Hot Mirror (IR-Cut Filter)
Definition
The infrared-blocking filter bonded in front of the digital sensor in consumer and professional cameras; must be removed or bypassed for NIR forensic imaging.
- Function
- Blocks infrared light reaching the digital sensor
- Location
- Bonded directly in front of the sensor stack
- Forensic action needed
- Must be removed or bypassed for NIR imaging
- Typical presence
- Standard in consumer and professional cameras
Common questions
Why do ordinary cameras need this filter removed for forensic work?+
Without the hot mirror, a camera's silicon sensor is naturally sensitive well into the near-infrared, which would distort ordinary colour photography, so manufacturers block it by default. Forensic NIR imaging needs that blocked infrared signal restored, so the filter is physically removed or a converted camera is used.
What happens if a photographer forgets the hot mirror is still in place during ALS or IR work?+
The filter will suppress most of the infrared signal the technique depends on, producing a weak or blank result even though visible light exposure looks normal, which can be mistaken for the absence of the target evidence rather than an equipment error.
Is filter removal permanent?+
It can be either. Some forensic imaging units use a camera permanently converted with the hot mirror removed and replaced with a clear or IR-pass filter, while others use interchangeable filter setups so the same body can shoot conventional and infrared images.
Related terms
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- Infrared Luminescence (IRL)
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- Stokes Shift
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- UV-a
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- UV Reflectance Imaging
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- Vogeley 2002 Protocol
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