Laser eyewear using dyed Plastic lenses have by far become the most popular format over the last 20 years. The main driver has of course been cost however laser filters based on plastic lenses also have other advantageous features such as low weight and good resilience in case they are dropped from the surgical bed! The downside is the transmission properties are quite often not desirable due to the broad-band absorption nature of the dyes used which introduce colour and also provide lower power handling capabilities. Traditional glass filter lenses such as Schott & Hoya filter glasses still have the upper-hand on the basis of general colour balance, transmission and power handling especially for standard IR laser used in surgery such as Nd:YAG (1064nm) and Holmium (2100nm). They also have an advantage in that they can be coated with optical thin- film stacks to increase laser power handling or wavelength coverage.
Glass filters make eyewear heavy and cumbersome for long medical procedures
For doctors operating lasers for critical applications such as tissue ablation, incision and coagulation consideration should be given to optical performance such as lens clarity, light transmission and colour balance. Glass filters are generally chosen for these critical applications however they can often make eyewear heavy and cumbersome especially in the case of a long medical procedure.
The ideal solution to this issue is to provide laser coatings onto plastic lenses, this can provide lower weight filters along with higher optical performance in terms of transmission and colour balance. Another important advantage of laser coated lenses is that they can be supplied with a prescription so that the user no longer has to wear over-glasses on top of their own prescription glasses. This reduces problems with ghost images from multiple internal reflections and lens aberrations and so improves visual clarity during these critical operations. However depositing thin, hard dielectric layers onto plastic lenses has always been a challenge and it is only in the last 10-15 years that standard AR coating for Ophthalmic lenses has become reliable on most plastics. Most Ophthalmic coatings use 6-8 thin-film layers to create the anti-reflection (AR) properties on the other hand laser blocking coatings often need in excess of 50-60 layers deposited to nanometre accuracy. There is a limit in processing temperatures used during coating due to the plastic substrate and so these large thin film stacks can be soft and have low laser damage thresholds without use of more advanced coating technology such as Ion Assisted Deposition or Plasma Ion Assisted Deposition.
Fig 1. A high powered Argon/Oxygen plasma source on left hand side and electron beam evaporation of metals on the right hand side. The pure materials are evaporated into a carefully controlled oxygen plasma before depositing on the lens surface as dielectric layers with nanometre accuracy.
There are a small number of products utilising coated plastics for Laser protection available generally in specialist medical areas such as Nd:YAG and Holmium laser surgery. However currently there is a limit to the number of layers which can be deposited and subsequently the performance in terms of power handling and laser blocking (Optical Density) due to the large levels of stress incorporated into the films during processing. Without careful management of materials and intrinsic film stress these types of coatings will always be prone to failure in standard use.
Fig 2. A coated plastic lens from Laser protection eyewear for Holmium medical lasers used in Urological surgery after standard post-surgical cleaning. Intrinsic stress within the deposited layer stacks causes complete coating failure with the introduction of just a small surface defect.
Advanced Plastic Laser Protection Lens technology
Brinell Vision has developed a laser coated plastic lens which surpasses standard thermal shock, abrasion tests and long term environmental exposure. The solution has been made by stripping all the production steps right back down from selection of the plastic pellets used in the lens molding process, advanced lens surface treatments to development of the proprietary coating materials and deposition method.
The Brinell Laser lens is molded and treated using a blend of specifically chosen materials to provide the capability to accept large stacks of dense dielectric thin films. Core to the deposition technology is the high energy plasma and custom designed advanced computer controlled tooling system which allows us to deposit the optical layers onto curved surfaces with nanometre accuracy. The deposition parameters have been optimised for limiting stress while effectively hardening the surface of the plastic. It is now possible to create plastic lenses with narrow notch filters in the visible for blue and green lasers using over 100 individual optical layers to create deep blocking capabilities. This development is a significant move forward in eyewear for medical laser surgery as the product combines the advantage of low weight plastic with the optical properties of high grade glass laser filters.
Fig 3. Shows Brinell Vision custom designed prescription eyewear for blocking green and NIR lasers. The base6 curved lens substrates are coated with over 100 individual dielectric thin films to provide maximum visible transmission and high levels of blocking for green (532nm) and NIR (1064 & 2100nm).
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