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Protective Eyewear

Published: 10th Jan 2009

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Preventing optical damage through welding

Several forms of eyewear exist for protection of the eye. Sunglasses are frequently used to reduce the amount of solar radiation reaching the eye. The main casual use of sunglasses is to reduce glare by decreasing the luminance of visible radiation reaching the eye. Sunglasses also attenuate ultraviolet (UV) radiation, but the degree of attenuation is not apparent by visual inspection of the lenses. Several countries have standards specifying the classification of sunglasses for the general use according to their UV transmittance.

In Europe, a specific standard also exists for sunglare filters used in the workplace. The design of sunglasses is important; “wrap-around” glasses that fit close to the eyes providing better protection than more open designs. For exposure to artificial UV radiation sources, greater levels of protection may be needed in the form of face-shields, e.g. during electric arc welding.

Optical radiation ranges

Optical radiation includes ultraviolet (UV) radiation, visible light and infrared (IR) radiation. Optical radiation belongs to the part of electromagnetic radiation that is not capable of producing ionisation, hence called non-ionising radiation (NIR).

Wavelength ranges of optical radiation
UV-C: 100 - 280 nm
dUV-B: 280 - 315 nm
UV-A: 315 - 400 nm
Light: 400 - 800 nm
IR-A: 800 - 1400 nm
IR-B: 1.4 - 3 μm
IR-C: 3 μm - 1 mm

Adverse effects of optical radiation on the eye

Optical radiation is absorbed by the external structures of the human body. Depending on radiation wavelength, the penetration depth into the skin ranges from a fraction of millimetre to a few millimetres. The eye is an exception. Its structures, the cornea, the aqueous humour, the lens and the vitreous humour are transparent so that the visible light and the near infrared radiation (IR-A) are transmitted to the retina where they are absorbed. The ocular media can be considered as a set of filters, each one absorbing in a particular spectral region and thus preventing the transmission of specific wavelengths to the following structure. The human cornea can transmit radiation with wavelengths as short as 300 nm, while the lens absorbs nearly all UV radiation with wavelengths shorter than about 380 nm.

Adverse biological effects on the eye vary with the spectral band of radiation. Human eye is well adapted to minimise the risk from overhead sources, like the sun, except in environments producing reflections such as a snow-field. Overexposure from viewing intense light sources is generally prevented by the aversion reflex and squinting. The cornea absorbs almost all incident radiation in the UV-C, UV-B, IR-B and IR-C spectral regions. The lens is the ocular structure absorbing most of the UV-A and in a lesser extent the IR-A.

It is important to stress that while the skin can actively react to UV exposure by a thickening of the horny layer and by delayed pigmentation, the eye has no known active defence capability. On the contrary, it may even become more sensitive with repeated exposures. The cornea and the lens are the two main targets at risk of UV radiation.

Effects of UV radiation on the cornea and conjunctiva

The primary acute effects of UV radiation on the cornea and on the conjunctiva are the photokeratitis and photo-conjunctivitis. Photokeratitis is the condition, often called “welder’s eye”, that almost all welders have experienced when they repeatedly ignite the arc prior to using the protective mask. The action spectrum for photokeratitis peaks at 270 nm, and the corresponding threshold value for the minimal damage is approximately 40 J m- 2 . Radiant exposures exceeding the threshold value, after a latency period ranging typically from 6 to 12 hours, may cause acute symptoms such as the sensation of “sand” on the eyes, and photophobia. Fortunately ocular pain, except in rare cases of very severe exposure, disappears in a couple of days.

Effects of UV radiation on the lens

Cataract is defined as opacity of the crystalline lens of the eye. Cataract is responsible for one half of cases of blindness worldwide. Recent epidemiological studies suggest that exposure to UV-B radiation is responsible for inducing or accelerating cataract.

Adverse ocular effects of visible radiation (“blue-light”)

Normally, the human eye is not exposed to prolonged bright visible radiation, since it is protected by the natural aversion response (blink reflex and head movement). However, in special situations, such as welding, the eyes can be exposed to high irradiances of bright light, and hence effective filters to protect the eyes are necessary.

Retinal injury from viewing bright light sources is of particular concern. The wavelength band of retinal risk extends from 385 to 700 nm. It is well known that direct observation of the solar disc during eclipses may result in the so-called “eclipse blindness” or solar retinitis. It is evident that retinal damage due to exposure to bright light sources, such as welding arcs, is mainly the result of oxidation processes. The action spectrum of photochemical damage to the retina peaks at 440 nm.

