Relative Safety of Filter Materials

For this article I tested a variety of samples. Photographic film samples were purchased from a local retailer, 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' stocks. Floppy disks were tested with the outer plastic casing removed.

There are numerous solar filters on the market that weren't evaluated here because of their similarity to other items tested. The purpose of this effort was to determine the general types of materials that make safe filters, not to compare similar designs by different manufacturers.

Not surprisingly, I found a wide disparity in the attenuation of visible light by these materials, even among the "safe" filters. For example, the differences in processing methods and chemistry gave varying optical densities for the silver-bearing black-and-white film emulsions. The double-layer filters had shade numbers ranging from 11 to 16.

I also found a wide range of optical densities between individual audio and data compact discs because of variations in manufacturing processes. Some CDs have aluminum films that are so thin they appear semitransparent at normal room illumination levels. These are obviously unsuitable for use as solar filters. Other CDs, however, are suitable if the aluminum coating is dense enough that the glowing filament of an incandescent light bulb is just barely visible through them.

Floppy disks have only a marginally safe infrared transmission and produce poor-quality images of the solar disk. The magnetic medium scatters visible light to such an extent that you see a dull red disk surrounded by a broad halo of red light. I would not recommend using this material for a solar filter.

Aluminized polyester and glass filter materials gave the most consistent performance. Most of the items specifically designed for eye protection easily met all of the transmittance criteria for safe filters. I would avoid aluminized polyester used in packaging for food products and collector cards because of the inconsistent optical quality, though the particular pop-tarts wrapper I tested performed surprisingly well. (It rated as marginally safe.)

Acceptable and Unacceptable Filters

Unsafe filters include any photographic emulsion bearing an image, 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 visible-light transmittance levels, they pass 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 percent of the infrared radiation!

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

Acceptable solar filters for unaided visual observations include aluminized polyester specifically designed for viewing the Sun, shade 12, 13, and 14 welding filters, black polymer filters, and two layers of fully exposed and developed silver-bearing black-and-white negative film.

For photographic and aided visual use, particularly with binoculars or telescopes, acceptable filters include aluminized polyester specifically designed for the purpose and Type 2-Plus glass filters. The Thousand Oaks Type 3-Plus filter should be used with extreme care for photographic use only.

Not recommended are metal-coated polyester film that is not specifically intended for solar observation, smoked glass, floppy disks, black color transparency (slide) film, chromogenic film (not tested here), and compact discs (because of the inconsistent quality of the metal coating).

My data and further comments on safe solar fillers appear on the NASA/GODDARD eclipse page.