Filter-based Multimode Microplate Reader Technology
Filter-based multimode microplate readers (Figure 1a) usually incorporate several sets of optical filters. White light is passed through a filter on the excitation side, which typically transmits at least 60% of the desired wavelength in the visible range to the sample well.
The light excites the sample, which in return emits a specific fluorescent signal according to its unique properties. An emission filter, coupled with a dichroic mirror in some cases, cleans the sample’s signal and typically transmits at least 60% of the desired wavelength to the detector. The high transmission efficiency on both the excitation and emission channels provides high sensitivity.
There are several advantages of filter-based readers when compared to monochromator-based microplate readers. First, filter-based readers are typically less expensive than monochromator-based microplate readers. Filter wheels or slides are less expensive components than monochromators, and the light source required to produce the same level of sensitivity does not need to be as powerful.
The second important advantage is that of sensitivity; a filter-based reader is more effective at delivering light to the sample and light blocking between the excitation and emission channels for superior sensitivity. Bandwidth selection is an advantage as a filter can be dedicated to a specific assay for maximum sensitivity and can have a bandwidth from a few nanometers to greater than 100 nm, which is necessary for low-level fluorescence assays.
Finally, filter-based microplate readers can rapidly switch between two wavelengths, or be designed with two measurement channels, for ratiometric-based assays while monochromator-based systems are typically much slower.
A disadvantage of filter-based microplate readers is that separate filter sets must be purchased and maintained for separate applications. Accordingly, the fixed wavelength of a filter also disallows the use of spectral scanning applications.