Flatness of Dichroic Filters

Imaging applications require maximum flatness as it affects image quality. If filters have high flatness, they do not deflect light rays in the system, thus preventing any pixel shift or distortion of the image created in the system detector.

The main characteristics of ultra-hard-coated filters are:

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

Dichroic Filters

Ultra-Hard-Coated Dichroic Filters are the Most Stable and Sophisticated Dichroic Kind of Filter one can Find

However, ultra-hard-coated dichroic filters also have more sensitivity towards substrate bending as the deposition process is very vigorous and usually happens on thin substrates (thickness of 1 mm). Stress is created by the stack of thin layers deposited upon the substrate surface during the process. The forces that originate from this stress can cause the coated substrate to bend.

Similar issues are faced by all types of production where the deposited film is densified by some sort of energetic production process.

Some filters where the substrate is thicker than the stack of thin layers do not bend. In other words, the stress effect created by the thin-film stack can be counteracted by the substrate thickness. On the other hand, in some filters, 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 high.

Delta Optical Thin Film knows about this issue and is involved in the development of a unique proprietary technique to counteract this effect.

Why can Ultra-Hard-Coated Dichroic Filters Bend?

Filter substrates can be purchased in accordance with any reasonable flatness requirements.

A filter can bend after the substrate has been through Delta Optical Thin Film’s Ultra-Hard-Coating process. Assisted by accelerated energetic ions, this process creates ultra-hard-coated thin film surfaces.

Which Flatness Specifications can Delta Optical Thin Film Meet for Dichroic Filters?

Flatness is different for each product. During the development process, individual flatness requirements should be discussed.

How does Delta Optical Thin Film Test the Flatness of a Filter?

A typical Fizeau interferometric set-up with a light source with an emission line @589.6 nm is used to measure the bending of a filter.

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

This information has been sourced, reviewed and adapted from materials provided by Delta Optical Thin Film A/S.

For more information on this source, please visit Delta Optical Thin Film A/S.


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