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                                  EXHIBIT (A)

                         THERMO INSTRUMENT SYSTEMS INC.

                                81 WYMAN STREET
                                 P.O. BOX 9046
                       WALTHAM, MASSACHUSETTS 02454-9046

Dear Thermo Optek Corporation Stockholder:

     On January 31, 2000, Thermo Electron Corporation and Thermo Instrument
Systems Inc. announced our intent to take Thermo Optek Corporation (the
"Company") private through a so called "short-form" merger. The purposes of this
letter and the Schedule 13e-3 Transaction Statement that accompanies this letter
are to:

        - tell you more about the merger,

        - explain why Thermo Electron and Thermo Instrument think that the
          $15.00 per share that you will receive in the merger is fair
          consideration for your shares and

        - let you know about your rights for an appraisal hearing under Delaware
          law.

     Neither you nor the Company's Board of Directors are being asked to approve
the merger. Under Delaware law, Thermo Electron and Thermo Instrument alone own
a sufficient number of shares to cause the merger to occur. After the merger,
Thermo Electron and Thermo Instrument will be the only stockholders of the
Company. In the merger, which we hope will occur on May 11, 2000 or as soon
thereafter as possible, you will receive $15.00 per share in cash. If you do not
believe that $15.00 is a fair price for your shares, you can follow the
procedures described in the Schedule 13E-3 Transaction Statement and exercise
appraisal rights under Delaware law. YOU SHOULD READ THE SCHEDULE 13E-3
TRANSACTION STATEMENT CAREFULLY BEFORE DECIDING WHETHER TO ACCEPT $15.00 PER
SHARE OR TO HAVE A DELAWARE COURT DETERMINE THE FAIR VALUE OF YOUR SHARES. The
amount determined by such court may be higher or lower than $15.00 per share.

     Stockholders of record on the date the merger becomes effective will be
mailed a Notice of Merger and Appraisal Rights and a Letter of Transmittal.
Stockholders receiving such documents should carefully read them. Detailed
instructions for surrendering your stock certificates, together with a detailed
description of statutory appraisal rights, will be set forth in the Notice of
Merger and Appraisal Rights and the Letter of Transmittal. Please do not submit
your stock certificates before you have received these documents.

     After the merger, the Company will not be publicly traded and will not be
listed on the American Stock Exchange. The Company also will not be required to
file reports with the Securities and Exchange Commission. In addition, the
merger will have federal income tax consequences for you, and you should consult
with your tax advisor in order to understand fully how the merger will affect
you.

                                          Sincerely,

                                          Earl R. Lewis
                                          President and Chief Executive Officer,
                                          Thermo Instrument Systems Inc.