Unfortunately, despite the best intentions, the news media often provide inaccurate information on safe observing techniques. This is especially true when the subject of protective filters for direct observation of the sun is raised. Over the past five years, I have been asked to comment on the safety of using devices such as floppy disk media, multiple layers of space blanket (a very thin type of aluminized polyester), compact disks (CDs) and metal coated polyester wrappers as solar filters. There are now several manufacturers of solar filters intended for both visual and photographic use who were not operating in 1981 when I first published solar filter data in Sky and Telescope (August, 1981).

An invitation to participate in a NATO-sponsored meeting on solar eclipse astronomy in June, 1996, prompted me to make spectrophotometric measurements of a variety of solar filter materials and assess whether these filters provide adequate protection for the eyes.

Retinal Burns

Solar radiation reaching the surface of the earth ranges from ultraviolet (UV) radiation at wavelengths longer than 290 nm to radio waves in the metre range. It is widely accepted that environmental exposure to high levels of solar UV radiation contributes to the accelerated ageing of the outer layers of the eye and skin, and the development of cataracts. However, observing the sun with inadequate or no eye protection results in "eclipse blindness" or retinal burns because the eye transmits most of the optical radiation between 380 nm and 1400 nm to the light-sensitive retina.

Exposure of the retina to high irradiance levels of visible light triggers a series of complex chemical reactions within the light-sensitive rod and cone cells. The resulting photoproducts damage the cells, impairing their ability to respond to light, and in extreme cases can destroy them. Depending on the severity of the damage, an affected observer experiences either a temporary or permanent loss of visual function. This photochemical retinal injury mainly occurs when the retina is exposed to blue and green light. Longer wavelengths of visible light and near-IR radiation are absorbed by the dark pigment epithelium below the retina, and converted into heat which can literally cook the exposed tissue. This thermal damage also occurs during extended exposure to blue and green light. Photocoagulation destroys the rods and cones, leaving a permanently blind area in the retina.

Each wavelength of optical radiation has an associated threshold retinal exposure level that must be exceeded in order for retinal damage to be observed - shorter wavelengths are more effective in that less energy is needed. The danger to vision of inadequately protected viewing of the sun is significant because light-induced retinal injuries occur without any feeling of pain (there are no pain receptors in the retina), and the visual effects do not occur for at least several hours after the damage is done.

Because the threshold exposure levels for photic damage to the retina at each wavelength of the optical spectrum are well known, it is fairly simple to calculate the maximum permissible filter transmittance that will provide adequate retinal protection from sunlight. The ratio between the threshold retinal irradiance for light damage to the solar spectral irradiance at each wavelength provides a starting point for this. The worst case scenario assumes that the sun is at the zenith in a clear sky (air mass of 1). The maximum permissible transmittance level of the filter at a given wavelength can be arbitrarily set at between one per cent and 0.1% of this ratio to provide a "safety" factor. When this is done for the waveband between 380 and 1400 nm, we find that a filter with a luminous transmittance of 0.0032% in the visible spectrum corresponding to a shade number of 12 provides "adequate" retinal protection during solar viewing. However, this does not take into account visual comfort; for comfortable viewing of the sun, a filter with a luminous transmittance of 0.0003% (shade number 14) is often preferable.

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Measuring Filter Transmittance

Filter materials that were tested are shown in Table 1. Solar filter materials were randomly selected from the manufacturers' stock. The photographic film samples were purchased from a local retailer, then exposed to full sunlight and developed to maximum density according to the manufacturers' instructions. The smoked glass filter was produced by depositing soot from a candle flame onto a glass microscope slide. The other materials were obtained by random selection from retailers' stock.

Transmittance measurements were made with a Cary 5 spectrophotometer at 5 nm intervals over the waveband 200 to 2500 nm, and the data were stored as Lotus spreadsheet files. A rear-beam attenuator accessory was used to reduce the noise level, but there was still a significant level of signal fluctuation in the infrared (IR) and ultraviolet (UV) regions of the spectrum. This is a common problem when measuring transmittance of high-density filter materials.

Microsoft Excel workbooks developed in the Ophthalmic Standards Laboratory at the School of Optometry, University of Waterloo, were used to calculate luminous transmittance, shade number, and mean transmittance in two UV wavebands (200 to 315 nm and 315 to 380 nm) and the near-IR (780 to 1400 nm). These calculations are specified in the American National Standard Practice for Occupational and Educational Eye and Face Protection (ANSI Z87.1-1989). The data are shown in Table 2. The spectral transmittance curves for these solar filter materials are shown in the accompanying figures.

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Relative Safety of Solar Filter Materials

Not surprisingly, there was a wide range in the attenuation of visible light by these filter materials. Even among the "safe" filters, there was considerable variation in transmission levels. For example, the differences in processing methods and chemistry resulted in considerable variation in optical density of the silver-bearing black-and-white film emulsions. The double-layer filters had shade numbers between 11 and 16.

I have recently also found a wide range of optical density between individual audio and data compact disks (CD and CD-ROM) because of variations in manufacturing processes. Some compact disks have aluminum films which are so thin that they appear semi-transparent at normal room illumination levels. These CDs are unsuitable for use as solar filters. Higher quality CDs are suitable for use if the aluminum coating is dense enough that the glowing filament of an incandescent light bulb is just barely visible through it.

Floppy disk media have a marginally safe infrared transmission, and produce poor quality images of the solar disk. The magnetic media scatters visible light to the extent that one sees a dull red disk surrounded by a broad halo of red light. I would not recommend using this material for a solar filter.

The most consistent performance was found with the polyester and glass filters. I would avoid aluminized polyester which is used in wrappers for food products and collector cards because of the inconsistent optical quality, but even my sample of Poptarts wrapper performed surprisingly well in terms of protection from optical radiation. (It rated as marginally safe.) However, most of the filter materials specifically designed for eye protection easily met all of the transmittance criteria for safe filters.

