Diffusion of Light

One of the most sensitive and complex measurements on protective eyewear is that of the diffusion of light through the lens. Diffusion must be tested for all eyewear certified to EN 166:2001. It occurs when light passing through a transparent ocular (lens) is scattered and spreads in all directions.

In the optical components of safety eyewear, diffusion has to be minimised to give clear vision and contrast for fine detail work. Excessive diffusion in eyewear gives a ‘soft’ image. The standard for safety eyewear recognises that higher amounts of diffusion must be accepted for certain applications. For instance, a welding screen is permitted to diffuse light twice as much as a basic ocular, and protectors against high-speed particles are permitted to diffuse by 1.5 times the basic ocular.

Methods specified by standard

There are two methods of measuring diffusion specified in EN 167:2001 – ‘basic’ and simplified’.

SATRA uses the ‘simplified method’ as it is applicable to both ordinary and corrective oculars. The equipment uses a laser focussed to a tiny point of 0.1 mm diameter. The point of light is then expanded and re-focussed through the lens onto the sample ocular. The intention is to give a very even spot of incident light with a narrow waveband that will be scattered by diffusion as it passes through the sample.

The laser light coming through the sample ocular is collected by a lens and detector placed behind one of two masks (dark coloured plates incorporating clear portions to allow a controlled passage of light). The masks are positioned 400 mm behind the sample and can be swung sideways along with the detector if the ocular acts as a prism and bends the laser beam in addition to diffusing it. It is the ability to move the masks and detector that enables corrective eyewear to be tested.

Measurements have to account for lose of light due to transmission through the sample as well as its diffusion spread. To allow for that, light collected by the detector is first registered with a mask having a 10 mm diameter hole in place.

Light is measured with and without the sample in position. Any difference in the readings is due mainly to transmission losses. Any diffused light scattered outside of the 10 mm masked area is not detected. The readings are then repeated with a second mask having an annular ring cut from it. The clear ring has a 21 mm inner diameter and a 28 mm outer diameter. The ring falls outside of the main laser beam, which is focussed on the blanked central portion so that any light that the ring lets through must have been scattered by the sample. Measurements are again made with and without the sample ocular present to account for any ambient light that may be detected.

A calculation of diffusion is made using a formula incorporating the four light readings. The method has proved to be extremely sensitive and can even detect the differences in light spread through a flat acrylic plate from areas with varying internal stress patterns.

European legislation

Safety eyewear must be assessed and CE marked before it can be supplied into the European market. It has been illegal for many years to place an item of PPE on the market in a European member state unless it carries the CE mark. In Europe, two pieces of legislation are closely related in terms of safety eyewear:

Directive 89/656/EEC – the ‘use’ directive, covers the use of PPE at work. It requires employers to provide suitable PPE to employees where a particular hazard cannot be eliminated by other means.

Directive 89/686/EEC – the Personal Protective Equipment directive covers minimum health and safety performance criteria and procedures for PPE placed on the market within the European Economic area to ensure it is fit for intended purpose. It specifies in general terms what features should be addressed in the design of PPE and how it should be tested and certified. Products meeting the requirements of the PPE directive should be marked with the CE symbol.

Three categories of PPE are recognised by the PPE Directive and are based on the risk, consequences and severity of injury likely to occur to someone not wearing adequate PPE. Each category requires a different level of involvement by a Notified Body. Safety eyewear is considered to be ‘category 2’ or ‘intermediate’ category PPE and as such is subject to EC type examination by a Notified Body.

To support the PPE Directive, CEN – the European standardisation organisation – has been mandated by the European Commission to produce technical standards written specifically for safety eyewear products. These published EN standards describe in detail how a particular type of product should be tested and what performance is required to achieve a satisfactory pass. The tests developed for the various standards are designed to assess safety eyewear against the requirements of the PPE Directive for the risks of the particular activity for which they are intended to be used.

The European Commission reviews standards and, if suitable, they become officially ‘harmonised’ throughout Europe, and have a ‘presumption of conformity.’ This means that products that are fully covered by and meet the standard requirements are deemed to satisfy the directive’s general safety requirements.

