Partial filtering

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Partial filters, produced by cementing optical colored glass filters onto partial areas; Use only in high quality photometer measuring heads.
Full filter , produced by cementing optical colored glass filters over the entire surface; Use in regular photometer measuring heads.

The partial filtering (English. Mosaic filtering) represents an extension of the full filtering of photometer measuring heads . In addition to the simulation of the active function " photopic " can also be produced " scotopic ". The motivation lies in the possibility of further improving the f1 'error. While an adjustment to f1 '( ) = 1.5% is possible in full filtering, the error of the spectral matching (spectral mismatch) can be reduced to approx. F1' ( ) = 0.5% in partial filtering . The resulting advantage (reduction of the measurement uncertainty) outweighs the additional manufacturing costs in special cases.

Filter structure

After a coarse filtering (identical to full filtering ), very small glass particles with an individual volume of a few cubic millimeters are cemented onto the coarse filter , whereby the number / volume, size, orientation and position of the glass particles and the spectral transmittance of the glass particles are critical. This manual activity requires forward thinking and intuition. Since the glass particles only have a local effect, the degree of transmission of the coarse filtering can be partially (spectrally) influenced and thus improved. The finished optical filter must be provided with a diffusing screen in order to prevent uneven (inhomogeneous) illumination of the partial filters.

Typical advantages

  • excellent f1 'fit (relative spectral fit)
  • minimized measurement uncertainty (with significantly reduced additional costs in subsequent process steps)

Typical disadvantages

  • lower sensitivity due to the need to use the homogenizing diffuser
  • higher costs due to additional material expenditure
  • higher costs due to individual manufacture / construction of the measuring head

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