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 made out of glass with a special coating to reflect light outside the designated wavelength range. Only a select range (in the illustration, the red light) can be transmitted.

In the Zetasizer, the narrow band filter is placed in front of the detector, so it experiences only very low light flux intensities. The standard Zetasizer Nano has a HeliumNeon laser with wavelength 632.8 nm . The corresponding narrow band filter 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]

This transmission profile is shown in the graph below.  Light at wavelengths other than the center wavelength of 633 nm is blocked by the narrow band filter, meaning only a narrow spectrum can pass.  This increases the quality of the detected light, but slightly reduces the number of photos (by up to about half). We recommend the narrow band filter to be used when fluorescence is a concern.


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