Relevant standards

One of the most important general eyewear standards is EN166: 2002 Personal eye- protection – specifications which gives basic requirements for general safety eyewear. EN167:2002 – Personal eye- protection – optical tests and EN168:2002 Personal eye- protection- non optical test methods provide test methods for optical and non-optical properties.

Other standards address more specialist applications, such as welding protection or ski goggles. However, many of the tests required are also the same basic test methods called up in EN166, EN167 and EN 168, but with different requirements relevant to specific hazards.

EN166 specifies functional requirements for various types of personal eye protectors used against typical hazards found in industry, laboratories and educational establishments which are likely to impair vision or damage the eye. EN166 is also applicable to personal eye protectors fitted with prescription lenses. The standard does not cover nuclear radiation, X-rays, laser beams and low temperature infra-red radiation.

Testing

Optical tests detailed in EN167 ensure that any form of protective eyewear does not unacceptably distort or restrict the wearer’s vision and includes checking for spherical, astigmatic and prismatic refractive powers. Tests need to be highly sensitive to these properties of the oculars (lens components).

The laboratory equipment must be sensitive enough to detect minor distortions that an optician would not consider necessary to measure or correct. Calibration of this equipment relies on the use of special lenses specially ground for the purpose as they have longer focal lengths – they are “weaker” lenses – than any generally commercially available.

Other test methods include assessment of light diffusion (two different tests depending on whether the lens is vision corrected or not) and variations in luminance transmittance.

Non-optical tests covered in EN168 include field of vision to ensure that frames do not unacceptably impede peripheral vision. This parameter is checked using laser equipment that projects beams of light onto the test sample which is mounted onto a head form.

Physical property tests ensure that the eyewear provides the mechanical protection claimed and remains fit for use after normal wear and tear. A variety of properties can be tested depending on the hazards against which protection is offered. Some tests such as checking for the lateral protection provided by articles such as goggles are quite simple. The goggles are mounted on a standard head form and a 2mm. diameter probe is used to attempt to touch defined areas around the eyes. Other tests can be more complex.

Most safety eyewear must be tested for resistance to ignition using a heated probe at 650oC. A steel rod is heated to the required temperature and pressed onto all parts of the test sample except elastic headbands and textile edgings.

As an absolute minimum for robustness, oculars must withstand pressure from a 122mm. diameter steel ball loaded with 100N (approximately 10kg) without breakage or excessive deflection. Where increased robustness is needed, the steel ball is projected to impact the defined points on the frame and oculars under high and low temperature conditions.

Corrosion resistance tests involve immersing the sample in a solution of sodium chloride (salt) at both boiling point and room temperature. The sample is then dried off and checked visually 24 hours later for any signs of corrosion.

Other tests include assessing the usability of the eye protector after exposure to high temperature, low temperature and ultra- violet light originating from strong sunlight or welding arcs.

More specialist eyewear has been developed to filter out excessive light from, for instance, welding arcs. Other applications include laser protection. Laser goggles conforming to EN207 and EN 208 work by dissipating laser energy, blocking off a narrow range of wavelengths, so allowing the user some degree of visibility. Laser safety glasses should not only absorb laser light of a given wavelength, but should also be able to withstand a direct hit from the laser without breaking or melting and withstand the effects of a continuous wave laser for 10 seconds or 100 pulses for a pulsed laser.

Regardless of the type of product, safety eyewear should be ‘personal issue.’ Eye size, bridge size and temple length all vary and wearers should ensure that the product fits properly and is comfortable. Safety glasses should fit comfortably over the ears, with the frame as close to the face as possible and adequately supported by the bridge of the nose.

It is important to look after and maintain safety eyewear. Dirty lenses impair vision, causing eye fatigue which can lead to accidents. Safety glasses should be cleaned regularly, following the manufacturer’s instructions. Lenses should be treated carefully to avoid scratching – which not only impairs vision but can weaken lenses. Damaged or badly fitting eyewear should be replaced, this being particularly important for scratched lenses or face shields if they become crazed or brittle with age.

European legislation and standards place obligations on employers and manufacturers to provide safety eyewear that is fit for purpose and provides adequate protection against a variety of hazards.

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Published: 10th Apr 2007 in Health and Safety International