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Ahrensburg, 01 November 2005 - Whether DVD-R can store and play back data depends on a number of factors. These include groove filling, groove geometry, and optical density, as well as birefringence, radial and tangential tilt, diffraction, and substrate thickness. Obviously, disc manufacturers prefer all measurements to be performed by one recordable offline tester. To fulfill this requirement most offline equipment suppliers use green or blue lasers in order to be capable of delivering valid groove filling and groove geometry data. The Basler REFERENCE proves that an intelligent measurement setup avoids costly additional green or blue lasers while it continues to fully rely on the mature and proven red laser technology.

The major challenge with DVD-R: capturing the 2nd order of diffraction

The major challenge for all physical offline DVD-R testers is getting a full-scale groove filling and groove geometry measurement integrated. For both parameters capturing the second order of diffraction is a vital precondition, otherwise no definite statements on groove geometry and groove filling can be made. With 0.6 mm substrates and the specific DVD-R track pitch, however, the commonly used red-laser setups, operating at 7°, do not produce a 2nd order signal. When comparing different offline testers that are currently out in the field you’ll find different ways out of this dilemma. Some inspection equipment suppliers decided to resort to separate offline testers for evaluating groove filling and geometry – of course a rather costly alternative. Others simply define and enter the groove width via software, assuming instead of actually measuring this parameter – not a safe approach to control this parameter. Yet another alternative is the integration of green or blue lasers. However, especially blue lasers are highly costly and have not yet reached full market maturity.

The Basler offline tester went a new way while staying with a familiar and proven technology. The special optical setup of the Basler REFERENCE allows to decouple a portion of the original red laser beam (635 nm). The capability of measuring the light intensity of the second order is derived from the different angle of incidence achieved with the decoupled beam – a flat 60° angle. With this setup the Basler REFERENCE achieves a resolution of 1 nm for groove width and depth after dye coating and sputtering (Wabs and Dabs).

Correlation tests with AFM prove: red is as accurate as blue

Correlation measurements with AFM prove the red laser approach right. In a thorough test procedure, blue and red laser-based offline test results have been compared to AFM measurements. The outcome: Both red and blue show a correlation near 90%, with blue lasers offering no significant gain in accuracy over red.
While the measurement accuracy is the same, prices differ widely between red and blue. Currently, blue lasers are about ten times more expensive than red. At the same time, they have not yet reached full market maturity with regard to lifetime. Presently, blue lasers must be replaced after less than a year compared to a lifetime of several years for red lasers. Another matter is the robustness of the optical setup - experience has shown that lenses in use with blue lasers show clearly reduced durability.
“With the high price pressure that manufacturers are facing in recordable business today, inspection equipment manufacturers must come up with solutions that are both highly accurate and cost-efficient”, explains Marko Grönwoldt, product manager for offline testing. “With our all-in-one tester for recordables we have proven that it’s possible to fulfill this requirement by consistently exploiting the potential of the existing red laser technology.”


For further information, please contact:

Dorothea Brandes
Market Communications
Basler Vision Technologies
Optical Media Inspection
An der Strusbek 60-62
22926 Ahrensburg Germany
Fon +49 4102 463-551
Fax +49 4102 463-309
e-mail