Fluorescence
Definition
Light emission following absorption of light at a shorter (higher-energy) wavelength. Fluorescein absorbs blue-green excitation light and emits yellow-green light. The emitted wavelength is always longer than the excitation wavelength.
- Common excitation wavelengths
- 450 nm (blue light) is widely used in forensic detection
- Typical emission lifetime
- 1 to 100 nanoseconds
- Wavelength rule
- Emitted light is always longer wavelength than excitation light
Common questions
What happens when you shine a blue light on a biological sample in forensics?+
Many biological materials like semen contain compounds that fluoresce. When exposed to shorter-wavelength light such as 450 nm blue light, they absorb that energy and re-emit it as light at a longer wavelength, typically appearing to glow under an orange filter. This property makes fluorescence useful for detecting biological evidence at crime scenes.
Why does the emitted light always look different from the excitation light?+
Fluorescence occurs because a material absorbs light at a shorter, higher-energy wavelength and re-emits it at a longer, lower-energy wavelength. For example, fluorescein absorbs blue-green light but glows yellow-green. This difference is not a defect but the core principle that makes fluorescence detectable and useful in forensic work.
How long does the glow last when fluorescence happens?+
Fluorescent emission is extremely fast. When an excited material drops back to its ground state and releases light, the entire process typically takes just 1 to 100 nanoseconds. This rapid emission is why the glow appears instantaneous to the human eye.
Related terms
- 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...
- 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...
- Chemiluminescence
- Light emission produced directly by a chemical reaction, without requiring an external light source. Luminol's reaction with haemoglobin in alkaline peroxide is...
- Detective Dye
- A fluorescent powder (typically anthracene or phenolphthalein-based) applied to banknotes or surfaces. Transfers to the suspect's hands on contact and fluoresces under...
- Electromagnetic Spectrum
- The full range of electromagnetic radiation by wavelength, from gamma rays (shortest) to radio waves (longest). Forensic devices use UV, visible, IR...
- Excitation Light
- The external light source used to excite a fluorescent molecule. For fluorescein the optimum excitation is around 490-510 nm (blue-green). The light...
- Haem Iron
- The iron atom at the centre of the porphyrin ring in haemoglobin. It catalyses the luminol reaction by cycling between oxidation states...
- IR Reflectance
- Imaging under near-infrared illumination (715-1000 nm) to differentiate inks by their IR absorption: carbon-based inks remain dark, many synthetic dye inks become...
- 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,...
- Latent Bloodstain
- A bloodstain that is no longer visible to the naked eye under normal lighting conditions, either because it has been diluted, cleaned,...
- Neutron Radiography
- Imaging technique using a thermal-neutron beam that penetrates dense metal and resolves light-element components like explosives, plastics and adhesives. Rare; used for...
- Phosphorescence
- Light emission from the triplet excited state (T1) via a spin-forbidden radiative transition; characterised by emission persisting after the excitation source is...
Explained in these topics
- Crime Detection Devices: UV, IR, X-Ray and Detective DyesThe re-emission of light at a longer wavelength after absorbing shorter-wavelength radiation. Why semen 'glows' under 450 nm blue light through an orange filte...
- Fluorescence and Phosphorescence SpectroscopySpin-allowed emission from the lowest excited singlet state S1 back to the ground singlet S0, with a typical lifetime of 1 to 100 nanoseconds.
- Luminol and FluoresceinLight emission following absorption of light at a shorter (higher-energy) wavelength. Fluorescein absorbs blue-green excitation light and emits yellow-green li...