Every photon is potentially precious, but nevertheless there are some that you don’t want anywhere near your sample or your detectors! Or maybe you want different photons to end up in different places, whatever that may actually mean. The goal of course is to conserve as many as possible of the photons that you do want, while rejecting as many as possible of the ones you don’t. This brings us to the fascinating world of filters and beamsplitters, where, thanks to the ingenuity of people like our friends at Chroma Technology, almost anything is possible.

The same applies to beamsplitting components, which are generally but incorrectly referred to as “dichroic mirrors”, or just “dichroics” for short. Dichroism actually refers to polarisation dependence, which is something you don’t want for many applications, although many “dichroics” do inadvertently live up to their name by their wavelength-dependence also varying somewhat with polarisation! But here again the performance can be improved by careful design, as Chroma have also done here.

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Beamsplitters work by transmitting some wavelengths and reflecting others, so they are effectively wavelength- and/or polarisation-dependent switches. As referral to the Chroma website will show, beamsplitters with multiple transmissive and reflective wavebands have been devised for a variety of specific applications, and as they share Cairn’s passion for “tinkering”, they are likely to respond positively to any suggestions for yet further variants!

So, it all comes down to optimising the selectivity, and this is something that Chroma are very good at. Thanks to their mastery of the technology of multilayer interference films, filters of almost arbitrary characteristics can be designed. Transmission levels on the order of 90% within the desired passband are typically achieved, with out-of-band rejections of typically several orders of magnitude (and perhaps more than that), combined with cutoff regions maybe just a few nanometers wide. They have a huge range of “stock” components, details of which are of course to be found on Chroma’s very comprehensive website, but European customers in particular may well be pleased to note that many of these are also stocked at Cairn, for whom it may therefore be quicker and/or easier to obtain them from us. Shipping costs from us may well be lower too, of course.

In fact, one of their product lines here was the direct result of a suggestion by Cairn. Before “infinity” fluorescence microscopes came widely into use, the “dichroic” that reflected the excitation light and transmitted the emission light was in the focussed imaging pathway from the objective. Since it was in this pathway at a 45 degree angle, its refractive index caused a degree of astigmatism, which was minimised by making it thin, typically just 1mm or so. However, “thin” things have a tendency to be less flat, which can have nasty implications for the reflected pathway. For the fluorescence excitation application that doesn’t matter, but in applications – such as in our image splitters! – where the reflected pathway is also an imaging one, any curvature of the reflective surface (whether within an “infinity” space or not) will have an unwanted focussing effect, resulting in image distortion. However, since we are generally in an infinity space here, we’ll be better off using a thicker and hence potentially flatter substrate, hence Chroma’s range of 2mm “ultraflat” components, for which the performance improvement is genuinely noticeable. Thank you Chroma!