Technique #1. Laser marking direct onto glass substrate
Simply this is marking directly on the glass without coatings or additives; without using any pre-mark surface preparation. From the manufacturing perspective this is the easiest to implement as it is simply using the heat from a laser beam to mark directly on the edge of the finished optical lens.
Method A. Using a CO2 laser to mark.
1. This ablates any coatings such as magnesium fluoride, silicon oxide, zinc sulphide…etc… on the edge of the glass and causes a controlled surface micro-fracturing directly on the glass at the lens edge. The result is often an opaque/white looking permanent mark with character widths typically between 200 micron to 300 micron (0.008 inch to 0.012 inch).
2. Using a CO2 offers the best dollar per watt value for laser processing. In some cases when laser marking of ultra thin optics or for those lenses used in harsh environment applications, this type of laser may create stress fractures around the heat affected zone.
Method B. Using a solid state 532nm (visible green) laser to mark.
1. This also ablates coatings but the 532nm wavelength puts less heat into the glass substrate reducing the micro-fracturing occurring by about 50%. The result is often an opaque looking permanent marked directly into the glass with character widths typically between 100 micron to 150 micron (0.004 inch to 0.006 inch).
2. Using a solid state 532nm is recommended when small spot size and a reduction of applied heat, (thereby, reducing micro-fracturing) is required.
Technique #2. Laser marking by laser bonding additives to the glass substrate
This technique uses a commercially available ceramic or glass fritz bonding agent such as TherMark or Cermark (www.cermarksales.com) that is applied before laser marking. Typically this technique is used with a lower powered CO2 laser which uses the heat of the laser not to mark directly into the glass substrate but rather to bond the ceramic fritz onto the edge surface of the glass substrate.
Laser bonding offers a solution for creating permanent, high contrast, high resolution marks on a wide variety of surfaces. There is a limited selection of bonding colors such as black, blue, red yellow and white.
Laser bonding offers the least possibility of micro fracturing of the glass substrate but requires the greatest pre-mark preparation and post-mark cleanup.
The decision to mark directly into the glass lens or using a bonding additive depends on the cosmetic look you are wanting (e.g. high contrast colors) to achieve and the end application of the optical lens. For instance lenses used in high vibration areas or in space applications where the lens is exposed to pressure and rapid temperature changes should be considered candidates for laser bonding,
The draw back to laser bonding and using the required bonding additives is the amount of manual preparation and cleanup labor needed on each individual lens.