A brief look back

The first optical filters were made of coloured glass. Although they are sometimes still used today, they are mostly replaced by modern thin film interference filters. Delta Optical Thin Film A/S suggested the use of interference filters for use in fluorescence microscopy in the late 1960s. The term thin film describes that these filters are built up of several or hundreds of thin layers with different refractive index (see image below). The word interference describes that the physical principal these filters are based on is destructive and constructive interference of light waves that travel through the different layers.


SEM cross section through an ultra-hard-coated filter.

The first thin film filters were produced with a technology that created so-called soft coatings. They can still be found in the market because of their low cost. But be aware that these filters are not durable and degrade over time. The next step were hard coatings that are mechanically stable. However, the layers are not fully densified and contain voids. In these voids, water vapour is absorbed depending on temperature. This changes the effective refractive index of the coating and makes the filters shift spectrally. Moreover, repeated absorption cycles degrade the filter over time and – like for soft coated filters – can lead to delamination of the coating (see image below).

Delamination of a traditional thin film coating.

Delamination of a traditional thin film coating.

Ultra-Hard-Coated filters are Delta Optical Thin Film's latest technical advance in optical thin films components

Already in 2005, Delta Optical Thin Film successfully managed to produce optical filters with the Ultra-Hard-Coating technology. Delta Optical Thin Film has then lead the way towards an ever increasing complexity and performance of optical filters. This fourth generation of optical filters is nowadays exclusively used for our filters.

What is an Ultra-Hard-Coated filter?

Ultra-Hard-Coated (UHC) filters are produced with an advanced plasma process that enables a much higher packing density than traditional hard coatings.

The coating is deposited on one single substrate and hence does not make use of glue. The resulting filter is exceptionally robust and can stand intense light and humidity without losing its performance.

The graph below shows the extreme stability and robustness of the Ultra-Hard-Coated filter. After having undergone an environmental test, the spectrum of our UHC filter is not altered.

UHC Test results

Test results before and after 34 days exposure to elevated temperature and high humidity.

The SEM (Scanning Electron Microscope) picture taken at a x31695 magnification shows the perfect surface and density of the UHC surface that makes Delta Optical Thin Film's Ultra-Hard-Coated filters the best and most robust filters you can find on the market.


SEM picture of the surface of a UHC filter.

The advantages of the smooth surface:

Reduced stray light.
No voids: shift-free coating.

The advantages of Ultra-Hard-Coated filters:

Minimal water uptake.
Best spectral stability (no spectral drift).
Increased lifetime.
Excellent adhesion of the coating on the substrate.
Possibility of depositing very thick layers and increased complex functions.
Optimal mechanical stability.
Possibility of dicing the filters to smaller sizes with 100% clear aperture after coating.

Did you know that Delta Optical Thin Film was amongst the first to introduce Ultra-Hard-Coated technology to optical filters?