…continuedSolar Filter Safety
Relative Safety of Filter Materials
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.
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!
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.