The D1 Filter has been replaced with the D2 filter. You can find the D2 filter HERE
Following from the highly successful IDAS P2 LPS, the D1 has been optimised for one-shot colour and modified DSLR cameras. The key concept of the D1 LPS is to reduce the reddish background cast whilst blocking as much light pollution as possible improving upon the already good colour balance of the P2 LPS.
Light pollution suppression (LPS) filters are designed to suppress the common emission lines generated by artificial lighting, yet allow the important nebula emission lines to pass, thus enhancing the contrast of astronomical objects, particularly emission nebulae (see filter plots).
Unlike other light pollution suppression filters, IDAS filters are specifically designed for balanced colour transmission using the IDAS unique Multi-Bandpass Technology (MBT) process. The balanced transmission allows colour photographs to be taken with minimal colour cast to broadband emission objects such as stars, galaxies and globular clusters.
IDAS LPS filters utilize the unique IDAS Ion Gun Assisted Deposition (IGAD) coating technology for superior coating durability (quartz hardness) and safer cleaning. IGAD coatings also improve temperature and humidity stability, reducing spectrum shifts down to +/-1nm from the +/-3 or 4nm shift of standard coatings.
CCD imaging can also benefit, because although CCD imagers can already shoot through light pollution to some extent, including an LPS filter to the setup gives an added (signal-to-noise) edge. The filter also blocks infrared so a separate IR blocking filter is not needed.
As the response curve indicates, LPS-D1 loses SII emission line while LPS-P2 didn t. So this is why it's basically intended to be used with one-shot color cameras.
Note, however, that light pollution suppression filters are not a perfect substitute for dark skies.