Chemiluminescence
Definition
Light emission produced directly by a chemical reaction, without requiring an external light source. Luminol's reaction with haemoglobin in alkaline peroxide is the most common forensic example.
- Definition
- Light from a chemical reaction without external light source
- Forensic example
- Luminol reacting with haemoglobin
- Conditions
- Alkaline peroxide solution
- Output
- Visible glow, photographed in darkness
Common questions
Why must luminol chemiluminescence be photographed in darkness?+
The light produced is faint compared to ambient light, so any room or camera-flash illumination washes it out, meaning the scene must be darkened and long-exposure photography used to capture the glow before it fades.
Does a chemiluminescent reaction confirm the presence of blood specifically?+
No, luminol reacts with any peroxidase-like activity, including some plant materials, certain metals, and household bleach, so a positive glow is presumptive only and must be followed by a confirmatory blood test.
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...
- 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...
- 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,...