Exhibit 99.1

Use of Non-GAAP Financial Measures

The non-GAAP financial measures used in this exhibit are non-GAAP gross profit, gross profit margin, income from operations, operating margin, income from continuing operations before income taxes, income from continuing operations, net of tax, diluted earnings per share from continuing operations and Adjusted EBITDA.

The Company believes that the non-GAAP financial measures provide useful and supplementary information to investors regarding the Company’s quarterly performance. It is Management’s belief that these non-GAAP financial measures are particularly useful to investors because of the significant changes that have occurred outside of the Company’s day-to-day business in accordance with the execution of the Company’s strategy. This strategy includes streamlining the Company’s existing operations through site and functional consolidations, strategic divestitures, expanding the Company’s business through significant internal investments, and broadening the Company’s product and service offerings through acquisition of innovative and complementary technologies and solutions. The financial impact of certain elements of these activities, particularly acquisitions, divestitures, and site and functional restructurings, are often large relative to the Company’s overall financial performance, which can adversely affect the comparability of its operating results and investors’ ability to analyze the business from period to period.

The Company’s Adjusted EBITDA, a non-GAAP financial measure, is used by management to evaluate operating performance, communicate financial results to the Board of Directors, benchmark results against historical performance and the performance of peers, and evaluate investment opportunities including acquisitions and divestitures. In addition, Adjusted EBITDA is used to determine bonus payments for senior management and employees. Accordingly, the Company believes that this non-GAAP measure provides greater transparency and insight into management’s method of analysis.

Non-GAAP financial measures should not be considered as substitutes for, or superior to, measures of financial performance prepared in accordance with GAAP. They exclude charges that have a material effect on the Company’s reported results and, therefore, should not be relied upon as the sole financial measures to evaluate the Company’s financial results. The non-GAAP financial measures are meant to supplement, and to be viewed in conjunction with, GAAP financial measures. Investors are encouraged to review the reconciliation of these non-GAAP financial measures to their most directly comparable GAAP financial measures as provided in the tables in this exhibit.

GSI GROUP INC.

Sales, Gross Profit and Operating Income by Reportable Segment

(UNAUDITED)

The following table sets forth the external sales, gross profit, gross profit margin and operating income for each of our reportable segments for the periods noted (dollars in thousands):

GSI GROUP INC.

Consolidated Statements of Operations

(In thousands of U.S. dollars or shares, except per share amounts)

(UNAUDITED)

GSI GROUP INC.

Reconciliation of GAAP to Non-GAAP Financial Measures

(In thousands of U.S. dollars, except per share amounts)

(UNAUDITED)

Adjusted EPS (Non-GAAP):

Non-GAAP Gross Profit and Gross Profit Margin

The calculation of non-GAAP gross profit and gross profit margin is displayed in the tables above. Non-GAAP gross profit and gross profit margin exclude the amortization of acquired intangible assets and acquisition fair value adjustments from business acquisitions because: (1) the amounts are non-cash; (2) the Company cannot influence the timing and amount of future expense recognition; and (3) excluding such expenses provides investors and management better visibility into the components of operating expenses.

Non-GAAP Income from Operations and Operating Margin

The calculation of non-GAAP income from operations and operating margin is displayed in the tables above. Non-GAAP income from operations and operating margin exclude the amortization of acquired intangible assets and acquisition fair value adjustments related to business acquisitions because: (1) the amounts are non-cash; (2) the Company cannot influence the timing and amount of future expense recognition; and (3) excluding such expenses provides investors and management better visibility into the components of operating expenses. The Company also excluded restructuring costs and other, and acquisition related costs from non-GAAP income from operations and operating margin due to the significant changes that have occurred outside of the Company’s day-to-day business in accordance with the execution of the Company’s strategy for the reasons described above in the introductory paragraphs of the “Use of Non-GAAP Financial Measures”.

Non-GAAP Income from Continuing Operations before Income Taxes

The calculation of non-GAAP income from continuing operations before income taxes is displayed in the tables above. The calculation of non-GAAP income from continuing operations before income taxes excludes amortization of acquired intangible assets and acquisition fair value adjustments related to business acquisitions, restructuring costs and other, and acquisition related costs for the reasons described for non-GAAP income from operations and operating margin above.

Non-GAAP Income from Continuing Operations, Net of Tax

The calculation of non-GAAP income from continuing operations, net of tax, is displayed in the tables above. Because pre-tax income is included in determining income from continuing operations, net of tax, the calculation of non-GAAP income from continuing operations, net of tax, also excludes amortization of acquired intangible assets and acquisition fair value adjustments related to business acquisitions, restructuring and other, and acquisition related costs for the reasons described for non-GAAP income from operations and operating margin above. In addition, the Company excluded significant non-recurring income tax expenses related to releases of valuation allowances, recognition of previously unrecognized income tax benefits due to expiration of statute of limitations, effects of changes in tax laws, effects of acquisition related tax planning actions on our effective tax rate, and the income tax effect of non-GAAP adjustments discussed above.

Non-GAAP Diluted EPS from Continuing Operations

The calculation of non-GAAP diluted EPS from continuing operations is displayed in the above tables. Because income from continuing operations, net of tax, is included in the diluted EPS calculation, the calculation of non-GAAP diluted EPS from continuing operations excludes amortization of acquired intangible assets and acquisition fair value adjustments related to business acquisitions, restructuring costs and other, acquisition related costs, significant non-recurring income tax expenses related to releases of valuation allowances, recognition of previously unrecognized income tax benefits due to expiration of statute of limitations, effects of changes in tax laws, effects of acquisition related tax planning actions on our effective tax rate, and the income tax effect of non-GAAP adjustments for the reasons described for non-GAAP income from operations, net of tax.

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