Adverse effects of IR radiation on the eyes

Ocular exposure to IR-A radiation (780 - 1400 nm) can cause ocular Hazards with substantial increase of temperature. This acute thermal damage requires, however, very high IR radiant exposures. Thermal Hazards from IR radiation appear primarily as opacities on crystalline lens associated with chronic exposure. This is commonly known as industrial heat cataract. High exposures to IR-radiation can also cause thermal injury of the cornea and conjunctiva (1400 nm - 1 mm). This happens almost exclusively during laser radiation exposure.

Safety guidelines for exposure to optical radiation

Exposure limits for UV radiation (180 - 400 nm)

The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has given guidelines for limiting exposure to UV radiation in the range of 180 - 400 nm. These limits apply both to the skin and the eyes. The biologically weighted irradiance UV radiation can be determined using the spectral weighting factors given in the guidelines. To evaluate the Hazards to the eyes and skin, the safe daily (8 h) exposure time can be calculated from the measured spectra using the following equation:

  • t UV = 30 J m- 2 /(ΣE(λ) S(λ) Δλ)

where

  • t UV = permitted exposure duration in seconds
  • E(λ) = measured spectral irradiance in W m- 2 nm- 1
  • S(λ) = relative UV spectral effectiveness (unitless)
  • Δλ = bandwidth in nanometers of the measurement intervals

Exposure limits for visible and infrared radiation

ICNIRP has also established specific guidelines for evaluation of the photochemical and the thermal Hazards to the eye. They are considered separately by adopting the “blue light” and “burn Hazard” functions, B(λ) and R(λ), respectively. The necessary spectral data include determination of the spectral irradiance E(λ) or spectral radiance L(λ) of the source.

Blue-light photochemical retinal Hazard (300-700 nm)

The guidelines recommended by ICNIRP to protect the eyes against photochemical injury require assessments of the blue-light weighted irradiance or radiance with the source size measured at the position of the eyes. For small sources subtending a plane angle less than 11 mrad, and for which the blue-light weighted irradiance EB exceeds

1 mW cm- 2 , the permissible exposure time t B (in seconds) is calculated by the following equation

  • t B = 10 mJ cm- 2 / E B

For extended sources, for which the blue-light weighted radiance L blue exceeds 100 W cm- 2 sr- 1 , the safe exposure time is

  • t B = 100 J cm- 2 sr- 1 /(ΣL(λ) B(λ) Δλ)

where

  • tB = permitted daily exposure in seconds
  • L(λ) = measured spectral radiance in W cm- 2 nm- 1 sr- 1
  • B(λ) = relative blue-light spectral effectiveness (unitless)
  • Δλ = bandwidth in nanometers of the measurement intervals

Retinal thermal Hazard (380 - 1400 nm)

For the assessment of the exposure limit for Hazardous radiance L Haz , the spectral weighting function R(λ) is used. The exposure limit for retinal thermal Hazard,

L Haz (in W cm- 2 sr- 1 ), is a function of the angular subtense α (in radians) of the source, and the exposure duration t (in seconds)

  • L Haz = 5/(α t 0.25 )

The weighted spectral radiance of the source must not exceed L Haz

  • ΣL(λ) R(λ) Δλ < L Haz

where

  • L(λ) = measured spectral radiance in W cm- 2 nm- 1 sr- 1
  • R(λ) = relative retinal thermal spectral effectiveness (unitless)
  • Δλ = bandwidth in nanometers of the measurement intervals

For t < 10 μs, the time-integrated radiance should not exceed L Haz for 10 μs. Similarly, for t > 10 s, the time-integrated radiance L Haz should not exceed the limit for 10 s.

Survey of optical properties of dual-filter welding face-shields

Electric welding processes emit remarkable amounts of optical radiation, and therefore welders have traditionally used opaque face-shields with a small dark filter in front of the eyes. Usually the welder has to knock down the face-shield by his neck at the ignition of the welding process. Use of these types of eye protectors can, however, cause photokeratitis if the motion is not rapid enough, so that the eyes are left unprotected repeatedly during a work day. This quick, repetitive motion may also cause severe chronic health problems of the neck. Some face-shields are also ergonomically uncomfortable and heavy to wear, and the visibility of the surrounding areas is very restricted.

In order to avoid the above mentioned problems, new types of welding face-shields have been developed. These contain a dual-shade eye protector with a dark filter to protect the eyes, and a light filter to protect the skin of the face. The dark filter is needed to attenuate bright visible radiation, ”blue light”, which can cause Hazards on the retina of the eye. For eliminating UV-radiation, a highly attenuative filter is also necessary, but it need not be dark. Visible light is harmful for the eyes but harmless for the face, and hence dark filters are needed only against ocular exposure.

