Excitation Light
Definition
The external light source used to excite a fluorescent molecule. For fluorescein the optimum excitation is around 490-510 nm (blue-green). The light is filtered or selected so that the observation wavelength (barrier filter) blocks the excitation and passes only the emission.
- Function
- Excites a fluorescent molecule to emit light
- Fluorescein optimum
- Roughly 490 to 510 nm, blue-green
- Filtering
- Barrier filter blocks excitation wavelength, passes emission
- Context
- Fluorescein blood detection alongside luminol
Common questions
Why is a barrier filter needed if the excitation wavelength is known?+
Without filtering, the excitation light itself, which is far more intense than the weak emitted fluorescence, would overwhelm the observer's view or a camera sensor, making the faint fluorescent signal impossible to see.
How does excitation light differ from the luminol reaction it is often paired with?+
Luminol produces its own light through a chemiluminescent reaction with blood and needs no external light source or camera filter. Fluorescein instead requires an external excitation light and a barrier filter to visualize the fluorescence, and is typically used after luminol to confirm faint results.
Related terms
- 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...
- Fluorescence
- 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...
- Haem Iron
- The iron atom at the centre of the porphyrin ring in haemoglobin. It catalyses the luminol reaction by cycling between oxidation states...
- Latent Bloodstain
- A bloodstain that is no longer visible to the naked eye under normal lighting conditions, either because it has been diluted, cleaned,...