CLAHE
Definition
Contrast Limited Adaptive Histogram Equalisation. Divides the image into local tiles, equalises each tile's contrast separately, and blends results. Recovers shadow detail without clipping bright regions.
- Full name
- Contrast Limited Adaptive Histogram Equalisation
- Method
- Equalises contrast per local tile, then blends tiles
- Advantage over global equalisation
- Recovers shadow detail without clipping bright regions
- Common use
- Enhancing low-contrast CCTV and scene images
Common questions
Why does CLAHE work better than global histogram equalisation on scene photos?+
Global equalisation applies one contrast curve to the whole image, which can wash out bright areas while trying to lift dark ones, whereas CLAHE processes small tiles independently so each region gets contrast enhancement suited to its own local brightness range.
What does the 'contrast limited' part of CLAHE actually do?+
It caps how much any single tile's histogram can be stretched before equalisation, preventing the noise amplification and blocky artefacts that unrestricted local equalisation would otherwise introduce, especially in near-uniform dark regions.
Does applying CLAHE count as altering evidence?+
Enhancement techniques like CLAHE are applied to a working copy alongside a preserved original, and the processing steps are documented, so the output is treated as an enhanced view for interpretation rather than a replacement for the original exhibit.
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