The first optical filters were composed of colored glass. Although they are still in use today, they are mostly substituted by advanced thin film interference filters. Delta Optical Thin Film A/S recommended the use of interference filters in fluorescence microscopy in the late 1960s. The term thin film describes that these filters are made up of several or hundreds of thin layers with different refractive index as shown in the image below. The word interference describes that the physical principal these filters are based on is constructive and destructive interference of light waves that move through the different layers.
SEM cross section through an ultra-hard-coated filter.
The first thin film filters were created with a technology that produced so-called soft coatings. They are still prevalent in the market due to their low cost. But be aware that these filters are not durable and tend to degrade over time. The next step was creating hard coatings that are mechanically stable. However, the layers are not completely densified and possess voids. In these voids, water vapor is absorbed depending on temperature. This alters the effective refractive index of the coating and makes the filters change spectrally. Furthermore, repeated absorption cycles degrade the filter over time and – like for soft coated filters – can result in delamination of the coating as shown in the image below.
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 effectively managed to create optical filters with the Ultra-Hard-Coating technology. Delta Optical Thin Film has since led the way towards an ever increasing performance and complexity of optical filters. This fourth generation of optical filters is currently exclusively used for the company’s filters.
What is an Ultra-Hard-Coated Filter?
Ultra-Hard-Coated (UHC) filters are generated with an advanced plasma process that enables a much higher packing density compared to traditional hard coatings.
The coating is placed on one single substrate and therefore does not need glue. The resulting filter is extremely robust and can withstand intense humidity and light without compromising its performance.
The graph below illustrates the extreme robustness and stability of the Ultra-Hard-Coated filter. After having undergone an environmental test, the spectrum of Delta Optical Thin Film’s UHC filter is not changed.
Test results before and after 34 days exposure to elevated temperature and high humidity.
The Scanning Electron Microscope (SEM) picture taken at x31695 magnification displays the perfect surface and density of the UHC surface that makes Delta Optical Thin Film's Ultra-Hard-Coated filters the best and most sturdy filters one 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
- Ideal spectral stability (no spectral drift)
- Optimal mechanical stability
- Superior adhesion of the coating on the substrate
- Possibility of depositing very thick layers and increased complex functions
- Increased lifetime
- Possibility of dicing the filters to smaller sizes with 100% clear aperture after coating
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.