Definition:
A notch filter selectively blocks or attenuates a specific range of wavelengths (the notch region) while allowing wavelengths either side of this range to pass through with minimal attenuation. These filters are commonly used when 2 regions of interest are separated by a region of no interest or a region with a particularly strong interfering or background signal, for instance in the MID IR a common request is to remove the influence of moisture or CO₂ from a measurement. Notch filters are often used in applications such as spectroscopy, laser systems, and medical instruments to remove unwanted wavelengths from a light source.
Design and Manufacture:
We design and manufacture Notch Filters between 300 nanometres to 6000 nanometres to any custom specification. Please contact us to discuss your specific requirements.
Supplying Optics:
We hold an extensive inventory of stock wafer substrates, including Glass, Sapphire, Silicon, Germanium, Fused Silica, Calcium Fluoride, and more, available in various thicknesses and ready for immediate coating. Post-coating, these substrates can be swiftly diced, or core drilled, significantly reducing lead times, often in less than two weeks. Additionally, we have a network of trusted optics suppliers, whom we regularly audit, to provide custom optics. This allows us to offer a comprehensive solution by delivering both the optics and coating as a complete package from a single supplier, simplifying your procurement process.

Case Studies

CO₂ Blocking Notch Filter

H₂0 Blocking Notch Filter

810 nm Notch Filter
Applications

Environmental Monitoring:
Notch filters are used in satellite and airborne sensors to block unwanted spectral bands and enhance the detection of specific environmental parameters, such as chlorophyll concentration in water bodies or vegetation health. Air Quality Monitoring also utilises notch filters in instruments that measure pollutants by blocking interfering wavelengths and allowing the detection of specific gases or particles.

Biomedical and Medical Imaging:
Fluorescence Microscopy uses Notch filters to block the excitation light while allowing the emitted fluorescence signal to pass through, improving contrast and image clarity. Laser Surgery also uses notch filters to protect sensitive optical components and detectors from laser light used in surgeries, allowing only specific diagnostic wavelengths to pass. In devices such as slit lamps and fundus cameras, used in Ophthalmology, notch filters block specific wavelengths to enhance retinal imaging.

Manufacturing and Industrial Applications:
Laser Cutting and Welding use notch filters to protect sensors and cameras from the laser’s operational wavelengths while allowing observation and measurement of the process. Quality Control utilises notch filters in optical inspection systems to enhance contrast and detect specific defects or features by blocking unwanted light.

Security and Defence:
Laser Protection utilises Notch filters to protect sensors and human eyes from laser threats by blocking specific laser wavelengths while maintaining visibility in other spectral regions. Night Vision Equipment is enhanced by notch filters blocking specific wavelengths that could interfere with the detection of infrared light.

Scientific Research:
Notch filters block the laser excitation wavelength while allowing Raman scattered light to pass, improving signal-to-noise ratio in Raman spectroscopy. Chemical Analysis uses notch filters inside spectrometers to isolate and detect specific chemical signatures by blocking unwanted wavelengths.
Spluttering Vs Evaporation
Traditional Thermal Evaporation
- Voids in coating.
- Poor environmental performance.
- High shift with temperature change.

Open structure of traditionally evaporated coatings with gaps and voids.

Sputter Deposition
- Voids eliminated.
- Excellent environmental performance.
- Extremely low temperature shift.

Densely packed sputtered coating with no gaps
or voids.

Coating Durability:
We carry out the following MIL-C-48497A tests on samples from every coating run to ensure all coatings are durable and will stand the test of time.
Adhesion Test:
The adhesion test is a guide as to how well the coating is bonded to the substrate. The test involves pressing ‘Scotch’ tape against the coating and then pulling it off, it should stay firmly attached and show no signs of removal.
Humidity Resistance Test:
The humidity resistance test assesses the performance of the coating in an atmosphere of 95% relative humidity for a 24 hour period. There should be no change in optical performance or physical integrity.
Mild Abrasion Test:
The mild abrasion test evaluates the resistance of the coating to surface wear. The filter is subjected to a dry rubbing cloth for 50 strokes. After a clean with acetone, there should be no evidence of marking visible when viewed with the naked eye.
Severe Abrasion Test:
The severe abrasion test is similar to the mild abrasion test but involves more aggressive abrasion conditions to simulate harsher wear scenarios. The coating is subjected to 10 strokes of abrasion with the Mil Spec abrader which is impregnated with grit. This test is particularly important for filters used in rugged environments or applications with high mechanical stress.
Cleanability Test:
The cleanability test ensures the filter can be cleaned with standard laboratory solvents such as propan-2-ol (IPA) and acetone. The coating should survive 10 minutes immersion in each solvent.
Water Solubility Test:
The filter is immersed in water for 24 hours and it should comfortably survive this with no signs of damage or delamination.
Notch Filter Key terms explained:
Transmission Bands:
Notch filters will have a minimum of 2 bands, either side of a blocked region and a Transmission value absolute or average will be assigned to each passband.
Half Power Point:
This is sometimes referred to as the 50% point and is measured in nm. It is the wavelength at which the transmission is half of the peak transmission of each sideband (If Peak transmission was 80% the half power point would be at 40% transmission point).
Passband Average Transmission:
This can be defined as the average transmission, in the passband, between the two points where transmission exceeds 80% of the peak value.
Notch/Blocking Range/Minimum Transmission:
This is where the filter needs to have a low transmittance percentage (or high Optical Density) to prevent the detector from picking up any signal in this region. It can be any width from very narrow of a few nm to a wide range of over 1000 nm.
Cut on Wavelength:
This refers to the wavelength at which a filter starts to transmit light. This can be specified at any percentage but is often between 1 and 10%. The diagram below shows it at 5%.
Cut off Wavelength:
This refers to the wavelength at which a filter stops transmitting light. This can be specified at any percentage but is often between 1 and 10%. The diagram below shows it at 5%.
Slope of a filter Left Hand side:
The left-hand slope is calculated as follows, (ʎ 80% of Peak T in nm-ʎ 5% of Peak T in nm)*100/ʎ 5% of Peak T in nm, see the diagram below.
Slope of a filter Right Hand side:
The right-hand slope is calculated as follows, (ʎ 80% of Peak T in nm-ʎ 5% of Peak T in nm)*100/ʎ 5% of Peak T in nm, see the diagram below.
Ripple:
This normally refers to the allowed variation between maximum and minimum in the pass band of a filter. For example, ripple variation to be< +/-5% form the passband average is a typical requirement.
Notch Filter Terms Explained Graph:
