Filter Hints from Experts: Troubleshooting Signal Interference in Optical Systems
Beam distortion, ghost images, disturbing background signals or background radiation are problems that are perceived as very disturbing by users in optics and photonics, fluorescence microscopy, high-resolution microscopy or in the development of handheld systems.
By Dr Ingrid Feuerbacher, AHF analysentechnik AG
Optical systems can consist of a combination of individual components such as bandpass, short pass, long pass, clean-up filters, beam splitters or dichroic filters (‘dichroics’). These must be selected appropriately and well matched to each other to achieve the desired functionality. Although the simulation of beam paths is carried out in advance in the R & D departments, unexpected effects occur during the transfer from theory to practice.
Note the angle of incidence
The user of optical filters must first be aware that the specification of the spectra is always recorded and displayed under an angle of incidence of zero degrees, usually with random polarized light. As soon as the angle of incidence deviates by about 5 degrees, the spectral characteristics of the filters change in such a way the spectrum shifts to shorter wavelengths. The blocking properties of the filters also change.
In contrast to excitation filters, emission filters or clean-up filters, beam splitters or dichroics are always used at an angle not equal to zero degrees. As a rule, the angle is 45 degrees, but there are also systems in which, for example, beam splitters with 11.25 degrees are used (octagon systems). The displayed beamsplitters or dichroic spectra on our website are always taken at an angle of 45 degrees.
Beam splitters don't replace blocking filters
In general, a beam splitter also differs from a band-pass filter in that it has no blocking properties. The difference between maximum and minimum transmission is about two size units. Therefore, additional blocking filters must always be installed in front of the detector. The beam splitter does not replace a blocking filter! This is often misjudged. Since in practice, especially with miniaturized systems, the light path cannot be parallelized as in the case of a complete microscope setup, different spectral properties must always be expected.
If the angles are sufficiently known, it may be possible to calculate back by simulation which edge the filter must have in order to ensure that the transmission and blocking properties are still fulfilled under real conditions.
Helpful trick: Use color glass filters
Another ‘trick’ can be the use of color glass filters. In contrast to interference filters, these are not dependent on the angle of irradiation. They retain their transmission properties even under oblique incident light.
At first sight very clever, but unfortunately these filters have the disadvantage that they show autofluorescence themselves. This is not acceptable for highly sensitive systems. Furthermore, these colored glasses are by far not as steep-edged as interference filters and the selection is smaller. Nevertheless, in some systems a very clever solution can be achieved by combining color glass and interference filters.
Small measure, big effect: Clean-up filters
Another reason for unwanted background radiation is the assumption that LEDs, for example, have a clearly defined spectrum. A very effective means of limiting spectral fluctuations in LEDs is to use so-called ‘clean-up’ or excitation filters. They provide a clear limitation of the spectral window in which the LED light source illuminates.
This small measure often proves to be very helpful, especially in fluorescence applications. Once the upper edge of the excitation filter is clearly defined, the lower edge of the blocking filter or emission filter can be defined more clearly.
Check the filter's blocking
For fluorescence measurements it is always desirable that the excitation filter and emission filter are spectrally separated from each other to such an extent that no excitation light can appear in the detector. Unfortunately, the term ‘no’ does not really exist – one speaks of orders of magnitude. If the rejection filter has a transmission of 0.0001% in the spectral range of the excitation filter, then this rejection filter has a blocking of OD6 (OD= -log T), whereby the transmission of the rejection filter is assumed here to be approx. 95%. The examination of the blocking data is always a focal point in the evaluation of optical systems.
We’re happy to advise you
The team of AHF analysentechnik will be pleased to support you in the selection and specification of the appropriate filters for your optical system and application. We can also provide you with data under different beam angles of filters, as well as under different ‘cone-half’ angles. Transmission data and blocking data over a wide wavelength range can also be shown. Furthermore, the transmission and blocking data can be displayed for s-polarized light, p-polarized light and randomly polarized light.