Sunday, April 13, 2008

Photochromic glasses dangerously reduce night vision

A worrying finding from investigation into a UK shipping accident is that photochromic glasses block so much light that they should not be used by ship's lookouts. The report on the loss of the yacht Ouzo and its crew of three, found that the lookout on the ship Pride of Bilbao, which collided with it, was wearing "reactolite" (photochromic or photoctomatic) prescription spectacles. These darken in reaction to ultra violet (UV) in daylight. At night these appear clear, but actually block 20% of the light (ordinary coated lenses only block 0.6% of the light). Perhaps there should be clearer warnings against the use of these lenses for other night activities, such as driving a car, or flying an aircraft.
The seaman lookout on board Pride of Bilbao at the time of the incident was 60 years of age. He had worked on board the vessel for 10 months and had sailed previously on board similar vessels for many years. He was, therefore, an experienced lookout.

He had a valid ENG 1 certificate of health, which includes a requirement for regular eyesight tests.

His eyes had been tested privately in 2005, after which he was prescribed glasses to adjust his slight short-sighted vision. As a consequence, he purchased a pair of reactolite, or photochromic lensed glasses, that he could wear both during the day and at night because they darkened only in reaction to daylight or ultra violet (UV) light.

Following the accident, the MAIB had the lookout’s eyes examined once again and his prescription was found to be still correct. His eyes were also tested for other defects or anomalies that might have affected his vision or night time adaptation, but none were found.

2.5.3 The seaman lookout’s glasses

... The lookout’s photochromic glasses were sent to University College London’s Institute of Ophthalmology to assess whether they might have had an adverse effect upon his night vision.

The glasses were examined and a report was prepared (Annex 1), which concluded that the optical transmission of the lenses was no more than 80% efficient and, taking into account all of the other known factors, was probably less at the time of the accident. This compares to 94.7% and 99.4% optical transmittance of ordinary uncoated and coated lenses, respectively. This was a startling result as the consequences of such a reduction in night vision had not been fully appreciated by opticians and ophthalmologists before the investigation of this accident.

The report also stated that it would be correct to assume that a uniform reduction in brightness due to the optical density of the lenses would decrease the likelihood that a subject would detect the lights of shipping vessels.

It appears, therefore, that the lookout’s glasses would have been a contributory factor when considering why Ouzo’s lights were not seen earlier. However, there are no rules or guidelines concerning the wearing of such glasses on the bridge of a vessel at night.

This incident has raised a serious concern that glasses fitted with photochromic lenses are inappropriate for use by lookouts on the bridge of merchant vessels. It also raises the question of applicability of use by operators in other modes of transport.

The MAIB also requested the Institute of Ophthalmology to test lenses from the major tinted photochromic lens manufacturers to determine whether the concerns raised in the initial report regarding the lookout’s glasses were widespread, and not just applicable to that particular pair or manufacturer (see Annex 2). The report concluded that all of the photochromic lenses tested showed significant reductions in the amount of transmitted light.

However the lenses of the glasses supplied for test by the MAIB were significantly inferior to the other currently commercially available lenses indicating that either manufacturers have improved the performance of their photochromic materials, or that the performance of photochromic glasses is reduced with time. As at least one manufacturer only guarantees the performance of lenses for 2 years, the latter reason may be the most likely.

This is obviously an additional concern regarding photochromic lens glasses, however it is outside the scope of this investigation. ...

From: Report on the investigation of the loss of the sailing yacht Ouzo and her three crew South of the Isle of Wight during the night of 20/21 August 2006, Report No 7/2007, Marine Accident Investigation Branch, United Kingdom, April 2007

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