Refractortelescopes

Dichroic mirror (DM) The dichroic mirror is the optical element that separates the excitation light from the fluorescence. Dichroic mirrors are special mirrors that reflect only a specific wavelength of light, allowing all other wavelengths to pass through (Figure 3). Dichroic mirrors used in epi-fluorescence microscope filter blocks are placed in a 45?incidence angle to light, creating a "stop band"of reflected light and a "pass band"of transmitted light. Light passing through the excitation filter is reflected 90?toward the objective and the specimen. Finally, light emanating from the specimen is passed through and directed toward the observer (or high-sensitivity camera). Barrier filters (BA) or emission (EM) filters Barrier filters are optical elements that separate fluorescence emanating from the fluorophore from other background light. As shown in Figure 4, the barrier filter transmits light of the fluorescence wavelength which passes through the dichroic mirror while blocking all other light leaking from the excitation lamp (reflected from the specimen or optical elements). This is necessary because the strength of the fluorescent light is weaker than the excitation light by a factor of more than 100,000:1. Most barrier filters are positioned at a slight angle to allow better fluorescent imaging by suppressing ghost images. Nikon's noise terminator Nikon's epi-fluorescence microscopes have a proprietary Nikon optical element called a “noise terminator?(on models TE2000 and later). As shown in Figure 1, the noise terminator allows efficient processing of light transmitted through the dichroic mirror by preventing the excitation light from scattering within the filter block and leaking into the observed image. This helps to eliminate background light noise and more effective fluorescent images.

As shown in Figure 1, filter blocks are constructed from 2 types of filters and 1 dichroic mirror. Selecting appropriate filters and mirrors for each use allows researchers to attain a high signal to noise (S/N) ratio between the fluorescence and background light. The functions of these optical elements are described below.

Telescope lens

Excitation filter (EX) The excitation filter is the optical element that passes only the wavelength of light necessary for excitation from the excitation light source (usually a mercury lamp) to the fluorophore. As shown in Figure 2, only the excitation wavelength passes through the filter, usually a "band pass filter".