<PAGE>

                                                                   EXHIBIT 10.12


                                 AMENDMENT TO
                 3-YEAR $15,000,000 REVOLVING CREDIT AGREEMENT

     AMENDMENT TO 3-YEAR $15,000,000 REVOLVING CREDIT AGREEMENT, dated as of May
31, 2000, among NEWPORT CORPORATION (herein referred to as the "Borrower") and
ABN AMRO BANK N.V. (herein referred to as the "Lender").

                             W I T N E S S E T H:

     WHEREAS, the Borrower and the Lender are parties to that certain 3-Year
$15,000,000 Revolving Credit Agreement, dated as of October 29, 1999 (the
"Credit Agreement"); and

     WHEREAS, the Borrower has requested that the Credit Agreement be amended in
certain respects; and

     WHEREAS, the Borrower has requested, among other things, that the Lender
increase the Commitment and extend the Termination Date; and

     WHEREAS, the Lender is willing to so amend the Credit Agreement, subject to
the terms and conditions hereinafter set forth;

     NOW, THEREFORE, the parties hereto, in consideration of their mutual
covenants and agreements hereinafter set forth and intending to be legally bound
thereby, covenant and agree as follows:

      1.  General.  All terms used herein which are not otherwise specifically
defined herein shall have the same meaning herein as defined in the Credit
Agreement as further amended hereby.

      2.  Cover Page.  The reference to the amount "$15,000,000" on the cover
page of the Credit Agreement shall be deleted.

      3.  Revolving Credit.  Section 1.1 of the Credit Agreement shall be hereby
amended by deleting the amount "$15,000,000" in line 8 thereof and substituting
in its place the amount of "$20,000,000" therefor.

      4.  Revolving Credit Loans.  Section 1.2 of the Credit Agreement shall be
amended by deleting the amount "$15,000,000" in line 6 thereof and substituting
in its place the amount of "$20,000,000" therefor.

      5.  Definitions.  Section 4.1 of the Credit Agreement shall be and is
hereby amended as follows:

               (a) The definition of "Adjusted LIBOR" shall be amended and
          restated in its entirety as follows:
<PAGE>

          "Adjusted LIBOR" means a rate per annum determined by the Bank in
          accordance with the following formula:

                     Adjusted LIBOR =          LIBOR
                                       -----------------------
                                       100%-Reserve Percentage

          "Reserve Percentage" means, for the purpose of computing Adjusted
          LIBOR, the daily average for the applicable Interest Period of the
          maximum rate at which reserves (including, without limitation, any
          marginal, emergency, supplemental or other special reserves) are
          imposed during such Interest Period by the Board of Governors of the
          Federal Reserve System (or any successor) under Regulation D on
          "eurocurrency liabilities" (as such term is defined in Regulation D)
          (or in respect of any other category of liabilities that includes
          deposits by reference to which the interest rate on LIBOR Portions is
          determined or any category of extensions of credit or other assets
          that include loans by non-United States offices of the Bank to United
          States residents), but subject to any amendments to such reserve
          requirement by such Board or its successor, and taking into account
          any transitional adjustments thereto becoming effective during such
          Interest Period.  For purposes of this definition, LIBOR Portions
          shall be deemed to be Eurocurrency liabilities as defined in
          Regulation D without benefit of or credit for prorations, exemptions
          or offsets under Regulation D. "LIBOR" means, for each Interest
          Period, (a) the LIBOR Index Rate for such Interest Period, if such
          rate is available, and (b) if the LIBOR Index Rate cannot be
          determined, the arithmetic average of the rates of interest per annum
          (rounded upward, if necessary, to the nearest 1/100th of 1%) at which
          deposits in U.S. Dollars in immediately available funds are offered to
          the Bank at 11:00 a.m. (London, England time) 2 Business Days before
          the beginning of such Interest Period by three (3) or more major banks
          in the interbank eurodollar market selected by the Bank for a period
          equal to such Interest Period and in an amount equal or comparable to
          the applicable LIBOR Portion scheduled to be outstanding from the Bank
          during such Interest Period.  "LIBOR Index Rate" means, for any
          Interest Period, the rate per annum (rounded upwards, if necessary, to
          the next higher one hundred-thousandth of a percentage point) for
          deposits in U.S. Dollars for a period equal to such Interest Period
          which appears on the Telerate Page 3750 as of 11:00 a.m. (London,
          England time) on the date 2 Business Days before the commencement of
          such Interest Period.  "Telerate Page 3750" means the display
          designated as "Page 3750" on the Dow Jones Telerate Service (or such
          other page as may replace Page 3750 on

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<PAGE>

          that service or such other service as may be nominated by the British
          Bankers' Association as the information vendor for the purpose of
          displaying British Bankers' Association Interest Settlement Rates for
          U.S. Dollar deposits). Each determination of LIBOR made by the Bank
          shall be conclusive and binding absent manifest error.

               (b) The definition "Termination Date" shall be amended by
          deleting the date "September 30, 2002" and substituting the date "May
          30, 2003" therefor.

               (c) The definition of "364-Day Agreement" shall be amended by (i)
          deleting the amount "10,000,000" therein and (iii) inserting
          immediately after the phrase "Revolving Credit Agreement" the phrase,
          "as amended," therein.

      5.  Tangible Net Worth.  Section 7.9 of the Credit Agreement shall be
amended by (i) deleting the amount "$50,465,000" therein and substituting the
amount "$56,677,000" therefor and (ii) deleting the date "December 31, 1998"
therein and substituting the date "December 31, 1999" therefor.

      6.  Revolving Note.  The Note found in Exhibit A to the Credit Agreement
shall be amended and restated in its entirety by replacing such with Exhibit A
attached hereto.

