Hamming Distance (HD)
Definition
The fraction of bit positions where two IrisCodes disagree, computed only over mutually unmasked bits. Genuine pairs centre around HD 0.10-0.15; impostor pairs centre near HD 0.45.
- Measures
- Fraction of disagreeing bit positions between IrisCodes
- Computed over
- Mutually unmasked bits only
- Genuine pair range
- Roughly HD 0.10 to 0.15
- Impostor pair centre
- Roughly HD 0.45
- Origin
- Daugman iris recognition algorithm
Common questions
Why is Hamming distance computed only over unmasked bits?+
Eyelashes, eyelids, and specular reflections corrupt parts of an iris image, and masking excludes those unreliable regions so the comparison reflects genuine iris texture rather than occlusion noise.
Why is the impostor distribution centred so far from 0.5 rather than at it?+
IrisCode bits are not fully independent, but the correlation between unrelated irises is low enough that they still average close to coin-flip disagreement, around 0.45 rather than the theoretical 0.5 of true randomness.
How does the system decide whether a comparison is a match?+
A threshold is set between the genuine and impostor distributions, and because the two barely overlap, even a strict low threshold still separates true matches from false ones with very low error rates.
Related terms
- DOD ABIS
- US Department of Defense Automated Biometric Identification System, administered by the Defense Forensics and Biometrics Agency, holding iris, fingerprint, face and DNA...
- HIIDE
- Hand-held Interagency Identity Detection Equipment: a handheld iris and fingerprint capture device used by US military forces in Iraq and Afghanistan for...
- Integro-Differential Operator
- Daugman's boundary-detection method: it maximises the magnitude of the line-integral intensity gradient around circular paths, locating the pupillary and limbic boundaries of...
- IrisCode
- The 2048-bit binary template produced by quantising the phase of 2D Gabor wavelet responses across the normalised iris strip. Phase encoding provides...
- NIST IREX
- NIST Iris Exchange: the series of iris recognition algorithm evaluations conducted by the National Institute of Standards and Technology since 2009, providing...
- Puttaswamy V. Union of India (2018)
- Supreme Court of India judgment upholding Aadhaar's constitutional validity while classifying iris and other biometric data as sensitive personal information within the...
- Rubber-Sheet Model
- The coordinate transform that maps the annular iris region (between pupillary and limbic circles) to a normalised rectangular strip of fixed dimensions,...
- UIDAI
- Unique Identification Authority of India, the statutory body created by the Aadhaar Act 2016 that operates the world's largest biometric identity system,...
- Usable Bit Fraction
- The proportion of IrisCode bits not masked for eyelid, eyelash, or specular reflection occlusion. Values below 0.7 reduce match confidence; values below...