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Illumination and Filters Foundations of Microscopy Series Amanda Combs Advanced Instrumentation and Physics
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Foundations of Microscopy2 Luminescence Emission of light from an excited electronic state Requires the absorption of a photon There are 2 types of luminescence Fluorescence Phosphorescence
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Foundations of Microscopy3 Absorption E photon > E transition Absorption is followed immediately by vibrational relaxation Occurs on the order of femtoseconds Use of light pulses on the order of fs can result in the absorption of more than one photon
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Foundations of Microscopy4 Fluorescence Emission from an excited singlet state E fluorescence < E absorption due to vibrational relaxation Spin of excited electron remains unchanged S 1 S 0 is an allowed transition Has a lifetime on the order of nanoseconds
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Foundations of Microscopy5 Intersystem Crossing The spin of the excited electron can ‘flip’ resulting in a move from the Singlet excited state to the Triplet excited state A relaxation process, not an emissive transition
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Foundations of Microscopy6 Phosphorescence Emission from an excited triplet state to the singlet ground state T 1 S 0 is not an allowed transition Has a lifetime on the order of milliseconds to seconds due to forbidden nature of the transition E phosphorescence < E fluorescence
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Foundations of Microscopy7 Phosphorescence
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Foundations of Microscopy8 Fluorescence Spectra Not a significant change in nuclear separation between ground state and first excited state Overlap between ground state and excited state vibrational levels doesn’t change significantly upon excitation Results in spectra that are nearly mirror reflections
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Foundations of Microscopy9 Fluorescence Spectra Emission spectrum is independent of the excitation wavelength because of rapid vibrational relaxation The spectral peak refers to the most probable transition Excitation at peak wavelength is most efficient No need to excite only at the peak
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Foundations of Microscopy10 Excited State Lifetime Average amount of time a fluorophore spends in an excited state Depends on the selection rules for the transition (allowed versus forbidden) back to the ground state Depends on the number of possible relaxation pathways The more non-radiative pathways possible, the shorter the fluorescence lifetime = 1/(k r +k nr )
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Foundations of Microscopy11 Quantum Yield A measure of the fluorescence efficiency The ratio of the number of photons emitted to the total number of photons absorbed Q=k r / (k r + k nr ) Q 1 as k nr 0 essentially every photon being absorbed is going towards fluorescence; no loss of fluorescence due to nonradiative decay
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Foundations of Microscopy12 Photobleaching Permanent loss of luminescent ability The triplet state can react to form new products Due to the highly reactive nature of the triplet configuration as well as the long lifetime of the triplet excited state
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Foundations of Microscopy13 Detecting Fluorescence The correct combination of filters is required to separate the fluorescence signal from the excitation light There are 3 important types of filters to consider Long pass / Short pass filters Bandpass filters Dichroic beamsplitters
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Foundations of Microscopy14 Bandpass Filters Allows a well defined range of wavelengths to transmit Other wavelengths are absorbed by the filter Called BP535/40 Bandpass filter Centered at 535 nm FWHM of 40 nm Allows 515 nm-555 nm to transmit
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Foundations of Microscopy15 Short and Long Pass Filters Allow wavelengths above (long pass) or below (short pass) a threshold value to transmit while the other wavelengths are absorbed Long pass version called LP515 Allows wavelengths greater than 515 nm to transmit (pictured) Short pass version called KP515 Allows wavelengths smaller than 515 nm to transmit (not pictured)
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Foundations of Microscopy16 Dichroic Beamsplitters Beamsplitters transmit and reflect light intensity according to some parameter Dichroics divide the light intensity according to color Transmit a range of wavelengths and reflect a range of wavelengths Plot shows only transmission 505 nm are transmitted through the optic Called FT505
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Foundations of Microscopy17 Choosing the Appropriate Filter Set Alexa 488 for example Excitation Filter: BP485/15 Dichroic: FT505 Emission Filter: BP530/40
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Foundations of Microscopy18 Fluorescence Filters in a Microscope Filter cubes are used in a microscope Excitation and emission filters can be either band pass or short/long pass Dichroic beamsplitter reflects the excitation light but transmits the emission light
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Foundations of Microscopy19 Stimulated vs. Spontaneous Emission Fluorescence is an example of spontaneous emission Directionally random Not dependent upon state populations Lasing is a result of stimulated emission Directional Requires a stimulating field Dependent upon the excited state population
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Foundations of Microscopy20 Continuous Wave Lasers 4 level system provides continuous lasing Can use electricity, light or a chemical reaction to pump Requires a population inversion of the lasing transition Excited state population is greater than ground state population Narrow lasing bandwidth due to discrete lasing level The cavity length takes stimulated emission to lasing Requires the existence of a standing wave (L=n/2)
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Foundations of Microscopy21 Pulsed Lasers Pulses come from the interference of wavelengths from the range of transitions The addition of more wavelengths (transitions) makes a shorter pulse in time Tunability comes from changing cavity length to ‘choose’ a transition
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Foundations of Microscopy22 Illumination--Halogen Lamp Used for bright field imaging Smooth spectrum provides nearly uniform illumination Not a good illumination source in the UV
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Foundations of Microscopy23 Illumination—HBO Lamp Peaks can give good excitation for certain dyes Must consider spectral structure to make quantitative conclusions
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Foundations of Microscopy24 Illumination—XBO Lamp More uniform illumination than the HBO May not excite as efficiently as HBO for some dyes
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Foundations of Microscopy25 Lamp Comparison with DAPI and FITC DAPI FITC
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Foundations of Microscopy26 Upcoming Seminars Schedule Friday, October 13th: No seminar Foundations of Microscopy: Winfried Wiegraebe, "Non-Linear Optics " Friday, October 20th from 1:00 - 2:00 p.m. in room 421 Foundations of Image Processing: Christopher Wood, "De- Convolution " Friday, October 27th from 1:00 - 2:00 p.m. in room 421 Foundations of Microscopy: Winfried Wiegraebe, "Fluorescence Lifetime Imaging Microscopy (FILM)“ Friday, November 3rd from 1:00 - 2:00 p.m. in room 421 FCS User Club: Joseph Huff, "Characterization of Fluorescent Proteins by FCS " Friday, November 10th from 1:00 - 2:00 p.m. in room 421
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