A recent discussion on the Solar Eclipse Mailing List caught my eye as something that should be shared with everyone. It is about one of those things that I thought had been put to rest a long time ago. The simple fact is: you cannot build your own SAFE solar filter by stacking sunglasses or neutral density (ND) filters.
The reason is these filters (sunglasses are in filters) are for visual light only and do little or nothing for radiation outside the visible spectrum. Ultraviolet and infrared radiation can damage your eyes.
For photographers, stacking filters presents a different problem - flares. Reflections between the filters will be picked up by the camera and look like flares in the images.
It is important to use the right tool for the job. For cameras, purchase a dedicated solar filter. This is even more important for telescope owners. And make sure the filter goes on the front of the optics - you don't want a filter over heating and ruining your vision! Click here for more information about filters and photographing the Sun.
A further consideration is that commercial ND filters are absorptive and can get warm when exposed to the Sun for a prolonged period of time. Gelatin filters may melt or deform under the heat.
Check out the Solar Filters for sale at Amazon.Com