuming the Merger
Agreement is approved, we will send you instructions concerning the surrender of
your shares.

    Thank you for your interest and participation.

                                          Yours very truly,

                                          BARRY S. HOWE
                                          PRESIDENT AND CHIEF EXECUTIVE OFFICER