The standard closest to a standard that I could find came from looking for filters in the front of the lens for solar photography, ie, say filters for use with telescopes (or telephoto lenses) specially designed to take pictures of the sun in white light.
For visual solar observation, the standard is ND5.0. Baader Planetarium also offers an ND3.8 version for use in high magnification (NON visual) imaging, and the Wikipedia entry for neutral density filters contains the following note:
Note: ND 3.8 is the correct value for exposure to solar CCD without risk of electronic damage.
But do not have a quote to explain where the number comes from.
On the other hand, there is a clear conflict between this and the daily experience with non-SLR digital cameras. It is not unusual to have the sun in the plane – either intentionally or during framing – and yet we do not have any traces burned through the sensor.
Part of the explanation may be due to the fact that mainstream cameras have integrated infrared blocking filters for color balance (unlike many astronomical CCDs). Another
a part can be that many point cameras use relatively small lenses – so as not to capture as much heat and light as a telescope,
which usually have lenses or mirrors several inches in diameter.
In addition, with conventional photography, at least for freehand shooting, you usually take a few seconds at a time, often moving as you frame the shot – so you normally not much time for the heat to increase. If you compare this with solar imaging, where you normally follow the sun for several minutes (or sometimes a few hours), with a much larger lens / mirror (and thus capturing more light / heat), you can see why a filter becomes more important.
With a total solar eclipse in the United States, it is probably worthwhile to add a safety warning regarding visual filters. You only have one eye, so do not take chances – use only filters specifically designed for solar use. Do not try to improvise from exposed films, CDs or others. Visual performance is not a safe guide to performance at invisible IR / UV wavelengths. The appropriate sunscreens are relatively inexpensive – a few dollars for a portable device that you can look at with the naked eye,
or $ 20 to $ 30 for a solar filter film sheet, or you can get predefined filters (the ND5.0 visual quality filters are also suitable for photography).
and as specified by Wikipedia's ND filter entry:
Note: ND 5.0 is the minimum required for direct solar observation in the eyes without retinal injury. An additional check must be made for the particular filter used, checking on the spectrogram that the UV and IR values are also attenuated with the same value.
Specially designed sunscreens are safe. Others are unknown and are not worth risking your sight.