      7.  Representations.  In order to induce the Lender to execute and deliver
this Amendment, the Borrower hereby represents to the Lender that as of the date
hereof the representations and warranties set forth in Section 5 of the Credit
Agreement are and shall be and remain true and correct (except that the
representations contained in Section 5.5 shall be deemed to refer to the most
recent financial statements of the Borrower delivered to the Lender) and the
Borrower is in compliance with the terms and conditions of the Credit Agreement
and no Default or Event of Default has occurred and is continuing under the
Credit Agreement or shall result after giving effect to this Amendment.

      8.  Effectiveness.  This Amendment shall become effective (i) when it
shall be executed by the Borrower and the Lender, (ii) the Borrower shall have
executed and delivered a new Note to the Lender as identified in Exhibit A
hereto, (iii) the Lender has received certified copies of the resolutions of the
Board of Directors of the Borrower approving the increase and extension
described herein, (iv) the Lender shall have received copies (executed or
certified, as may be appropriate) of all legal documents or proceedings taken in
connection with the execution and delivery of this Amendment to the extent the
Lender or its counsel may reasonably request.  This Amendment may be executed in
separate counterparts, all of which taken together shall constitute one and the
same instrument.  This agreement shall be construed and determined in accordance
with the laws of the State of California.  Except as herein specifically
amended, the Credit Agreement shall be and remain in full force and effect and
wherever reference is made in any note, document, letter or other communication
to the Credit Agreement, such reference shall, without more, be deemed to refer
to the Credit Agreement as amended hereby.

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<PAGE>

     IN WITNESS WHEREOF, the parties hereto, by their officers thereunto duly
authorized, have executed and delivered this Agreement as of the date first
above written.


                                 NEWPORT CORPORATION, as Borrower



                                 By /s/    ROBERT C. HEWITT
                                    ----------------------------------------
                                    Title: Vice President, CFO and Secretary

                                 ABN AMRO BANK N.V., as Lender



                                 By /s/    JOHN A. MILLER
                                    ----------------------------------------
                                           John A. Miller
                                    Title: Senior Vice President



                                 By /s/    MITSOO IRAVANI
                                    ----------------------------------------
                                           Mitsoo Iravani
                                    Title: Assistant Vice President

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<PAGE>

                                   EXHIBIT A

                              NEWPORT CORPORATION

                             REVOLVING CREDIT NOTE

$20,000,000                                                         May 31, 2000

     On the Termination Date, for value received, the undersigned, Newport
Corporation, a Nevada corporation (the "Company"), hereby promises to pay to the
order of ABN AMRO BANK N.V. (the "Bank") at its office at 300 South Grand
Avenue, Los Angeles, California, the principal sum of (i) TWENTY MILLION and
no/100 DOLLARS ($20,000,000), or (ii) such lesser amount as may at the time of
the maturity hereof, whether by acceleration or otherwise, be the aggregate
unpaid principal amount of all Loans owing from the Company to the Bank under
the Revolving Credit provided for in the Credit Agreement hereinafter mentioned.

     This Note evidences Loans made and to be made to the Company by the Bank
under the Revolving Credit provided for under that certain 3-Year Revolving
Credit Agreement, as amended, dated as of October 29, 1999 between the Company
and the Bank (said Credit Agreement, as the same may be amended, modified or
restated from time to time, being referred to herein as the "Credit Agreement"),
and the Company hereby promises to pay interest at the office described above on
such Loans evidenced hereby at the rates and at the times and in the manner
specified therefor in the Credit Agreement.

     Each Loan made under the Revolving Credit against this Note, any repayment
of principal hereon, the status of each such Loan from time to time as part of
the Domestic Rate Portion or a LIBOR Portion and, in the case of any Fixed Rate
Portion, the interest rate and Interest Period applicable thereto shall be
endorsed by the holder hereof on a schedule to this Note or recorded on the
books and records of the holder hereof (provided that such entries shall be
endorsed on a schedule to this Note prior to any negotiation hereof).  The
Company agrees that in any action or proceeding instituted to collect or enforce
collection of this Note, the entries endorsed on a schedule to this Note or
recorded on the books and records of the holder hereof shall be prima facie
evidence of the unpaid principal balance of this Note, the status of each Loan
from time to time as part of the Domestic Rate Portion or a LIBOR Portion and,
in the case of any Fixed Rate Portion, the interest rate and Interest Period
applicable thereto.

     This Note is issued by the Company under the terms and provisions of the
Credit Agreement and this Note and the holder hereof are entitled to all of the
benefits and security provided for thereby or referred to therein, to which
reference is hereby made for a statement thereof.  This Note may be declared to
be, or be and become, due prior to its expressed maturity, voluntary prepayments
may be made hereon, and certain prepayments are required to be made hereon, all
in the events, on the terms and with the effects provided in the Credit
Agreement.  All capitalized terms used herein without definition shall have the
same meanings herein as such terms are defined in the Credit Agreement.
<PAGE>

     The Company hereby promises to pay all costs and expenses (including
attorneys' fees) suffered or incurred by the holder hereof in collecting this
Note or enforcing any rights in any collateral therefor.  The Company hereby
waives presentment for payment and demand.  This Note shall be construed in
accordance with, and governed by, the internal laws of the State of California
without regard to principles of conflicts of laws.


                                 NEWPORT CORPORATION

                                 By: /s/ ROBERT C. HEWITT
                                    -----------------------------------
                                 Name: Robert C. Hewitt
                                      ---------------------------------
                                 Its:  Vice President and Chief
                                     ----------------------------------
                                       Financial Officer
                                     ----------------------------------


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