Long exposures with digital cameras present a particular challenge due to the sensitivity of digital sensors to Infra Red.
Digital cameras have internal filters which block IR to varying degrees - there is no hard and fast rule as to what wavelength the IR spectrum should be cut off at, and different dye formulations act at different wavelengths, in order to get really rich reds the cutoff is normally pushed slightly into the infrared region - the aim is to give the best possible image at normal expire ranges. Standard Neutral Density filters have traditionally only been designed and tested to limit the visible spectrum as IR was not an issue for film.
If you use a 10 stop standard ND filter for a long exposure then the visible light is reduced by about 1000x, however the IR is not effected so in effect the IR going through to the sensor is now about 1000x more in comparison to the visible light. A proportion of this excess IR will get through the internal IR filter and cause a colour cast on the image which cannot be corrected in post software as it is across all channels.
To cater for varying scenarios Tiffen have produced 3 different 10-stop filters for long exposures:
AXent (standard ND Filter) - works in visible spectrum only - for film cameras or digital camera in Black and White Mode. As it is basically allowing much more IR though than visible light it can be used for IR photography when used with IR-capable lenses on FUJI X-Trans APS-C sensor cameras.
ADVANTIX IR ND FIlter - designed to remove the near IR polluting light that the AXent standard ND lets though by adding an active absorbing element - this element adds a slight greenish cast which can be adjusted for, either in your camera’s white balance settings or in your post software afterwards.
The APEX is equipped with a standard near infrared blocker and a Tiffen Hot Mirror to eliminate all IR pollution that long exposure photography is susceptible to. These two features allow you to capture your long exposures without the risk of compromising the color in your image. If you require a heightened red response in your image, either manually reset your white balance or adjust in your post-production software.