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                           OFFER TO PURCHASE FOR CASH
                     ALL OUTSTANDING SHARES OF COMMON STOCK

                                       OF

                            TREX MEDICAL CORPORATION
                                       AT

                              $2.15 NET PER SHARE

                                       BY

                         TREX MEDICAL ACQUISITION, INC.
                          A WHOLLY-OWNED SUBSIDIARY OF

                          THERMO ELECTRON CORPORATION

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  THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY
      TIME, ON WEDNESDAY, NOVEMBER 22, 2000, UNLESS THE OFFER IS EXTENDED.
- --------------------------------------------------------------------------------

To Our Clients:

     Enclosed for your consideration is an Offer to Purchase dated October 25,
2000 (the "Offer to Purchase") and the related Letter of Transmittal (which,
together with any amendments or supplements thereto, collectively constitute the
"Offer") relating to an offer by Trex Medical Acquisition, Inc., a Delaware
corporation (the "Purchaser") and a wholly-owned subsidiary of Thermo Electron
Corporation, a Delaware corporation ("Thermo Electron"), to purchase all
outstanding shares of common stock, par value $0.01 per share (the "Shares"), of
Trex Medical Corporation, a Delaware corporation (the "Company"), at a purchase
price of $2.15 per Share, net to the seller in cash, without interest thereon,
upon the terms and subject to the conditions set forth in the Offer.

     We are the holder of record of Shares held by us for your account. A TENDER
OF SUCH SHARES CAN BE MADE ONLY BY US AS THE HOLDER OF RECORD AND PURSUANT TO
YOUR INSTRUCTIONS. THE LETTER OF TRANSMITTAL IS FURNISHED TO YOU FOR YOUR
INFORMATION ONLY AND CANNOT BE USED BY YOU TO TENDER SHARES HELD BY US FOR YOUR
ACCOUNT.

     We request instructions as to whether you wish to have us tender on your
behalf any or all of the Shares held by us for your account, pursuant to the
terms and subject to the conditions set forth in the Offer.

     Your attention is directed to the following:

     - The tender price is $2.15 per Share, net to the seller in cash, without
       interest thereon.

     - The Offer and withdrawal rights will expire at 12:00 midnight, New York
       City time, on Wednesday, November 22, 2000, unless the Offer is extended.

     - The Offer is made for all of the outstanding Shares.

     - The Offer is conditioned upon, among other things described in the Offer
       to Purchase, there being validly tendered and not withdrawn prior to the
       expiration of the Offer that number of Shares which, together with Shares
       owned by Thermo Electron and the Purchaser, constitutes at least ninety
       percent (90%) of the outstanding Shares on the Expiration Date (as
       defined in the section of the Offer to Purchase captioned "The Tender
       Offer -- Terms Of The Offer; Expiration Date"). The Offer is also subject
       to other important terms and conditions contained in the Offer to
       Purchase.
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     - Tendering stockholders will not be obligated to pay brokerage fees or
       commissions or, except as set forth in Instruction 6 of the Letter of
       Transmittal, transfer taxes on the purchase of Shares by the Purchaser
       pursuant to the Offer.

     - In all cases, payment for Shares tendered and accepted for payment
       pursuant to the Offer will be made only after timely receipt by American
       Stock Transfer & Trust Company (the "Depositary") of (i) certificates
       evidencing Shares or timely confirmation of a book-entry transfer of such
       Shares into the Depositary's account at the Book-Entry Transfer Facility
       (as defined in the section of the Offer to Purchase captioned "The Tender
       Offer -- Acceptance For Payment And Payment For Shares") pursuant to the
       procedures set forth in the section of the Offer to Purchase captioned
       "The Tender Offer -- Procedures For Accepting The Offer And Tendering
       Shares," (ii) the Letter of Transmittal (or a facsimile thereof),
       properly completed and duly executed, with any required signature
       guarantees, or an Agent's Message (as defined in the section of the Offer
       to Purchase captioned "The Tender Offer -- Acceptance For Payment And
       Payment For Shares") in connection with a book-entry transfer and (iii)
       any other documents required by the Letter of Transmittal.

     The Offer is being made solely by the Offer to Purchase and the related
Letter of Transmittal and is being made to all holders of Shares. The Offer is
not being made to (nor will tenders be accepted from or on behalf of) holders of
Shares in any jurisdiction in which the making of the Offer or the acceptance
thereof would not be in compliance with the laws of such jurisdiction. In any
jurisdiction where the securities, blue sky or other laws require the Offer to
be made by a licensed broker or dealer, the Offer shall be deemed to be made on
behalf of the Purchaser by J.P. Morgan Securities Inc. or one or more registered
brokers or dealers licensed under the laws of such jurisdiction.

     If you wish to have us tender any or all of the Shares held by us for your
account, please instruct us by completing, executing and returning to us the
instruction form contained in this letter. If you authorize a tender of your
Shares, all such Shares will be tendered unless otherwise specified in such
instruction form. YOUR INSTRUCTIONS SHOULD BE FORWARDED TO US IN AMPLE TIME TO
PERMIT US TO SUBMIT A TENDER ON YOUR BEHALF PRIOR TO THE EXPIRATION OF THE
OFFER.

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                        INSTRUCTIONS WITH RESPECT TO THE
                           OFFER TO PURCHASE FOR CASH
                     ALL OUTSTANDING SHARES OF COMMON STOCK
                                       OF

                            TREX MEDICAL CORPORATION

     The undersigned acknowledge(s) receipt of your letter enclosing the Offer
to Purchase dated October 25, 2000 (the "Offer to Purchase") and the related
Letter of Transmittal (which, together with any amendments or supplements
thereto, collectively constitute the "Offer") pursuant to an offer by Trex
Medical Acquisition, Inc., a Delaware corporation and a wholly-owned subsidiary
of Thermo Electron Corporation, a Delaware corporation, to purchase all of the
outstanding shares of common stock, par value $0.01 per share (the "Shares"), of
Trex Medical Corporation, a Delaware corporation.

     This will instruct you to tender the number of Shares indicated below (or,
if no number is indicated below, all Shares) that are held by you for the
account of the undersigned, upon the terms and subject to the conditions set
forth in the Offer.



                                                   
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     ------------------------------------------------
     Number of Shares to be
     Tendered(1):
     ------------------------------------------------

     Dated: ------------------------------, 2000

                                          SIGN HERE      x
                                                         ------------------------------------------------
                                                         Signature(s)

                                                         ------------------------------------------------
                                                         Please type or print name(s)

                                                         ------------------------------------------------
                                                         Address

                                                         ------------------------------------------------
                                                         Telephone Number (including Area Code)

                                                         ------------------------------------------------
                                                         Tax Identification or Social Security Number

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  (1) Unless otherwise indicated, it will be assumed that all of the Shares held by us for your account
      are to be tendered.


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