Laboratories in four countries have
designed and evaluated an objective method for testing the resistance
to fogging of industrial eye protectors. A sample eye protector is
attached to a model human head equipped to simulate perspiration.
A TV camera in one eye views a target of closely-spaced stripes. As
condensation forms on the lens of the eye protector, the visibility
of the stripes falls. The times for the visibility to fall to 75%
and 50% are recorded and used to derive a 'fogging resistance'. The
method is expected to form part of a future European Standard.
Virtually all industrial injuries to the
eye are avoidable if workers wear suitable eye protection, such
as goggles or safety spectacles. Yet despite the greater awareness
of health and safety issues at work, eye protection is often unpopular
with the people it is designed to safeguard. Eye protectors can
be hot and sticky, and rapid fogging of the lenses due to condensation
is a safety hazard in itself.
Research in the UK in the 1980s showed that fogging is the main
reason why workers do not wear eye protectors in situations where
they are desirable.
Internal fogging is caused by water vapour evaporated from warm
skin enclosed by the eye protector condensing on the cooler lenses.
The moisture forms a mist of small droplets on the lens which impairs
vision. As condensation proceeds, the small droplets merge to make
bigger droplets, reducing misting but causing refractive distortions.
There are several methods which manufacturers of eye protectors
use in attempts to reduce fogging. The simplest and cheapest is
to include ventilation holes or slots in the frame of the protector
to allow the moist air inside to be replaced with dryer air from
outside. Fogging may also be reduced by special 'anti-mist' coatings
on the lenses. Hydrophilic coatings attract water and cause it to
condense in an even film of moisture, rather than small droplets.
Being of even thickness, the film is transparent and does not impair
vision until further condensation distorts it. Hydrophobic coatings,
in contrast, repel water and delay condensation. When condensation
finally occurs, it does so in large drops, which are easily shed
from the lens.
Until now there has been no objective means of testing eye protectors
for their resistance to fogging. As a result, little guidance is
available on the best type for use in different circumstances. In
an attempt to establish an agreed method for such testing, national
health and safety institutes and manufacturers of safety equipment
in the UK, France, Spain and Finland have come together under the
EU's Standards, Measurement and Testing programme. This project
aims to develop a draft European Standard for testing and classifying
eye protectors according to resistance to internal fogging.
The first stage of the project, to devise a suitable test method,
was carried out by the UK Health and Safety Laboratory (HSL) in
Sheffield. Earlier work by HSL had already established that fogging
depends on the construction of the complete eye protector, not just
the optical surfaces.
The apparatus was based on a life-size model 'head', known as PETE
(Protective Equipment Test Effigy), which was already specified
in EN 168, a European Standard for eye protectors. The head was
modified to simulate perspiration by fitting it with heaters and
covering the face in felt which was kept moist by controlled injection
of water. Eye protectors were fitted to the head in exactly the
same way as to a human head. It was then placed in a duct through
which air flowed at a controlled speed to achieve a constant rate
HSL evaluated ten methods for measuring the fogging of the lenses
of the eye protectors. Some were direct, in the form of optical
measurements, while others were indirect, monitoring physical characteristics
associated with fogging, such as temperature and humidity.
In the chosen method, a miniature camera is placed in one of the
eye sockets of the head, recording the view through the eye protector.
The camera is focused on a target consisting of a number of black
and white stripes of various spacings. The camera scans the target
producing a video signal containing a large number of pulses of
a definite frequency. As the eye protector starts to mist, the stripes
become blurred and the frequency of pulses drops. A drop to 75%
corresponds to noticeable fogging and 50% is judged to be the point
at which the protector becomes unusable.
The method is not suitable for full-face protectors, because they
suffer fogging by condensation of moisture in exhaled breath as
well as moisture from the skin.
In the second phase of the project, HSL supervised the construction
of identical test equipment by the participating laboratories. They
also obtained 300 samples of commercially available eye protectors
of various types, and supplied each laboratory with five samples
of each of ten types.
Each laboratory then conducted a series of tests on the samples,
to determine the times at which visibility fell to 75% and 50% of
the starting value. Each sample was tested with the head at temperatures
of 5°C, 10°C and 15°C above the temperature of the air. As expected,
the greater the temperature difference the more rapidly fogging
The results showed that the behaviour of the protectors varies considerably
according to their design. Uncoated lenses become fogged and stay
fogged, while some coated lenses clear after an initial period of
fogging. The more fog-resistant eye protectors stayed clear for
more than 15 minutes, the maximum duration of the test.
Less expensive eye protectors, ventilated by perforations in the
frame, showed considerable variability between samples both because
the holes were not of consistent size and because of the difficulty
of forming a good seal to the face.
By statistical analysis of the results, each type of protector was
assigned a 'fogging resistance' (FR) which is the fogging time in
seconds which will be exceeded by 95% of samples.
HSL have conducted further tests in which a human subject wears
the eye protectors while walking on a treadmill. They show generally
good agreement with the objective tests.
The partners hope that the results of their work will be used by
manufacturers to make a new generation of eye protectors which are
more resistant to fogging.
The participants are now refining the test procedure before submitting
it to the European Committee for Standardisation (CEN) for consideration
as a future European Standard. The FR values could be used to classify
eye protectors on a simple pass/fail basis, or, more usefully, could
provide three or more grades of fogging resistance according to
performance at different temperatures. This will help users take
account of the large climatic differences between northern and southern
Europe - for example, a simple ventilated protector that performs
well in Greece may fog rapidly in Finland.