A cube containing the filters and mirror used in epi-fluorescence microscopy to separate fluorescence excitation and emission light
The function of a filter block is to separate fluorescence light returning from the specimen from the light used to excite the specimen so that the fluorescence light can be observed on a dark back ground. The filter block contains an excitation filter, dichroic beamsplitter (mirror) and a barrier / emission filter:
The excitation filter, usually a bandpass filter, allows only wavelengths of light necessary for excitation to pass through to the specimen.
The emission / barrier filter separates fluorescence emanating from the specimen from other background light.
The dichroic mirror separates excitation light from fluorescence by reflecting light of one range of wavelengths and transmitting only light of another range of wavelengths.
The excitation filter, barrier filter and dichroic mirror need to be matched to the excitation and emission characteristics of the fluorescent probe to ensure a high signal to noise ratio between the fluorescence and background light. The ideal combination of barrier filters and excitation filters is one that lets no light pass when combined. However, most filter combinations are not 100% efficient in preventing stray light from reaching the eyes / detector(s). Nikon's proprietary 'Noise Terminator' technology reduces stray light from filter blocks to improve signal-to-noise ratios.
Filter blocks are used in all epi-fluorescence imaging applications. The choice of a specific filter block will depend on the fluorescent probe(s) being used in the imaging application. For a quick and simple-to-use Nikon filter block selector matched to specific fluorescent probes visit:
Introduction to fluorescence microscopy: [microscopyu]
The filter block is housed in a filter cube holder that can be rotated or moved sideways to select the appropriate filter set for imaging. Different microscopes accommodate varying numbers of filter blocks.