Neutral Density Filters
What is an ND Filter and how does it work?
It is a Neutral Density filter that reduces light. Photographers use this filter when they want to use a large aperture in bright light with 35mm cameras. The filter reduces all visible light but maintains colour balance. An ND4X filter allows one quarter the light, and an ND8X allows only one eighth the light. Tiffen filters have their own system, with them an ND0.6 is a ND4X and a ND0.8 is a ND6X, etc. When you combine two filters it has a multiplying effect. For example, two ND4X filters are the same as an ND16X, NOT an ND8X.
What does all this mean for X-ray?
People who have fully modified Sony Handycams have the problem of not being able to control exposure so the picture whites out. This filter allows a person to reduce the light entering the camera and thus give proper exposure but not affect the infrared light entering the camera. Also it can save you money if you need a filter for every occasion but can only afford one.
The above picture show an x-ray picture of a large grassy field on a bright sunny day in a wide open area using a Hoya RM90 filter. The camcorder is a Sony TRV-315 digital 8.
Whilst the RM90 is fine for indoors and in the shade or overcast days it can be too washed out when used in very bright sunlight with no shade on a fully modified Sony Handycam.
The above picture shows the same scene only with a Hoya RM100 filter. Now this is ideal and would be fine, but of course this would mean having both filters as the RM100 is only suitable in bright light so that would be expensive. But there is an alternative - using a neutral density filter.
The above picture shows the same scene again only this time its with a Hoya RM90 filter plus a Hoya ND8X filter. Its slightly darker than the Hoya RM100 filter alone but still within usable limits and the ND filter only costs about $12 (Canadian dollars) which is much less than a Hoya RM100 filter.
I have found that the ND8X filter is the only useful neutral density filter for the newer camcorders. With that filter you can see outside on a sunny day but anything less is too bright and anything more is too dark.
In conclusion, having both the Hoya RM90 and Hoya RM100 filters is the best idea but if you can't afford both then a Hoya RM90 and a Hoya ND8X filter is the next best thing.