One of the main components of a fluorescent microscope is better known as a dichroic mirror. This mirror uses the concept of separating the excitation and emission light path. Both of the light captured from the sample share the same optics from within the objective. The sample is actually imaged and illuminated from below the stage on an inverted fluorescence microscope.

The dichroic mirror separates the light paths in separate ways. The excitation light is reflected off the surface of the dichroic mirror and into the objective whereas the fluorescence emission passes through the dichroic mirror onto the oculars or in some other cases a digital camera or any system of image capture. The dichroic mirror has special reflective characteristics allowing the separation of the two light rays. A set wavelength value is given to a dichroic mirror commonly known as the transition wavelength value. This is the wavelength f 50% transmission. The dichroic mirror reflects the wavelengths below the transition wavelength value and at the same time transmits the wavelengths of light above this value. Thus, the name “dichroic”, meaning two colors, is given to this type of mirror. Think of it as the glass of a tainted car window which can reduce the glaring effect of the sunlight as you drive around on a sunny day. In the case of a dichroic mirror, the wavelengths it is given to is usually between the wavelengths utilized for emission and excitation.

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The dichroic mirror can be regarded as the key element within a fluorescent microscope system but the entire task in fluorescent microscopy cannot be performed by this system alone. In a distinctive environment, a small amount of both the light waves can be scattered from within the objectives as about 90% of the light at wavelengths below the dichroic mirror’s transition wavelength values are reflected and about the same percentage above the mirror’s transition wavelength values are also transmitted through it. When the excitation light illuminates the sample, tiny amounts of excitation light is reflected off at angles away from the optical elements within the objective and can tend to scatter the actual light that should enter the objective. This is where another significant element in a fluorescent microscope comes into play, the emission filters. This tool allows selective transmission of light waves entering the objectives thus eliminating the contaminating reflected light for correction. Read more on this subject