What is a narrow band filter?

For fluorescent samples (like quantum dots) the Zetasizer has an option that can minimize the effect of (incoherent and thus undesirable for dynamic light scattering) fluorescent light: the narrow band filter eliminates most light that is not very close to the laser wavelength.

Narrow band filter – how it works and specifications

The narrow band filter is an optical component. It consists of glass with a special coating to reflect light outside the designated wavelength range. Therefore it only allows a select wave length range (in the illustration, the red light) for transmission.

In the Zetasizer, the narrow band filter sits in front of the detector. In this way, it experiences only very low light flux intensities. For example, the standard Zetasizer Nano has a HeliumNeon laser with wavelength 632.8 nm . The corresponding narrow bandfilter has a center wavelength of 633 nm with a tolerance of ±2.5 nm. The bandwidth of this filter is 10 nm with a tolerance of ±2.5 nm. The typical peak transmittance of this filter (not necessarily occurring at the center wavelength) is 75%.

Estimates of transmittance near the passband

• 10% of peak        at 633 nm ± 7 nm    [i.e. 626 nm and 640 nm]
• 1% of peak          at 633 nm ± 10 nm  [i.e. 623 nm and 643 nm]
• 0.1% of peak      at 633 nm ± 15 nm  [i.e. 618 nm and 648 nm]
• 0.01% of peak    at 633 nm ± 22 nm  [i.e. 611 nm and 655 nm]

The graph above shows the transmission profile.  The narrowband filter blocks light at wavelengths other than the center wavelength of 633 nm. This means that only a narrow spectrum can pass.  This increases the quality of the detected light, but slightly reduces the number of photons (by up to about half). We recommend the narrow bandfilter when fluorescence is a concern.

NB: For zeta potential measurements, the majority of the light to the detector is a portion of the laser beam. The laser beam light is typically much stronger than any fluorescent component from the sample. Thus, for electrophoretic light scattering a narrow bandfilter is not necessary. For an example see this application note on quantum dots.

Previously

• How many scientists at Malvern?
• 50,000 Zetasizer references
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• Pittcon short courses offered by Malvern scientists

If you have any questions, please email me at ulf.nobbmann@malvern.com. Thanks! While opinions are generally those of the author, our editorial team may have modified some parts.