Full filtration

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

The full filtering is a production type of optical filters , mainly in the photometer are used -Messköpfen. It differs significantly from partial filtering , which is used far less often. The filter itself is in communication with a Si - photodiode used as the light or radiation receiver. Together they enable, for example, the measurement of the irradiance in lux or similar quantities. From a mathematical point of view, this is essentially a convolution of the (spectral) transmittance of the (cemented) glass filters and the (spectral) absorption of the photodiode.

Use on the example of the light intensity measuring head: Filters influence the spectral characteristics of the measuring head, primarily those of the detector. While a light intensity measuring head consists of a Si photodiode, a full filter, possibly a diffuser, possibly thermostatic control and a housing, all components between the emitter (e.g. light source) and the Si photodiode determine the spectral characteristics. The aim of the adaptation (number, type and thickness of the glass filters) is to come as close as possible to the requirements of the CIE with regard to the spectral adaptation and to minimize the error (spectral mismatch). The quality of the adaptation (the so-called f1 'error) of the measuring head directly determines the quality of the measuring head and thus the measurement result ( uncertainty of the measurement result ).

construction

Filter packages created with full filtering consist of 1 to 4 types of glass of different thicknesses, which are cemented and are larger than the diameter / area of ​​the downstream Si photodiode. For example, photodiodes with a diameter of approx. 11.3 mm are used in conjunction with a 15 mm (free aperture) full filter to produce photometer heads with an active area of ​​1 cm². The types of glass are typically Schott colored glass filters which, in interaction with the photodiode , result in the best possible adaptation to the effective function to be achieved .

Typical advantages

  • higher sensitivity due to the omission of the homogenizing lens
  • lower costs due to less material
  • lower costs due to identical (reproducible) structure

Typical disadvantages

  • less good f1 'fit (typ. 4%, better 1.5% mismatch)
  • higher measurement uncertainty (with considerable additional costs in process steps)

Web links