Excitation and Emission Filters of an Inverted Fluorescence Microscope: Their Ultimate Purpose
Microscopes that have frames in inverted styles are primarily designed for applications relating to tissue culture. They are capable of creating illumination through fluorescence through two kinds of pathways: optical and episcopic pathways.
Epi-illuminators are normally composed of a laser system, a lamphouse with xenon or mercury, attached to a port found at the back of a microscope’s frame. The illumination coming from the lamp goes through collector lens and then into a filter cube that possesses interference filters. Interference filters may include barrier filter, dichroic mirror, and excitation and emission filters of an inverted fluorescence microscope. After the specimen excitation, secondary fluorescence is then collected through the objective.
Essential Requirements of Fluorescence Microscopes
Almost all types of fluorescence microscopes—inverted microscopes included—utilize objective lens for two purposes:
1. Collect fluorescence light emitted.
This kind of configuration for excitation and emission—where both emission and excitation light goes through the objective—is known as epifluorescence. The optics principle of the epifluorescence microscope is to separation excitation or illumination light from emission light coming from the sample. To collect an emission image without extreme illumination background, the optical elements used to separate two components of light should be highly efficient. The same optics should be used when measuring fluorescence emission minus background “noise.”
2. Concentrate the excitation or illumination light to the specimen.
To excite fluorescent light in the sample, the optical elements of the fluorescent microscope should concentrate the excitation or illumination light to the sample to a much greater extent accomplished by conventional condenser lenses you can see and used as illumination light path of a standard microscope.
Dichroic Mirror
In an inverted fluorescence microscope, the device being used to separate both emission and excitation light paths is called dichroic mirror. This is usually found at the bottom of the sample. The sample then is imaged and illuminated from below the specimen platform or stage.
Two Light Paths being Separated
There are two kinds of light paths being separated by a dichroic mirror: The excitation light is being reflected into the objective from dichroic mirror’s surface while the emission light path gores through the dichroic mirror and into the detection system or the eyepiece.
The unique reflective properties of a dichroic mirror make it possible for the component to divide the light paths. Each of them has its own wavelength value sets referred to as transition wavelength value. Light wavelengths of below transition value are reflected into the mirror, which, in turn, transmits light in above its wavelength value. Normally, the dichroic mirror’s wavelength is selected to be along the lines of wavelengths utilized for emission and excitation.
One of the primary components of a fluorescence microscope, the dichroic mirror, however, is not capable of performing all the necessary optical functions independently. Usually, only 90 percent of the wavelength of light belonging to below transition wavelength value is being reflected while 90 percent above the value is being transmitted through the dichroic mirror.
Once the excitation light will illuminate the sample, a little portion of the excitation light is reflected from the elements of optics within the objective. Some of the excitation light can be transmitted by the dichroic mirror together with the longer light wavelength coming from the sample.
Contaminating light can definitely find its way into the detection system. However, you can prevent that by using another selective element for your inverted fluorescence microscope: the emission filter.
Emission and Excitation Filters
The dichroic mirror functions together with emission and excitation filters. The excitation filter helps select the excitation wavelength. It is placed before the dichroic mirror. The emission filter, on the other hand, selects emission wavelengths coming from the sample, then takes away any excitation light traces. It is found after the dichroic mirror.
These filters belong to what is commonly known as interference filters. They have the capacity to block any form of out-of-band transmission. They also exhibit a very low transmission that is out of their natural bandpass. Hence, they become very effective in choosing the most appropriate emission and excitation wavelengths.
Filter Cube
A filter cube is an optical block from which the dichroic mirror is usually mounted. It also contains the two types of filters, namely, the emission and the excitation filters. This cube gives you a convenient method of changing the dichroic mirror without having to directly handle the filters or the mirror.
Indeed, excitation and emission filters of an inverted fluorescence microscope are highly essential if you want to view or capture a specimen’s image in its utmost clarity and sharpness.