Scratch Resistance
Applying a varnish to the surface of BBGR lenses calls for a series of meticulous workshop procedures to guarantee their quality.
The main stages in applying a varnish
Each substrate-varnish combination demands an individual preparation process. First of all, the lenses are thoroughly cleaned by workshop operators. They are then subjected to chemical coating in a machine. The cycle continues by applying the varnish, followed by a pre-polymerisation process, which dries the varnish sufficiently to check its cosmetic quality.
Workshop processes
BBGR has a department specialised in varnish coatings at its Provins plant. These varnishes feature qualities which enable them to resist abrasion and wear over a period of time while retaining their impeccable appearance.
The final stage
If their appearance is perfect, the lenses are sent to the baking ovens where the varnish undergoes a total polymerisation process, during which it acquires its definitive scratch-resistant properties.
Anti-reflection
Vacuum coating is used to add an anti-reflection coating, such as BBGR's Neva Max, to organic and mineral lenses. This complex and delicate operation calls for a great deal of dexterity, since even the slightest touch of a finger can compromise final product performance.
Lens preparation before vacuum coating
Organic (plastic) lenses subjected to vacuum coating are always varnished. They must be kept scrupulously clean, otherwise their appearance and coating adherence will not meet the standards demanded by opticians.
Positioning the lenses on their bases
After the preparation phase, the lenses are set out on dome-shaped bases. Depending on the type of machine, the number of lenses that can be treated at the same time ranges from 90 to 150. One side of the lens is treated at a time. The operator then has to manually turn each lens for the second side to be treated. Handling the lenses is a very delicate operation, since the surfaces must not be touched by the operator.
Vacuum coating: threecoating phases for each surface
Pumping
Pressure inside the machine is lowered from 1,015 (average atmospheric pressure) to 0.00001 mbar.
Deposit
The antireflective coating is obtained by accumulating several thin layers of different materials with a purity rating of over 99.999%. The thickness of each layer is precisely measured in nanometres, an operation carried out by extremely accurate, computer-controlled equipment. The solid materials to be deposited are heated to over 2,500�C by an electron beam, causing them to vaporise before subsequently condensing on the lenses. This is known as the evaporation phase.
Opening the machine
Once the final layer has been deposited, atmospheric pressure must be restored inside the machine. This is not a simple operation, since it takes several minutes. The temperature must first be stabilised and the air allowed to enter slowly to avoid excessive movements of air which could dislodge the lenses. The lenses are then turned over so that the second surface can be treated.
Quality-control
Sample lenses are added to each production cycle for testing purposes. These lenses, positioned on different parts of the dome, are subsequently subjected to quality control tests.
Reflection measurement of each surface, using a spectrophotometer. Software then supplies residual reflection colorimetric characterisation data.
A series of destructive tests to check that the process has been successfully completed -corrosion resistance and adherence tests. All coated lenses are then checked for their appearance, i.e.: to ensure they have no cosmetic faults before being packaged.