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).
For more information contact:
info@brinellvision.com
www.brinellvision.com
