Definition:
Dual bandpass filters are optical filters designed to transmit 2 separate specific ranges or wavelengths and blocking unwanted wavelengths. These filters can be positioned anywhere in the light spectrum to suit the application. The main benefit of dual bandpass filters is the potential cost savings they can offer in optical instruments. Whilst they are more expensive than standard single bandpass filters, they allow 2 spectral regions to be analysed with only one detector. Detectors are usually the most expensive element in any system so potential savings are significant. Another benefit is that they can allow for more compact systems when space is at a premium, i.e. a system needs only one detector or removing the need for a filter wheel and the mechanical components involved.
Design and Manufacture:
At Vortex we can manufacture any custom dual bandpass filter between 1000nm to 5000 nm. We hold a wide range of substrates in stock such as Glass, Quartz, Silicon, Sapphire ready to coat at anytime helping us to offer much shorter lead times than the industry standard. Please contact us to discuss your specific requirements.
All of our dual bandpass filters are deposited using sputter deposition technology which typically has a process energy 1000 times higher than thermal evaporation. This means that all our coatings are highly durable in challenging environments as well as very stable with temperature change. More information about this can be found below.
We are also able to provide modelling data of how dual band filters perform within a system, taking into account many variables including angular range for instance.

Case Studies

Dual Band Pass Filter

Dual Band Pass Filter
Applications
Dual Bandpass filters can be used in a wide range of applications in the same way that standard bandpass filters are used, we have highlighted some of these below:
Visible Range 300 – 700 nm:

Machine Vision and Robotics

Spectroscopy and scientific analysis

Fluorescence Microscopy

Optical Communications

Security, Surveillance and Forensic Science
Near Infrared Range 700 – 2500 nm:

Industrial Monitoring

Food Quality Inspection

Bio medical Imaging

Non-Contact Temperature Measurement
Mid Infrared Range 2500 – 6000 nm:

Gas Detection and Analysis

Aerospace

Environmental Monitoring

Security and Defence
Dual Bandpass Filter Key terms explained:
Centre Wavelength:
This is the respective centre point of each narrow band section of the filter; it is calculated by adding together the appropriate two half maximum points and dividing by two. See diagram below.
Band 1 and Band 2:
Band 1 is the narrow bandpass filter on the left at the lower wavelength and band 2 is the narrow bandpass on the right of the filter at the higher wavelength.
Peak Transmission:
This is the point where the filter reaches its highest transmittance percentage.
Bandwidth/Full Width Half Maximum (FWHM):
This is the width of the filter at half of the peak Transmission. This is calculated from the difference in wavelength between the two points. See diagram below.
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.
Blocking Range:
This is where the filter needs to have a low transmittance to prevent the detector from picking up any inappropriate out of band signals. This should be the sensitivity range of the detector plus a small safety margin.
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%, this is the most common value in specifications.
Cut off Wavelength:
This is the exact opposite to cut on wavelength and 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 5% of Peak T in nm-ʎ 80% of Peak T in nm)*100/ʎ 80% 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, a typical ripple variation spec. may be< +/-5% form the passband average.
Dual Bandpass Filter Terms Explained Graph:

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 an indicator of how well adhered the coating is to the substrate. The test involves pressing 3M scotch tape against the coating and then pulling it off, there should be no coating removal.
Humidity Resistance Test:
The humidity test assesses the durability of the coating when subjected to 24 hours humidity. The filter is exposed to 95% relative humidity, there should be no delamination or damage to the coating or change in optical performance. Sputtering deposition is key to ensuring high levels of durability and physical integrity in humid environments.
Mild Abrasion Test:
The mild abrasion test evaluates the resistance of the coating to surface wear. The filter is subjected to the 50 cycles of dry rubbing with a loaded cheesecloth abrader tool. After cleaning there should be no change in appearance or coating performance. This test helps determine the filter’s ability to withstand everyday handling.
Severe Abrasion Test:
The severe abrasion test is similar to the mild abrasion test but involves 10 cycles of rubbing with a more aggressive abrader, again after cleaning the coating should show no damage or change in performance. This test is particularly important for filters used in tough environments or applications with high mechanical stress.
Cleanability Test:
The cleanability test assesses the ease of cleaning the filter surface with a range of lab standard cleaning agents such as acetone and isopropanol. In assembly filters and coatings will require cleaning, the test helps ensure the filter is comfortable with this type of handling.
Water Solubility Test:
Surviving 24 hours in water without deterioration ensures the coating is also suitable for cleaning with water-based cleaning agents without compromising performance.