Hi Ken,
"Why do we think blocking IR beyond 1400nm is an important issue in solar filter design?"
For one reason it seems most of the solar filter manufacturers think it's important, and spend additional funds and increase the costs of their products to provide blocking to 3000 nm or beyond.
https://luntsolarsystems.com/lunt-solar-safety/
"This region is heat not light and well beyond the sensitivity of the eye or current imaging sensors."
There appear to be other structures of the eye besides the retina that are susceptible to mid and long IR:
- Occular absorbtion.jpg (162.2 KiB) Viewed 8713 times
Here's what our friend Mark Wagner of Solar Spectrum filters had to say in this regard:
The ITF is used to block what the main bandpass doesn't, the one that only lets one peak from the etalon through. The bandpass will normally turn back on at around 800 nm. So the ITF is designed to block any light from 690 nm to the FIR (past 2500 nm). It is not uncommon to be still blocking past 3000 nm. They will use a red glass (RG630) to help block the short side (lower wavelengths). These filters will be spec’d as blocked from X-ray to FIR with a of OD4. This is how a standard bandpass keeps the level of IR low for safe viewing.
Filters that are blocked to only 1000 nm (like most stock filters) are not safe for viewing the sun, but maybe fine for imaging depending on the camera. To decide how far in the IR is safe for viewing I would say nothing shorter then 2000 nm. I wanted to see what a filter looked like with a dim etalon that I only blocked to about 900 nm. I did this test because the hard coated filters I use have more transmission than a standard one. And it would be closer in brightness. I only looked for about 5 minutes. When I was done I noticed that I couldn't focus the eye I had been using. It took about 45 minutes before I could focus with that eye. Didn't get hurt but got scared that I had damaged my eye. So don't try viewing with filters that are blocked anything shorter then 2000 nm on the sun.
KG glass is also used in blocking the IR.
Mark's description sounds very similar to what one expects with corneal edema (swelling) and irritation likely from IR exposure.
George9 followed up with a well-regarded expert in eclipse filter safety:
Thanks, Mark. Dr. Ralph Chou was nice enough to answer a couple of email questions in this regard, too. He let me quote him here.
First, with regard to solar eclipse eyeglasses: "You are correct that the specification for solar filters does not need to go beyond 1500 nm since water within the ocular media prevent longer wavelengths from reaching the retina. As for the cornea, the thresholds for damage by infrared radiation are extremely high, and even with a telescope, the danger to the cornea from IR between 1500 and 2500 nm while observing the Sun is not significant. The solar filter materials do block IR quite well out to about 3000 nm, so the eye is well protected across the entire UV-visible- NIR waveband." There is an existing eclipse eyeglass standard (EN1836) and a new draft ISO standard (ISO 12312-2), for which Dr. Chou is lead writer.
But with regard to telescope filters: "I expect that the manufacturers are well aware that they need to ensure that all of the optical waveband in sunlight (280 to 3000+ nm) has to be attenuated for eye safety, no matter what their technology is."
In other words, while the cornea is relatively resistant to damage in the 1500-3000nm range and should be fine without magnification, you still really need to make sure it is blocked in a telescope. And it would be possible to create a filter that is perfectly safe for solar eyeglasses but would not be a great idea to use by itself in front of a large telescope.
If we do a little math, we can see that my 4 inch (102 mm) telescope will present 650 times the amount of light (hence IR) than the unaided (naked eye) alone:
Area = pi r^2
Pupil D = 4 mm, then A = 3.14 x 2^2 = 12.56 mm^2
Objective D = 102 mm, the A = 3.14 x 51^2 = 8167 mm^2
8167/12.56 = 650.2
Applying this methodology to H alpha filters, we can see there is also potential here as well. For example, if the etalon has a free spectral range of 10 Å, and a FWHM (50%) transmission of 0.7 Å, the native 650 times the pupil transmission would be:
0.7/10 = 0.07
0.07 x 650 = 45.5 x 0.50 = 23
So if a solar H alpha blocking filter is not blocked at all beyond 1400 nm, in the case of a 102 mm refractor it could present the eye with about 23 times as much mid and far IR as the pupil would receive alone, and additionally is focused at the eye itself. And who is to say what an individuals specific susceptibility to any form of radiation could be based on their genetics and previous exposures, which generally are cumulative?
"I accept that extended and prolonged exposure to IR can be harmful, but this is not the case with the average solar observer."
Why take the chance, especially when a KG3 filter (or ITF, BG38, etc.) is relatively inexpensive insurance? And if for no other reason, in our litigious society, it wouldn't take much for a casual observer at one of my solar outreach events to show up and claim my solar filters were not safe and caused their eye issues. We go to court, and I state because Ken Harrison said it was safe I didn't use any IR blocking beyond 1400 nm. The plaintiff's attorney points out with documentation that every solar filter and H alpha telescope manufacturer incorporates IR blocking to 3000 nm or more. He or she also quotes the above information from Wagner and Chou, readily discoverable on the internet. The plaintiffs then ad you to their lawsuit for contributory liability. Who do you think the jury or judge is more likely to side with?
If you truly believe there is no harm from 1400 nm and beyond, this should be the acid test:
Get yourself a THORLABS FEL 1400 1 inch filter, which is only $76 USD
- FEL1400_Transmission.gif (7.91 KiB) Viewed 8713 times
https://www.thorlabs.com/thorproduct.cf ... er=FEL1400
This filter is blocked OD 4 from 200 nm to the cut-on of 1400 nm. Focus your equatorial mounted 6 inch f 8 refractor on the sun using a standard piece of Baader astrosolar filter material. Add the FEL 1400 to your eyepiece, remove the Baader astrosolar material, and "observe"
only with your non-dominate eye for couple of hours every week for a year or two, and let us know how it goes. I hope you are right.