We achieved record revenue, 75.2% greater than last year, driven by organic growth of 11.9% and the remaining 62.3% of sales increase contributed by Teledyne FLIR.
Revenue increased organically in every major business group but was especially strong in our commercial imaging and electronic test and measurement instrumentation businesses where organic growth for each was greater than 20% in the quarter.
Furthermore, orders exceeded sales for the fourth consecutive quarter with the third quarter book-to-bill of 1.1 GAAP earnings per share of $2.81 increased 13.3% compared to last year, and was $0.03 less than our record GAAP third quarter earnings achieved in 2019.
However, excluding acquisition-related charges, earnings were $4.34 per share in the third quarter, an increase of 61.9% on a comparable basis from 2020.
Cash flow was a third quarter record allowing repayment of $300 million of debt while our leverage ratio declined to 3.3 from 3.7 at the end of the second quarter.
We continue to accelerate the pace of plant synergies and currently expect to achieve our annualized cost saving target of $80 million before the middle of 2022 as opposed to the end of 2022 as we described in our July earnings call, and compared with 2024 as noted when we announced the transaction in January of 2021.
On a full year basis, we now think a reasonable outlook for organic sales growth in 2021 is approximately 7% to 7.5%, led by forecasted growth of almost 13% in digital imaging, which excludes Teledyne FLIR.
This translates to total sales of $4.59 billion with contribution of $2.4 billion from digital imaging, including FLIR.
In our Digital Imaging segment, third quarter sales increased 217.3% largely due to the FLIR acquisition but organic growth in our combined commercial and government imaging businesses was also very strong at 17.9%.
GAAP segment operating margin was 12.5%, but adjusted for transaction costs and purchase accounting segment margin was 23.9%.
In our Instrumentation segment overall quarter sales increased 9% versus last year.
Sales of test and electronic test and measurement systems, which includes oscilloscopes and protocol analyzers were exceptionally strong and increased 20.8% year-over-year to record levels.
Sales of environmental instruments increased 7.6% from last year with sales related to human health and safety market such as drug discovery and gas and flame detection being strongest in the quarter.
Sales of marine instrumentation increased 3.2% in the quarter.
In addition, orders were the strongest in the last six quarters with a quarter book-to-bill of 1.13.
Overall, Instrumentation segment operating profit increased 24.3% with segment operating margin increasing 270 basis points or 247 basis points, excluding intangible asset amortization.
In the Aerospace and Defense Electronics segment, third quarter sales increased 11.7% driven by 8.4% growth in defense, space, and industrial sales combined with a 27% increase in sales of commercial aerospace products versus last year's pandemic-related tough quarter.
GAAP operating profit increased 34.5% with margin 375 basis points greater than last year.
Finally in the Engineered System segment, third quarter revenue increased 1.4% but operating profit and margin declined slightly since we exited the higher margin turbine engine business earlier this year.
For several years, we've been on a journey to move our overall operating margin from the low-teens to over 20%.
Over the last 2.5 years, we made tremendous progress with it -- with stand -- notwithstanding the pandemic and the recent supply chain and inflationary pressures.
To date, the approximate $1 increase in our earnings outlook is primarily the result of further improvement in our full-year 2021 forecasted operating margin which excluding acquisition-related charges is 100 basis points better at approximately 21% from our 20% forecast in July.
In the third quarter, cash flow from operating activities was $192.8 million including all acquisition-related costs.
Excluding acquisition-related cash costs, net of tax, cash from operations was $194.9 million compared with cash flow of $150.3 million for the same period of 2020.
Free cash flow that is cash from operating activities, less capital expenditures excluding acquisition-related costs was $165.7 million in the third quarter of 2021 compared with $135.1 million in 2020.
Capital expenditures were $29.2 million in the third quarter compared to $15.2 million for the same period of 2020.
Depreciation and amortization expense was $90.2 million for the third quarter of 2021 compared to $29.2 million in 2020.
In addition, non-cash inventory step-up expense for the third quarter of 2021 was $35.2 million.
We ended the quarter with approximately $3.89 billion of net debt that is approximately $4.44 billion of debt less cash of $551.8 million.
Stock option compensation expense was $5.8 million for the third quarter of 2021 compared to $5.7 million for the same period of 2020.
Resulting from the FLIR acquisition, restricted stock unit expense for FLIR employees was $1.8 million in the third quarter of 2021.
Management currently believes that GAAP earnings per share in the fourth quarter of 2021 will be in the range of $2.53 to $2.69 per share, with non-GAAP earnings in the range of $4.07 to $4.17.
And for the full year 2021, our GAAP earnings per share outlook is $9.13 to $9.29 and on a non-GAAP basis $16.35 to $16.45 compared with our prior outlook of $15.25 to $15.50.
The 2021 full year estimated tax rate, excluding discrete items is expected to be 23.9%.

Summary:
We achieved record revenue, 75.2% greater than last year, driven by organic growth of 11.9% and the remaining 62.3% of sales increase contributed by Teledyne FLIR.
Furthermore, orders exceeded sales for the fourth consecutive quarter with the third quarter book-to-bill of 1.1 GAAP earnings per share of $2.81 increased 13.3% compared to last year, and was $0.03 less than our record GAAP third quarter earnings achieved in 2019.
However, excluding acquisition-related charges, earnings were $4.34 per share in the third quarter, an increase of 61.9% on a comparable basis from 2020.
Management currently believes that GAAP earnings per share in the fourth quarter of 2021 will be in the range of $2.53 to $2.69 per share, with non-GAAP earnings in the range of $4.07 to $4.17.
And for the full year 2021, our GAAP earnings per share outlook is $9.13 to $9.29 and on a non-GAAP basis $16.35 to $16.45 compared with our prior outlook of $15.25 to $15.50.