Stokes Shift
Definition
The wavelength difference between the excitation peak and the emission peak of a fluorophore. A positive Stokes shift (emission at longer wavelength than excitation) is universal and is the physical basis for separating excitation from emission in fluorescence microscopy.
- Definition
- Wavelength difference between excitation (absorbed) light and fluorescence emission light
- Direction
- Always positive: emission occurs at longer wavelength than excitation
- Physical cause
- Vibrational relaxation dissipates absorbed energy as heat before emission
Common questions
What causes the Stokes shift in a fluorophore?+
When a fluorophore absorbs light, the molecule jumps to an excited state with extra energy. Before it emits light, the excited molecule releases some of that energy as heat through vibrational relaxation. Because the molecule has less energy left, it emits light at a longer wavelength than it absorbed. That wavelength gap is the Stokes shift.
Why does the Stokes shift matter in forensic imaging?+
The Stokes shift tells you the minimum gap between the excitation wavelength and the emission wavelength. This gap is exactly what you need to design barrier filters for alternate light sources and fluorescence microscopes. Without it, the excitation light would drown out the faint emission signal you're trying to detect.
Is the Stokes shift always positive?+
Yes. A positive Stokes shift means the emitted light always has a longer wavelength (lower energy) than the excitation light. This is universal for fluorophores and is the physical foundation that makes fluorescence detection possible at all.
Related terms
- Jablonski Diagram
- An energy-level diagram showing the ground state, singlet excited states, and triplet state of a molecule, with arrows indicating absorption, internal conversion,...
- Phosphorescence
- Light emission from the triplet excited state (T1) via a spin-forbidden radiative transition; characterised by emission persisting after the excitation source is...
- Absorption
- The purification technique in which a crude antiserum is mixed with antigen preparations from cross-reacting species. Cross-reactive antibodies bind those antigens and...
- ALS (Alternate Light Source)
- An adjustable-wavelength high-intensity light source with interchangeable excitation filters, used for scene examination across UV, visible, and NIR bands; examples include the...
- Alternate Light Source (ALS)
- A device emitting light at selected wavelengths (typically 415 to 535 nm) used to detect biological fluids that fluoresce or absorb light...
- Autofluorescence
- Native fluorescence of endogenous biological molecules (collagen, elastin, NADH, flavins) that produces background signal in fluorescence microscopy, potentially mimicking specific probe signals....
- Bandpass Filter
- An optical filter transmitting a defined wavelength range (typically 20-40 nm FWHM) while blocking all others; the excitation filter in an ALS...
- Barrier Filter
- An optical long-pass filter placed over the camera lens and worn as goggles by the examiner during ALS examination; blocks the excitation...
- Beer-Lambert Law
- A = εbc. Absorbance equals molar absorptivity times path length times concentration. The linear relation between absorbance and concentration that makes UV-Vis...
- Birefringence
- The difference between the highest and lowest refractive indices of an anisotropic mineral, seen as a characteristic interference colour between crossed polars....
- Chromophore
- The molecular subunit responsible for a dye's colour, typically a conjugated double-bond system; chemical erasure destroys the chromophore while leaving the decolourised...
- Conoscopic Illumination
- A convergent polarised beam mode in which a range of angles is passed through the specimen simultaneously, producing an interference figure (optical...
Explained in these topics
- Fluorescence and Phosphorescence SpectroscopyThe wavelength gap between the absorption maximum and the emission maximum. It exists because vibrational relaxation in S1 dissipates part of the absorbed ener...
- Forensic Light Sources and Alternate Light ExaminationThe wavelength difference between the excitation maximum and the fluorescence emission maximum of a fluorophore; determines the minimum filter gap needed to se...
- Light-Matter Interaction: Absorption, Reflection, FluorescenceThe wavelength difference between the excitation (absorption) maximum and the fluorescence emission maximum of a fluorophore; the physical basis for the ALS ba...
- Polarising and Fluorescence MicroscopesThe wavelength difference between the excitation peak and the emission peak of a fluorophore. A positive Stokes shift (emission at longer wavelength than excit...
- Specialised Imaging: UV, IR, Laser and ALSThe difference in wavelength between the absorbed excitation light and the emitted fluorescent light; the shift makes fluorescent emission detectable through a...