Barrier Filter
Definition
An optical long-pass filter placed over the camera lens and worn as goggles by the examiner during ALS examination; blocks the excitation wavelength reflection and passes only the longer-wavelength emitted fluorescence, making the dark-background fluorescence image possible.
- Filter type
- Long-pass or bandpass optical filter
- What it blocks
- Excitation wavelength (the source light)
- What it passes
- Longer-wavelength fluorescent emission
- Common colors with ALS
- Orange or yellow filters used with blue-green excitation for fluorescein
Common questions
What does a barrier filter do during ALS examination?+
A barrier filter blocks the bright excitation wavelength (the light source) from reaching the examiner's eyes or camera lens. This allows only the weaker fluorescent emission from the sample to pass through. Without this filter, the intense excitation light would overwhelm and mask the faint fluorescence, making the evidence invisible.
How does the barrier filter create the dark-background effect?+
By removing the excitation wavelength and passing only the longer-wavelength fluorescence, the background appears dark while the fluorescing evidence glows. This high contrast makes trace evidence stand out clearly against the dark background.
Where is the barrier filter positioned during imaging?+
The filter can be worn as goggles by the examiner for visual examination, or placed over the camera lens during photography. Either way, it sits between the sample and the observer to filter out the excitation light before it reaches the eyes or sensor.
Related terms
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Explained in these topics
- Cyanoacrylate Fuming, Alternate Light and Fluorescent StainsAn optical long-pass filter placed over the camera lens and worn as goggles by the examiner during ALS examination; blocks the excitation wavelength reflection...
- Luminol and FluoresceinA coloured filter worn by the examiner or placed in front of the camera lens to block the excitation wavelength and pass only the longer fluorescent emission....
- Specialised Imaging: UV, IR, Laser and ALSA long-pass or bandpass filter worn over the examiner's eyes or camera lens that blocks the excitation wavelength while passing the longer-wavelength fluoresce...