Narrowband Filter Selection Guide: How to Determine Center Wavelength, Bandwidth, and Cut off Depth
In recent years, I have received the most inquiries, all starting with the same sentence: "Please recommend a narrowband filter for me." However, narrowband filters are not recommended, they are calculated. There are only five parameters: center wavelength, bandwidth, cutoff depth, incident angle, and size. Speak clearly one by one, and after reading it, you can make your own decision.
1、 Center wavelength: Follow the light source, do not follow the directory
The center wavelength is determined by your light source or detection target, not by selecting a good-looking one from the manufacturer's catalog.
Three common scenarios:
Fluorescence detection. Align the center wavelength of the emission plate with the emission peak of fluorescein, and align the excitation plate with the excitation peak. For example, FITC and other commonly used fluoresceins have an excitation peak around 490nm and an emission peak around 520nm. To ensure that the excitation light and emission light do not intersect, a 470nm excitation plate and a 520nm emission plate are used. The emission spectrum of fluorescein is included in the data manual, don't guess.
Laser system. For laser cleaning or laser protection, align the center wavelength directly with the output wavelength of your laser -1064nm fiber laser is equipped with 1064 ± 2nm, with tolerance left according to the wavelength drift of the laser.
Infrared monitoring. 850nm and 940nm are two mainstream infrared bands. It depends on which infrared light bulb you are using. If you install the wrong band, the night vision image will turn black directly. We have encountered this mistake more than once.
2、 Bandwidth: Narrower is not better
Bandwidth refers to the full width at half maximum (FWHM) of the passband. The first reaction of many engineers is that the narrower the better, which is a typical misconception.
The correct algorithm for bandwidth: Take 1.5 to 2 times the spectral width of your signal.
Why leave a margin? Two reasons. One reason is that the narrower the bandwidth, the lower the light flux within the passband - if you buy narrowband chips at a higher price, the imaging may become darker and the signal-to-noise ratio may decrease instead of increasing. The second reason is that the emission peak of fluorescein drifts in different environments, causing the bandwidth to be too stiff and the signal to disappear outside the passband.
On the other hand, a bandwidth that is too wide is also not acceptable: stray light follows in and the background cannot be suppressed. The conventional method for fluorescence detection is 10-30nm; for spectrometers or Raman spectroscopy that require precise separation of spectral lines, narrow specifications below 10nm are needed. In most application scenarios, "appropriate" is more valuable than "extremely narrow".
3、 Cut off depth: calculated backwards based on background light intensity
The cut-off depth is represented by OD value, where OD4 is the attenuation of light outside the passband to one ten thousandth. Don't pat your head when setting OD, push back:
- Conventional fluorescence detection, excitation light is the main source of stray light, starting from OD6
- Laser system, strong light environment, background light several orders of magnitude stronger than the signal, starting from OD6
- For ordinary imaging and security applications, OD3~OD4 is usually sufficient
What are the consequences of light leakage? The background of the fluorescent image appears white and green, with weak signals completely submerged. Some people think it's because the camera sensitivity is not enough, changing the camera, adding exposure, and tinkering around, but in fact, a filter with insufficient OD is the root cause.
4、 Incident angle: the most easily missed one
The interference filter is coated according to the designed incident angle, and when the angle deviates, the center wavelength shifts towards shorter wavelengths. The dichroic mirror used at 45 ° and the narrowband plate used for parallel light are completely different film systems.
The measured data can refer to: a narrowband chip designed at 0 ° has a center wavelength drift of approximately 2% when tilted to 15 °. When placing an order, please clearly state whether your light path is parallel light or converging light, and what the maximum half angle is. Attention should also be paid to the converging light path: light from different angles passing through the same filter has different center wavelengths, and the passband will be "widened". The bandwidth index should be re evaluated according to the angle range.
5、 Temperature drift and size: to be clarified before delivery
Warm float. The typical temperature drift of the hard film process (magnetron sputtering with ion assistance) can be controlled within 0.02nm/℃, which is one order of magnitude higher for ordinary soft films. Outdoor equipment and high and low temperature conditions must specify the operating temperature range in the order, and the shipment inspection report should include temperature drift measurements.
Dimensional tolerance. Conventional ± 0.1mm and high-precision ± 0.02mm need to be marked on the drawing, and the prices of the two are different. The aperture of the light passage should be increased by 1-2mm according to the diameter of the light spot. Do not cut the material according to the size of the light spot - light hitting the edge of the lens will produce scattered stray light.
6、 List of parameters before placing an order (copy available)
Fill out the following form and send it to any manufacturer for direct quotation and evaluation:
- Center wavelength (or trans reflective band): ____ nm
- Bandwidth FWHM: ____ nm
- Cut off depth: OD__ (specify the cut-off band range together)
- Transmittance requirement for passband: __%
- Incident angle: 0 °/45 °/Other __ (indicate half angle for converging light)
- Dimensions and tolerances: mm ±
- Substrate preference: with or without specification (recommended by wavelength band without specification)
- Working temperature range: __ ℃~__ ℃
- One sentence for usage scenario:______
Conclusion
The selection of narrowband filters ultimately involves four steps: "determining the light source, calculating the bandwidth, reverse calculating the OD, and reporting the angle". If the parameters are uncertain, send the light source and detector models to the manufacturer and let the engineering team calculate them for you - we do this every day, and the evaluation is free of charge.
Ruicheng Optoelectronics: Established in 2010, equipped with magnetron sputtering and ion assisted hard film technology, 42 standard specifications of narrowband filters, with delivery records from UV 200nm to near-infrared 2000nm. Customized according to drawings and samples, samples can be shipped within 3-7 working days without a minimum order quantity threshold. Selection consultation: 15112422758 (Ms. Liu)/ruiccn@163.com / www.ruiccn.com