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Medicine and Health Title

An end to misty eyes

   
 
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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.

Reducing fogging

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.

Perspiring PETE

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 of evaporation.
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.

Coordinated testing

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 5C, 10C and 15C above the temperature of the air. As expected, the greater the temperature difference the more rapidly fogging set in.
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.

 

 

Project Title:  
Development of an objective method for assessing the fogging of complete eye protectors

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: MAT1-CT94-0042

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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