Unsafe practices - looking at the sun:
(1) directly
(2) through a telescope, camera or binoculars without a special protective filter designed specifically for these instruments
(3) through sunglasses
(4) through an unsuitable makeshift filter
(4a) CD with a thin film of aluminum
(4b) aluminized polyester used as wrapping material (coating may be uneven)
(4c) polarized filters
(4d) film other than silver-coated black and white film
(4e) smoked glass
Safe practices:
(1) using a pinhole projector (piece of paper with a small hole that allows sunlight to fall onto a second piece of white paper held at a short distance)
(2) reflecting the sun onto a piece of white paper with a mirror.
(3) using filters approved for solar viewing, typically #14-16 scale filters of EN 169/1992 (while #12-13 can be used these may be too bright for comfortable viewing), corresponding to a luminous transmittance <= 0.0003% (#14).
(4) using a makeshift filter with a suitable metal coated material:
(4a) 2 layers of silver-coated black and white film that has been exposed and developed as negatives
(4b) aluminized mylar
(5) through a telescope, camera or binoculars with a special protective filter designed specifically for these instruments
