Maximum flatness is required in imaging applications because it affects image quality. A high flatness ensures that the filter does not deflect the light rays in the system and hence prevents any pixel shift or distortion of the image created in the system detector.

The main characteristics of Ultra-Hard-Coated filters are:

High spectral stability. The filter spectrum does not shift because of air uptake or release.
Advanced coating: with high transmission bands, edge steepness and broad blocking.
High reproducibility of the production process.

AOI tolerant_dichroic_500

Ultra-Hard-Coated dichroic filters are the most advanced and stable dichroic type one can find

However, it also is more sensitive towards substrate bending because the deposition process is very energetic and typically takes places on thin substrates (thickness of 1mm). The stack of thin layers deposited upon the surface of a substrate during the process creates stress. The forces originating from this stress can force the coated substrate to bend.

This issue is the same for all types of production involving some sort of energetic process to densify the deposited film.

Some filters do not bend because the stack of thin layers is not thick compared to the thickness of the substrate. In other terms, the substrate thickness can counteract the stress effect created by the thin-film stack. For some others, the required function and/or mechanical constraints of the substrate thickness imply that the coated thin-film layer stack deposited will be so thick that the risk of bending is increased.

Delta Optical Thin Film is aware of this issue and has been developing a unique proprietary technique to counteract this effect.

Why can Ultra-Hard-Coated dichroic filters bend?

Filter substrates can be bought according to any reasonable flatness requirements.

The bending of a filter can happen after the substrate has been through our Ultra-Hard-Coating process. This process is assisted by accelerated energetic ions and creates Ultra-Hard-Coated thin film surfaces.

Which flatness specifications can Delta Optical Thin Film meet for dichroic filters?

Flatness varies from product to product.

Individual flatness requirements have to be discussed during the development process.

How does Delta Optical Thin Film test the flatness of a filter?

The bending of a filter is measured in a classic Fizeau interferometric set-up with a light source with an emission line @589.6 nm.

The filter to be tested is placed on top of a reference flat, and the resulting interferogram (Newton's rings pattern) seen by the eye gives a measure of the flatness of the filter.