Case Study: 710 nm Long Pass Filter

Case Study: Careful Substrate Selection to reduce cost of Long Wave Pass Filters.

Customer Challenge: The customer needed a steep blocking filter that transmits in the near infrared up to 900 nm but blocked for the visible range equivalent to a catalogue supplied item. The filter needed a slope<2% but had to be within a challenging budget. Long pass filters are made by joining coating “stacks” which make up the blocking part. Substrate selection can be used to reduce the number of stacks thus reducing coating time which in this case allowed the filter to meet the ‘budget challenge’. This approach ensured an excellent result, as good as the catalogue supplied filter but at a much lower cost.

Vortex’s Solution: Graph A below shows the long pass filter coating on a clear glass test piece (red line), it blocks down to 560 nm, the performance of Schott RG 665 in black. It can be seen that the combination will produce a long wave pass filter that blocks from the UV to 700 nm.  Graph B demonstrates another approach which involves depositing 2 further coating stacks onto a clear glass, this also performs well but is more costly due to a much larger amount of optical coating.

The Result: This solution utilises relatively cheap substrate glass (when compared to the cost of coating) to take care of most of the blocking so that key features such as slope, and specification tolerances can be optimised without compromise in performance.