In Europe, the basic requirements for eye and face protectors have been given by the Directive 89/686/EEC on Personal Protective Equipment (PPE). Specifically for eye and face-shields for welders, EN 169:1992 includes the transmittance requirements for welding filters. The standard restricts, however, utilizing of dual filter masks by the requirement that the difference in scale numbers between the light and dark zones must not be more than 5. According to EN 169, scale numbers of welding filters to be used for typical arc welding processes are 11 - 13. For dual shade filters, it means that the scale number of the light zone cannot be less than 6, with the consequence that the luminous transmittance will be less than 1 %. Hence, the basic idea of good visibility will be lost. Because EN 169 gives no biological justification for the maximum difference of 5, a European project was formed to evaluate whether a higher difference between the scale numbers would be arguable on the basis of biophysical safety assessments.

Spectroradiometric measurements of welding spectra and exposure evaluation

In order to evaluate suitability and safety of various dual filter combinations, optical radiation emitted by different welding processes was measured spectrally from 210 to 800 nm at 5 nm steps. This data was used to assess safe exposure times for the unprotected eyes and face, and the same spectra were furthermore combined with the spectral transmittance values of various welding filters to determine safe exposure times when using various filters and dual filter combinations.

Welding equipment and parameters

The welding machines used were ESS Pulsarc 550 for MIG/MAG, ESS Squerearc 306 for TIG, and Elin SGE 315 for electrode welding. The arc currents ranged from 50 to 467 A for MIG/MAG and TIG welding, and from 60 to 250 A for electrode welding.

An industrial robot, Reis Model RV6 with 6 axis of freedom, was used for MIG/MAG and TIG processes. Electrode welding was performed manually by a welder instructor from SZA. The shape of the base material was either a tube or a plate. The welding nozzle mounted to the robot arm can be seen on top of the tube. By rotating the tube and moving the welding nozzle in the axial direction of the tube it was possible to weld long uninterrupted sequences with only a minor change in the position of the arc.

Spectral measurements and exposure evaluation

A double monochromator spectroradiometer, Optronic Model 742, was used for measuring the optical spectra. The measurements were taken at 2.6 m from the welding arc, but for manual welding processes the working distance was estimated to be 0.5 m. Therefore, to be able to evaluate the exposure Hazards at the working distance, the measured spectral irradiance were multiplied with a correction factor to mathematically transfer the spectral data from the measurement distance (r = 2.6 m) to the normal working distance (r = 0.5 m).

Safe exposure times to UV radiation and blue-light were determined for the unprotected eye and skin using the Hazard functions established by ICNIRP. The respective times when using various welding filters were calculated by multiplying the measured spectral irradiance, E(λ), by the spectral transmittance, τ(λ), of the various filters.

For the calculation of the retinal thermal Hazard limit (LHaz), the angular subtense (α) of the source at the viewing distance and the exposure time must be assessed. For exposure times greater than 10 s, L Haz is 280 W cm- 2 sr- 1

Safe exposure times with and without eye and face protection

Safe daily exposure times of the eyes and face to UV radiation ranged from 0.3 s to 3 s without any protection, whereas it was more than 8 h for all welding processes with any of the filters included in this study.

Calculations of ocular exposure to blue-light indicate that safe exposure times for unprotected eyes varied from a minimum of 3 seconds to a maximum of 7 minutes depending on the welding parameters used. The corresponding times with light filters (scale numbers 1.4 to 6) ranged from 0.5 min to more than 8 h. Correspondingly, the calculated thermal retinal radiance (L Haz ) for unprotected eyes varied from 4 to 390 W cm- 2 sr- 1 . However, with any of the filters studied, L Haz was below the limit value relevant for this study (280 W cm- 2 sr- 1 ).

Conclusion

It is evident that without any welding filter, safe exposure times to UV and visible radiation are very short, from less than one second to a few minutes. For protection against UV radiation, on the other hand, all of the tested filters were adequate, so that the safe daily exposure times were more than 8 h for all welding processes.

The corresponding safe times for exposure of unprotected eyes to blue-light varied from 3 seconds to 7 minutes, depending on the welding parameters used. The corresponding times with light filters (1.4 to 6) ranged from 0.5 min to more than 8 h. The preferred scale numbers for the light filter were 2 to 5. The scale number 1.7 was regarded acceptable, except for aluminium welding, where it was assessed to be too light. The dual filter combinations that were in accordance with EN 169 (scale number difference 5 or less) were not regarded as comfortable to use as those with bigger difference between the scale numbers. It is important, however, to recognise that the best dual-filter combination depends greatly on the welding parameters used, and hence no universal recommendation applicable for all situations can be given.

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Published: 10th Jan 2009 in Health and Safety International

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