Latent Bloodstain
Definition
A bloodstain that is no longer visible to the naked eye under normal lighting conditions, either because it has been diluted, cleaned, dried and faded, or deposited on a dark substrate. Luminol and fluorescein both target latent stains.
- Visibility
- Not visible under normal lighting
- Common causes
- Dilution, cleaning, drying/fading, dark substrate
- Detection reagents
- Luminol, fluorescein
- Detection mechanism
- Chemiluminescence or fluorescence from haem catalysis
Common questions
How does an examiner decide between luminol and fluorescein for a scene?+
Luminol requires darkness and produces an immediate but brief chemiluminescent glow, while fluorescein needs an alternate light source and stains persist longer, making the choice depend on ambient lighting control and how much time is available for documentation.
Can luminol or fluorescein testing damage a latent stain for later DNA analysis?+
Both reagents can dilute or degrade the stain to some degree, so standard protocol documents and photographs the reaction first and collects a confirmed sample promptly afterward, since delayed collection risks further loss of usable DNA.
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...
- 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...