Unsafe filters include any image-bearing photographic emulsion, chromogenic (non-silver-bearing) black-and-white film, black processed color film, photographic neutral density filters and polarizing filters. Although these materials have very low luminous transmittance levels, they transmit an unacceptably high level of near-infrared radiation. The black color film is a good example, having a shade number of 15 for visible light, but transmitting almost 50% of the infrared radiation!

Infrared transmittance levels shown in Table 2 should be regarded as the upper limit of transmittance in the waveband 780 to 1400 nm. The signal-to-noise ratio for low-level measurements in this waveband is extremely low, and thus these data are less reliable than those in the shorter wavebands. Note that even some glass filters with very good safety performance histories such as the Questar and Thousand Oaks glass filters showed infrared transmission levels up to 0.4%.

Smoked glass had very good performance in terms of transmission of visible light and infrared radiation. However, it is a dangerous filter material for two reasons. First, it is very difficult to produce a heavy uniform coating of soot on glass. Second, the coating is very fragile. It is very easy to destroy the filter by handling it. Much of the soot on my sample came off because of contact with its protective wrapping. It also made quite a mess.

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Moving toward an International Standard?

As manufacturers of protective solar filters try to expand their markets beyond North America, they inevitably encounter skepticism from the medical community and government, as well as regulatory obstacles. For example, a company began efforts to distribute polyester solar filters in the United Kingdom in advance of the partial solar eclipse of 12 October 1996. However, as a member of the European economic union, the United Kingdom required that solar eclipse "glasses" incorporating polyester filter materials bear the "CE" mark. This required testing to certify the eclipse glasses as meeting the Basic Health and Safety Requirements (Annex II) of the European Community Directive 89/686/EEC on Personal Protective Equipment. I prepared a technical specification for "Protective Filters for Direct Visual Observation of the Sun" which specifies materials, filter transmittance, filter mounting and labelling requirements for both filters intended for unaided visual observations of the sun and direct solar observations with optical aids. The complete technical specification was reviewed and accepted by a British certifying agency as compliant with the EC directive.

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Filters intended for unaided visual observation of the sun

Material

Filters may be made of tempered glass (minimum thickness 3 mm), polycarbonate, polyester film, or any other material which provides a suitable substrate for an absorptive filter, or a vacuum-deposited metallic reflective coating, which meets the requirements for filter transmittance.

Filter Transmittance

The luminous transmittance of the filter, when determined as described in clause 6 of EN167, shall not exceed 0.0032%. Filter transmittance in the waveband 280 to 380 nm (ultraviolet radiation) shall not exceed 0.003% at any wavelength. Transmittance in the near infrared waveband (780 to 1400 nm) shall not exceed 0.027% at any wavelength. Filters with luminous transmittance (in the waveband 380 to 780 nm) equivalent to scale number 12 to 16 as specified in Table 1 of EN169:1992 are considered suitable for direct observation of the sun. It should be noted that many observers will find the solar image uncomfortably bright when filters with scale numbers of 12 or 13 are used.

Filter Mounting

Filters may be made with or without a mounting. A mounting shall hold the filter securely so that it cannot be displaced by normal handling or by gusts of wind. Mountings may be handheld, or shaped in the form of spectacles to be worn on the face in front of any corrective (spectacle or contact) lenses worn by the user. The filter or filter and mounting assembly shall be of a size sufficient to cover both eyes of the user simultaneously, and in no case shall have overall dimensions less than 115 mm in width and 35 mm in depth in the plane parallel to the facial plane. Spectacle shaped mountings may have a triangular cut-away area to accommodate the crest of the nose, not to exceed 15 mm in apical height and 35 mm width at the base. The filter and mounting shall be free from roughness, sharp edges, projections or other defects which could cause discomfort or injury during use. No part of the filter or mounting which is in contact with the wearer shall be made of materials which are known to cause any skin irritation.

Labelling

The filter and/or its packaging shall show the following information: a) name and address of manufacturer and/or distributor of the product; b) instructions for use in looking at the sun or a solar eclipse; c) warnings that filters that are damaged or separated from their mountings should be discarded; and d) warnings against the use of the filter with optical devices such as binoculars, telescopes or cameras; e) advice on storage, cleaning and maintenance, as appropriate. f) the relevant protection-factor number of the filter. g) the obsolescence deadline or period of obsolescence, as appropriate.

When the stringent requirements of the European Community Directive are considered, this specification may be suitable as an international performance standard for all solar filters. Adoption of such a "standard" would give astronomers who are asked to advise government authorities on safe solar viewing techniques a reference which addresses the public safety concerns over these devices.

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Filters for visual and photographic use

Acceptable filters for unaided visual observations include: aluminized polyester specifically designed for solar viewing, shade 12 and 14 welding filters, black polymer filters (Thousand Oaks Solar Shield 2000 and Rainbow Symphony Polymer), and two layers of fully exposed and developed silver-bearing black and white film negative. For photographic and visual use, particularly with binoculars or telescopes, acceptable filters include: aluminized polyester specifically designed for the purpose, and Questar and Thousand Oaks T1 and T2 glass filters. The Thousand Oaks T3 filter should be used with extreme care for photographic use only.

Not recommended are: metal-coated polyester that is not specifically intended for solar observation, smoked glass, floppy disk media, black colour transparency (slide) film, floppy disk media, and compact disks (because of the inconsistent quality of the metal coating).

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Additional Reading

My data and comments on safe solar filters will appear in the NASA solar eclipse publications by Fred Espenak and Jay Anderson, beginning with NASA Reference Publication 1398 for the total solar eclipse of 11 August 1999.