It would be helpful if manufacturers would actually post which bandwidths are passed by each component of their system, in addition to the final through-put. Of course, it would also help if they used consistent terminology. How much of their different terminologies indicates science versus simple label variation for marketing differentiation? For example, is there a real difference between a Blocking Filter vs an Interference Filter, etc?)
You should be able to look up their patent white paper if you want to see what's in there. You can also contact the manufacturer and ask for proof of safety so that there's something to go off, other than speculation or sheer trust.
See the links below for best terminology (we use common words like blocking filter, but it breaks down to more specific terms really).
A blocking filter is just a narrowband filter system and has at least 2 filters usually. It usually will be as tight as 2A or as wide as 10~12A even, many are in that 5~6A range. That's all they do is block out of band frequencies other than what they're centered on, in this case, 656nm. There's nothing special beyond this about these filters. They're expensive because they're so narrow (a few hundred bucks) and are small for that reason too. Making a hard coated filter that's very narrow, closer to 1A, is super expensive and very difficult to consistently produce. The out of band harmonic output frequencies of an etalon are separated enough (it's a comb filter, so it outputs lots of frequencies with a comb pattern with peaks and troughs) that a blocking filter only needs to be wide enough to pass one frequency and block the other surrounding frequencies beyond the 656nm peak and the two troughs on the blue/red wing sides of it, and 6A typically is sufficient for most applications for this.
An induced transmission filter (ITF) is a pre-filter that mostly cuts IR and long IR. Some work via reflection, some work via absorption. And the substrate its on, such as RG610 glass will already block lower frequencies. This filter is important and is often the filter that degrades first in some systems (such as Coronado/Meade commonly) if it's not hard coated or well made with its coatings and degrades to dim the image. But it's purpose is to grind transmission of off band frequencies and long IR, so less energy and less high transmission off-band frequencies are meeting the blocking filter after it, and then only a single wavelength is coming out of the final blocking filter (656nm). This is an important filter for visual because of the long IR blocking of course. For imaging, well, I won't get into it too specifically, but sensors are not bothered by IR and it can be handled in other ways. But just think of it as a safety filter to protect the far more expensive blocking filter and keeping long IR from your eyes potentially.
Almost every blocking filter is essentially made of those two components. The bandpass is not actually all that important, nor the transmission. All of it is way down load for visual levels. The only thing that matters is preventing any UV in any significant amount and the longer IR in any significant amount. The ITF does that. The blocking filter then limits the wavelength passing to one central peak on 656nm. This is why these blocking filters work on other systems, such as putting a Coronado into a Lunt, a Lunt into a Coronado, and why a Quark works on both of them with its own blocking filter too. I cannot speak for every design of course. There are some exotic ones out there. But these common entry level blocking filters are mostly the same with some variation in degree of filter quality and transmission of the particular frequencies and work. It's best practice to of course follow manufacture suggestion of not mixing them from a visual standpoint, if you have any doubt at all or question at all about it (again they cannot endorse something they didn't make, nor suggest mixing products when pointing them at the sun). It's totally irrelevant from a camera point of view though.
Anyhow, not trying to suggest everyone mix and match stuff and put their eyes up to it. Safety first. But since you asked for information, just expanding on the idea for discussion purposes. People should know more about what they're subjecting their vision to with these systems and why or why not they're safe.
A white paper and transmission profile would be great. Otherwise, again, just look to the patent paper.
Good reading here:
Edited by MalVeauX, 20 September 2021 - 10:00 AM.