https://www.edmundoptics.com/f/uv-nir-neutral-density-filters/14850/

There has clearly been a great deal of patient study of the effects from various combinations of polarising, ND, IR, and colour filters upon the image obtained. In very general summary, my impression is that parts of your study, with the “plastic” Hitech ND 0.9 (3 stop) filter in particular, confirm my observations that IR light is allowed through (effectively 100% in your view) while visible light is diminished. So, the effect is not confined to Cokin ND filters.

I note also the welder’s glass experiment and I’m not surprised that there is no IR light transmitted. While open to further testing, I’m sticking with the view that glass ND filters will not transmitted much or any IR light, as the substances used to reduce visible light (metal oxides and/or carbon) will also block IR light.

On the matter of polarising filters, you seem to have partial success I seeing effects, but remain unconvinced on any benefit in IR work. This is a fair enough assessment from the work done, but I would make two points. First, I am not sure about the position of the Sun during the experiments. I seems to have been on the left and moderately high in the sky, but I’m guessing. With polarisation effects the position of the Sun is normally critical. It needs to be a right angles to the direction of the shot. Ideally, this means having the Sun shining from low on the horizon towards your left or right shoulder. If this is the case, rotating the polarising filter will cause the sky to darken if there is some colour present, and glare and reflections to disappear. Now, the true comparison is between the two extremes – maximum and minimum effects from turning the filter. When I have carried this out I usually get a substantial change in IR shots, just as with visible light. Sometimes, I get odd effects that I have not satisfactorily explained. For example, Some reflected light seems to be affected, but other such light is not. Possibly, the angle of polarisation for reflected IR versus visible light is different. However, I do not really know. Secondly, the use of a polarising filter for creative effects is always a matter for the photographer. I came across a professional once who just did not believe in the use of such filters at all (in visible light) – to him they did nothing! Fair enough – that was his view. But, to me, the effects in early morning light in visible and IR light can be very striking.

I have heard of the issue about placing plastic filters in front of a polarising filter, but I’m not surprised by your result that it makes no difference. I think that the issue arises because the plastic used can itself cause some polarisation of the light as it passes through – stress polarisation. So, in principal, there might be odd effects from polarised light being re-polarised. But, I doubt that in practice this is a problem.

Finally, I note all the various colour filter experiments, some results of which look very interesting, for example the Lee 25 red polarised edited (#43) and the Lee 12 yellow polarised SOOC (#29) appealed to me. However, we are again into the realm of creative expression and personal tastes.

I have managed to buy a set of coloured filters and graduated colour filters from Amazon (ca. £30) with an even wider range of hues than you have looked at – so I guess I have some experimentation to do to follow up this valuable contribution.

Many Thanks

David