<PAGE>

                                                               EXHIBIT 10.12 (a)

                                 FIRST AMENDMENT
                            DATED AS OF JUNE 28, 2002
                                       TO
                           RECEIVABLES SALE AGREEMENT
                          DATED AS OF DECEMBER 21,2001

         This FIRST AMENDMENT (the "Amendment"), dated as of June 28, 2002, is
entered into among PerkinElmer Receivables Company, as Seller (the
"Seller"), PerkinElmer, Inc., as Initial Collection Agent (the "Initial
Collection Agent," and together with any successor thereto, the "Collection
Agents"), the committed purchasers party thereto (the "Committed Purchasers"),
Windmill Funding Corporation ("Windmill"), and ABN AMRO Bank N.V., as agent for
the Purchasers (the "Agent")

                                   WITNESSETH:

         WHEREAS, the Seller, the Initial Collection Agent, the Agent, the
Committed Purchasers and Windmill have heretofore executed and delivered a
Receivables Sale Agreement, dated as of December 21, 2001 (as amended,
supplemented or otherwise modified through the date hereof, the "Sale
Agreement"),

         WHEREAS, the parties hereto desire to amend the Sale Agreement as
provided herein;

         NOW, THEREFORE, for good and valuable consideration, the receipt and
adequacy of which are hereby acknowledged, the parties hereto hereby agree that
the Sale Agreement shall be and is hereby amended as follows:

         Section 1.        The defined term "Dilution Reserve Multiple"
appearing in Schedule I to the Sale Agreement is hereby amended in its entirety
and as so amended shall read as follows:

                           "Dilution Reserve Multiple" shall be determined
                  according to the following table based upon the senior
                  unsecured debt rating of the Parent from S&P and Moody's:

<TABLE>
<CAPTION>
   Debt Rating
(Senior Unsecured)                     Multiple
<S>                                    <C>
BBB+ and Baa1 and higher                 1.0

   BBB or Baa2 or lower                  1.5
</TABLE>

                           ; provided, however, (a) if the Parent has a split
                  rating, the applicable rating will be the lower of the two,
                  and (b) if the Parent has no such rating, or such rating has
                  been suspended by either Moody's or S&P, the applicable
                  Dilution Reserve Multiple shall be the one set forth in the
                  last line of the table above.

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         Section 2.        The defined term "Loss Reserve Multiple" appearing
in Schedule I to the Sale Agreement is hereby amended in its entirety and as so
amended shall read as follows:

                           "Loss Reserve Multiple" shall be determined
                  according to the following table based upon the senior
                  unsecured debt rating of the Parent from Moody's and S&P:

<TABLE>
<CAPTION>
   Debt Rating
(Senior Unsecured)                         Multiple
<S>                                        <C>
BBB+ and Baa1 and higher                     1.0

  BBB or Baa2 or lower                       1.5
</TABLE>

                  ; provided, however, that (a) if the Parent has a split
                  rating, the applicable rating will be the lower of the two,
                  (b) if the Parent has no such rating, or such rating has been
                  suspended by either Moody's or S&P, the applicable Loss
                  Reserve Multiple shall be the one set forth in the last line
                  of the table above.

         Section 3.        Anything in the Sale Agreement to the contrary
notwithstanding, the Seller may request Incremental Purchases from the date of
this Amendment to the date the next Periodic Report must be delivered based upon
an interim receivables report (the "Interim Receivables Report") in form and
substance satisfactory to the Agent.

         Section 4.        This Amendment shall become effective on the date the
Agent (i) has received counterparts hereof executed by the Seller, Initial
Collection Agent, each Purchaser and the Agent, (ii) has received the Interim
Receivables Report, and (iii) has received a $27,500 Amendment Fee.

         Section 5.1.      To induce the Agent and the Purchasers to enter into
this Amendment, the Seller and Initial Collection Agent represent and warrant to
the Agent and the Purchasers that: (a) the representations and warranties
contained in the Transaction Documents, are true and correct in all material
respects as of the date hereof with the same effect as though made on the date
hereof (it being understood and agreed that any representation or warranty which
by its terms is made as of a specified date shall be required to be true and
correct in all material respects only as of such specified date); (b) no
Potential Termination Event exists; (c) this Amendment has been duly authorized
by all necessary corporate proceedings and duly executed and delivered by each
of the Seller and the Initial Collection Agent, and the Sale Agreement, as
amended by this Amendment, and each of the other Transaction Documents are the
legal, valid and binding obligations of the Seller and the Initial Collection
Agent, enforceable against the Seller and the Initial Collection Agent in
accordance with their respective terms, except as enforceability may be limited
by bankruptcy, insolvency or other similar laws of general application affecting
the enforcement of creditors' rights or by general principles of equity; and
(d) no consent, approval, authorization, order, registration or qualification
with any governmental authority is required for, and in the absence of which
would adversely effect, the

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legal and valid execution and delivery or performance by the Seller or the
Initial Collection Agent of this Amendment or the performance by the Seller or
the Initial Collection Agent of the Sale Agreement, as amended by this
Amendment, or any other Transaction Document to which they are a party.

         Section 5.2.      This Amendment may be executed in any number of
counterparts and by the different parties on separate counterparts and each such
counterpart shall be deemed to be an original, but all such counterparts shall
together constitute but one and the same Amendment.

         Section 5.3.      Except as specifically provided above, the Sale
Agreement and the other Transaction Documents shall remain in full force and
effect and are hereby ratified and confirmed in all respects. The execution,
delivery, and effectiveness of this Amendment shall not operate as a waiver of
any right, power, or remedy of any Agent or any Purchaser under the Sale
Agreement or any of the other Transaction Documents, nor constitute a waiver or
modification of any provision of any of the other Transaction Documents. All
defined terms used herein and not defined herein shall have the same meaning
herein as in the Sale Agreement. The Seller agrees to pay on demand all costs
and expenses (including reasonable fees and expenses of counsel) of or incurred
by the Agent and each Purchaser Agent in connection with the negotiation,
preparation, execution and delivery of this Amendment.

         Section 5.4.      This Amendment and the rights and obligations of the
parties hereunder shall be construed in accordance with and be governed by the
law of the State of Illinois.

<PAGE>

         IN WITNESS WHEREOF, the parties have caused this Amendment to be
executed and delivered by their duly authorized officers as of the date first
above written.

                                   ABN AMRO BANK N.V., as the Agent, as the
                                     Committed Purchaser

                                   By: /s/ Patricia Luken
                                       ----------------------------------------
                                   Title: GVP

                                   By: /s/ Nancy C. Buhe
                                       ----------------------------------------
                                   Title: GVP

                                   WINDMILL FUNDING CORPORATION

                                   By: /s/ Andrew L. Stidd
                                       ----------------------------------------
                                   Title: President

                                   PERKINELMER RECEIVABLES COMPANY, as
                                     Seller

                                   By: /s/ Jeffrey D. Capello
                                       ----------------------------------------
                                   Title: Treasurer

                                   PERKINELMER, INC. as Initial Collection Agent

                                   By: /s/ David C. Francisco
                                       ----------------------------------------
                                   Title: Assistant Treasurer

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