Fluorescence Quantum Yield (Phi)
Definition
The fraction of absorbed photons that result in fluorescence emission (0 to 1); determines the brightness of a fluorescent signal and is a key parameter in ALS method validation.
- Symbol
- Phi (quantum yield)
- Range
- 0 to 1 (fraction of absorbed photons re-emitted as fluorescence)
- Determines
- Brightness of the fluorescent signal
- Forensic relevance
- Key parameter in alternate light source (ALS) method validation
Common questions
Why does a high quantum yield matter for choosing an ALS wavelength in casework?+
A fluorophore with a low quantum yield emits weakly even under strong excitation, which can make trace evidence like fibres or bodily fluids hard to detect against background. Validating an ALS protocol includes confirming the target evidence class has a quantum yield high enough to produce a visible signal at practical exposure settings.
Does a quantum yield of 1 mean every absorbed photon is visible to the examiner?+
It means every absorbed photon is re-emitted as a fluorescence photon, but not all of that emitted light necessarily reaches or is detected by the examiner, since factors like emission direction, filter transmission and instrument sensitivity also affect what is actually observed.
Why do some absorbed photons not produce fluorescence at all?+
The remaining fraction, 1 minus phi, is lost through competing non-radiative decay pathways such as heat release or energy transfer to nearby molecules, rather than being re-emitted as light.
Related terms
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- Chromophore
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- Fluorescence Lifetime
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- 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,...
- Kubelka-Munk Model
- A diffuse-reflectance theory relating the measured reflectance of a scattering-plus-absorbing layer to its absorption and scattering coefficients; used in forensic paint and...
- Molar Extinction Coefficient (ε)
- The intrinsic absorptivity of a molecule at a given wavelength, in L/(mol·cm); oxyhaemoglobin at 415 nm has ε ≈ 125,000, making it...
- Phosphorescence
- Light emission from the triplet excited state (T1) via a spin-forbidden radiative transition; characterised by emission persisting after the excitation source is...
- Raman Scattering
- Inelastic scattering of a photon from a molecular vibration; the scattered photon carries a frequency shift equal to the vibrational frequency, producing...
- Specular Reflection
- Mirror-like reflection from a smooth surface where angle of incidence equals angle of reflection; minimised in forensic photography with crossed polarisers to...
- Stokes Shift
- The wavelength difference between the excitation peak and the emission peak of a fluorophore. A positive Stokes shift (emission at longer wavelength...