B. Ralph Chou, MSc, OD
Associate Professor
School of Optometry, University of Waterloo
Waterloo, Ontario, Canada
N2L 3G1January 1999
Tel: 519-888-4567 x3741 Fax: 519-725-0784
e-mail: bchou@sciborg.uwaterloo.ca
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 orwithout the use of an optical aid such as binoculars or a telescope. Requirements for filtermaterial, 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, mylar
film, or any other material which provides a suitable substrate for an absorptive filter, orvacuum-deposited metallic reflective coating, which meets the requirements for filtertransmittance.
A.2. Filter transmittance
The luminous transmittance of the filter, when determined as described in clause 6 ofEN167, 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 anywavelength.
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 behandheld, 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 inwidth 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, notto 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 otherdefects 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: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 theirmountings should be discarded; andd)
warnings against the use of the filter with optical devices such asbinoculars, telescopes or camerase)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.

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, mylar
film, or any other material which provides a suitable substrate for an absorptive filter, orvacuum-deposited metallic reflective coating, which meets the requirements for filtertransmittance.
B.2. Filter transmittance
The luminous transmittance of the filter, when determined as described in clause 6 ofEN167, 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 anywavelength.
Filters with luminous transmittance (in the waveband 380 to 780 nm) equivalent to scale
number 12 to 16 as specified in Table 1 of EN69:1992 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: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 theirmountings should be discarded; andd)
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.Rationale
C.1. Materials
Materials recommended for use in filters for direct observation of the sun include welder’sfilters (shade or scale numbers 12 to 16 with glass or polycarbonate substrates), metalcoated glass or mylar, dyed plastic with high infrared absorption (Thousand Oaks BlackPolymer), and silver-bearing black and white photographic emulsion (exposed to light anddevelopment to maximum density) (Chou, 1981, 1998; Chou and Abel, 1993). This
specification accommodates these recommended filter substrates, as well as new materials
which may be developed with similar or superior physical characteristics.

C.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 forthermal retinal damage. Because of the relatively low levels of ultraviolet A radiation andshort wavelength (blue) visible light in solar radiation, the requirements for transmittanceof UV radiation (200 to 380 nm) and visible light between 400 and 480 nm by solar filtersneed 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 sunlighttransmitted 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 (Boettnerand Wolter, 1962). Filters should attenuate solar radiation in this waveband to levels ofexposure below the threshold dosages for photochemical and thermal retinal damage (Chou,1981).
C.3. Filter mounting
Where used, mountings should retain the filter securely in front of the face or opticalsystem to prevent solar radiation from reaching the eyes and/or optical system, except bytransmission through the filter. The specification permits the use of mountings held infront of the face, or worn as spectacle frames directly in front of the user’s eyes andcorrective lenses for direct viewing of the sun without an optical aid. Filters used in frontof 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.
C.4. Labelling
The requirements for labelling provide the end user with information on the source and thesafe use of the product, as well as warnings against improper use.
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 solareclipses. 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, 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.