29 April 2015

Advances in lightweight eyewear

Laser eyewear using dyed Plastic lenses have become by far the most popular format over the last 20 years. The main driver has been the lower cost however laser filters based on plastic lenses also offer low weight and good resilience in the event that they are dropped from the Lab bench or operating table! The downside is that the visible transmission can be poor, with strong colour filtering due to the characteristic broad-band absorption of the dyes used. A colour balanced view can be critical when working with various types of lighting scenarios and for higher power laser applications and critical procedures like medical laser surgery or flying an aircraft and physical damage to plastics needs to be considered carefully.

Traditional glass filter lenses such as those by Schott & Hoya still have the upper-hand in terms of general colour balance, transmission and power handling especially for standard IR lasers. They also have an advantage in that they can be coated with large stacks of optical thin films to increase laser power handling and/or wavelength coverage. For some critical applications, optical performance considerations (e.g. lens clarity, light transmission and colour balance) do indeed lead to glass filters often being generally chosen but heavy and cumbersome eyewear can be a significant consideration if it is to be worn for long period of time.

The ideal solution is to provide laser coatings on plastic lenses, providing 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 critical operations.

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. Most Ophthalmic coatings use only 6-8 thin-film layers to create the anti-reflection (AR) properties whereas for laser-blocking coatings in excess of 60 layers deposited to nanometre accuracy is often required. Also when coating plastics there is a relatively low limit to processing temperature and large thin film stacks can be unstable and have low laser damage thresholds. The introduction of more energetic advanced coating technology such as Ion Assisted Deposition or Plasma Ion Assisted Deposition (see figure) improves the film quality and the inherent film stresses.

There are a small number of protective eyewear products available that utilise coated plastics for laser protection, for applications mainly in the medical and defence sectors but without careful management of materials and intrinsic film stress these types of coatings are prone to failure in standard use.

Brinell Vision has developed a laser coated plastic lens that surpasses standard thermal shock, abrasion tests and long term environmental exposure. The lens format is already being utilised in other critical applications such as pilot protection from laser strikes. The solution has been made by stripping all the production steps right back down from the careful selection of the plastic pellets used in the lens moulding process, advanced lens surface treatments to development of the proprietary coating materials and deposition method.

The Brinell Laser lens is moulded 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 use of high energy plasma deposition 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.

Image: 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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