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B. Ralph Chou, MSc, OD
Associate Professor
School of Optometry, University of Waterloo
Waterloo, Ontario, Canada N2L 3G1
Tel: 519-888-4567 x33741 Fax: 519-725-0784
e-mail: bchou@uwaterloo.ca

October 2000

Protective Filters for Direct Visual Observation of the Sun Technical Specification

This technical specification applies to filters intended to protect the eyes during direct visual observations of the sun (e.g. during partial and annular eclipses of the sun), with or without the use of an optical aid such as binoculars or a telescope. Requirements for filter material, transmittance of optical radiation, mounting and labels are provided. This specification was prepared in accordance with Annex II (Basic Health and Safety Requirements) of the European Community Directive on Personal Protective Equipment.

A. Filters intended for unaided visual observation of the sun

A.1. 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 vacuum-deposited metallic reflective coating, which meets the requirements for filter transmittance.

A.2. Filter transmittance

The luminous transmittance of the filter, when determined as described in clause 6 of EN167, shall no exceed 0.0032%. Filter transmittance in the waveband 290 to 315 nm (ultraviolet B radiation) shall not exceed 0.003% at any wavelength. For ultraviolet A radiation (315 to 385 nm) the transmittance shall not exceed 0.02% at any wavelength. Transmittance in the near infrared waveband (780 to 1400 nm) shall not exceed 0.5% 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.

A.3. Filter mounting

Filters may be made with or without a mounting. A mounting shall hold the filter securely so that it cannot be dislodged 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.

A.4. Labelling

The filter and/or its packaging shall show the following information:

  1. name and address of manufacturer and/or distributor of the product;
  2. instructions for use in looking at the sun or a solar eclipse;
  3. warnings that filters that are damaged or separated from their mountings should be discarded;
  4. warnings against the use of the filter with optical devices such as binoculars, telescopes or cameras;
  5. advice on storage, cleaning, and maintenance, as appropriate;
  6. the relevant protection-factor number of the filter; and
  7. the obsolescence deadline or period of obsolescence, as appropriate.

B. Filters intended for direct observation of the sun with an optical aid

B.1. Material

Filters may be made of tempered glass (minimum thickness 2 mm), polycarbonate, polyester film, or any other material which provides a suitable substrate for an absorptive filter, or vacuum-deposited metallic reflective coating, which meets the requirements for filter transmittance.

B.2. Filter transmittance

The luminous transmittance of the filter, when determined as described in clause 6 of EN167, shall no exceed 0.0032%. Filter transmittance in the waveband 290 to 315 nm (ultraviolet B radiation) shall not exceed 0.003% at any wavelength. For ultraviolet A radiation (315 to 385 nm) the transmittance shall not exceed 0.02% at any wavelength. Transmittance in the near infrared waveband (780 to 1400 nm) shall not exceed 0.5% 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.

B.3. Labelling

The filter and/or its packaging shall show the following information:

  1. name and address of manufacturer and/or distributor of the product;
  2. instructions for use in looking at the sun or a solar eclipse;
  3. warnings that filters that are damaged or separated from their mountings should be discarded;
  4. warnings against use of the filter at the eyepiece of the optical device, which may cause failure or breakage due to extreme heating, and result in eye injury.

C. Quality Control of Metal Coated Filter Materials

C.1. Defects in metal coated filter materials

Metal coated filter materials shall not exhibit more than one (1) pinhole defect not greater than 200 µm in average diameter within any 5 mm zone.

C.2. Examination of filter materials

The filter shall be illuminated from one side by an intense white light source (e.g. projector beam or light table) and the opposite side viewed through a low power magnifying lens. Filters showing visible pinhole defects shall be examined in a light microscope at 25 to 40 X magnification. Filters with defects with an average diameter (edge-to-edge dimension) of 200 µm or greater shall be deemed unsuitable for use and be destroyed.

D. Rationale

D.1. Materials

Materials recommended for use in filters for direct observation of the sun include welder’s filters (shade or scale numbers 12 to 16 with glass or polycarbonate substrates), metal-coated glass or polyester film, polymer sheet with high infrared absorption, and silver-bearing black and white photographic emulsion (exposed to light and development to maximum density) (Chou, 1981, 1998; Chou and Abel, 1993). This specification accommodates these recommended filter substrates, as well as new materials that may be developed with similar or superior physical characteristics.

