Case Study: 710nm Short Pass Filter

Case Study: How short pass filter stacks are combined to extend blocking

Customer Challenge: For this application, the customer was using a silicon photo-diode detector within the imaging system and needed out of band blocking from 720 – 1200 nm in the near infrared. Unlike for long pass filters, it is often more difficult to select a substrate to block longer wavelengths. The blocking of materials such as Schott  BG38 or KG5 have shortened transmission bands in the visible range, this can often mean losses in the red channel, giving unbalanced colour images. Therefore, we had to design a filter that would have a high transmission band through the visible range (425-675 nm) but also dense blocking from 720 to 1200 nm.

Vortex’s Solution: Since the required blocking range was so wide, we produced a short pass filter “stack” on each side of the filter. Side 1 can be seen in isolation in red on graph A, this short pass stack forms the filters “critical edge”. In blue (on graph A) is the side 2 coating. The key to this process is to make sure that the 2 coatings are correctly aligned, graph A shows this, it can be seen they overlap at 930 nm. When done correctly, this gives a high quality fully blocked filter as seen in graph B. If the coatings are not aligned correctly, the result is a “leak” in between the stacks, as seen in graph C.

The Outcome: Vortex were successfully able to produce both coatings and deliver a filter that met all aspects, the visible transmission and near IR blocking with no leaks. The customers optical system could then produce accurate colour images.