D.2. Filter transmittance

Filters with luminous transmittance levels equivalent to scale number 12 or higher attenuate visible light in sunlight to a level several orders of magnitude below the threshold level for thermal retinal damage. Because of the relatively low levels of ultraviolet A radiation and short wavelength (blue) visible light in solar radiation, the requirements for transmittance of UV radiation (200 to 380 nm) and visible light between 400 and 480 nm by solar filters need not be as restrictive as those specified in EN169:1992 for welding filters. Welding filters are intended to protect against the emission of UV radiation from artificial sources (electric welding arcs) which is several orders of magnitude greater in the UV wavebands than sunlight. The levels of UVB (280 to 315 nm) and UVA (315 to 380 nm) in sunlight transmitted by filters commonly used by solar observers is insufficient to cause photokeratitis (welder’s flash) or other anterior eye damage (Chou, 1981, 1996; Pitts, 1993).

The normal human eye transmits radiation between 400 and 1400 nm to the retina (Boettner and Wolter, 1962). Filters should attenuate solar radiation in this waveband to levels of exposure below the threshold dosages for photochemical and thermal retinal damage (Chou, 1981).

D.3. Filter mounting

Where used, mountings should retain the filter securely in front of the face or optical system to prevent solar radiation from reaching the eyes and/or optical system, except by transmission through the filter. The specification permits the use of mountings held in front of the face, or worn as spectacle frames directly in front of the user’s eyes and corrective lenses for direct viewing of the sun without an optical aid. Filters used in front of the objective lens(es) of an optical aid may be mounted as full-aperture or reduced aperture filters for the purpose of direct visual solar observation.

D.4. Labelling

The requirements for labelling provide the end user with information on the source and the safe use of the product, as well as warnings against improper use.

D.5. Quality control of metal-coated filter materials

Concerns have been raised over the safety of metal-coated filter materials that exhibit pinhole defects in the coatings when viewed against a bright light source. The presence of such defects has been recognized in both metal-coated glass and polymer filters as an unavoidable artefact of the manufacturing process. The typical defect is seen under the optical microscope as a "pore" of approximately 20 to 50 µm in diameter. (Chou, 1999) These defects do not pose a hazard to the eye.

It has been shown that in rare instances larger defects may be produced in aluminized polyester film as a result of the manufacturing process. Although the manufacturers’ quality control procedures should prevent such defective product from being made into solar filters, there is a small chance that solar eclipse viewers with larger defects in their coatings may reach the distributor.

Coating defects in aluminized polyester solar filters are the main concern because of the large numbers produced for a given solar eclipse. It has been demonstrated (Chou, 1999) that these defects exist in one of the two layers of coated polyester film that are laminated together to produce solar filter material. While defects on the order of 500 µm in diameter may cause a degradation in image contrast, there is insufficient optical radiation reaching the eye to be a retinal hazard. Defects greater than 1 mm in extent may result in retinal irradiance levels approaching the photochemical damage threshold in visible light. The criteria for the size and number of defects present in a solar filter made of metal-coated glass or polyester film allows for the presence of small artefacts of manufacturing, while providing criteria for the maximum allowable size and number of coating defects that may be present in a filter without degrading its optical performance and safety.

References

Chou BR (1981) Safe solar filters. Sky and Tel; 62(2):119-121.

Chou BR (1996) Eye safety during solar eclipses - myths and realities. in Madourian Z, Maris G, Stavinschi M, eds. Theoretical and observational problems related to solar eclipses. Proceedings of a NATO Advanced Research Workshop. NATO ARW Series C, Vol. 494. Dordrecht: Kluwer Academic Publishers. pp 243-247.

Chou BR (1998) Solar filter safety. Sky and Telescope 95(2): 36-40.

Chou BR (1999) Ocular hazards of pinhole defects in mylar solar filters. Presented at the 1999 General Assembly of the Royal Astronomical Society of Canada, Toronto, Ontario, July 1999.

Chou BR, Abel K (1993) Your complete guide to the solar eclipse of May 10, 1994. Carnegie, PA: ABELexpress, pp 19-43.

Boettner EA, Wolter JR (1962) Transmission of the ocular media. Invest Ophthalmol Vis Sci; 1(6):776-783.

Pitts DG (1993) Ocular effects of radiant energy. in Pitts DG, Kleinstein RN, eds. Environmental vision: Interactions of the eye, vision, and the environment. Toronto: Butterworth-Heinemann, pp. 151-220.

Copyright © 1999 University of Waterloo