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Latent Print Examination » Questions from FP Experts for FP Experts... Processing, Testimony, and Technical Matters » LP Examiner Vision Questions (shades of gray and spatial frequency of ridges)? « Previous Next »

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Webservant (Member)
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Post Number: 223
Registered: 03-1997
Posted on Monday, February 26, 2007 - 06:58 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

You seem to pose two questions from the ophthalmologist as follows:
- What is the number of shades of gray Latent Print Examiners can perceive?

- What is the average distance between ridges in friction ridge impressions ? (Already asked and answered here)
Please tell the ophthalmologist that Latent Print Examiners should be considered as having normal human vision, which they almost always supplement with various enhancement tools.

Shades of Gray
Humans can perceive between 16 and 64 shades of gray. Those limitations are overcome to varying extents depending on the knowledge, ability and tools available to each expert. For example, typical digital image capture of friction ridge impressions in compliance with SWGFAST Digital Imaging Guidelines records 256 shades of gray. Through use of computer image processing such as contrast adjustment, histogram equalization, and pseudo color assignment (humans can detect thousands of varying colors), Latent Print Examiners can discern all 256 shades of gray in the images. Digital image processing has been used in US forensic laboratories since the early 1980s. Before then, photographic techniques such as high contrast film/paper/filters were used to enhance friction ridge detail contrast. Bas relief photography is one example of a darkroom precursor to digital image processing edge detection techniques such as laplacian filtering. See chapter five in the 1991 edition of Advances in Fingerprint Technology for more information. Also see the information IBM has posted online about human vision abilities regarding contrast and color.

Distance between Ridges (Acuity)
This query from the ophthalmologist seems to perhaps be related to the ability of Latent Print Examiners to discern line pairs per millimeter (visual acuity). Please be sure to inform the ophthalmologist that almost 100% of latent print examination is performed using tools that compensate for visual acuity limitations, including fingerprint magnifiers, photographic enlargements, and computer image processing (zooming, enlargement, etc.). See IBM's explanation about human visual acuity online here.
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Jaci Durr (Unregistered Guest)
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Posted on Friday, February 16, 2007 - 03:08 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

I am working on a research paper that involves studying one's ability to detect differences between various shades of gray. In order to make the test more accurate the ophthalmologist I am working with needs this information to create a test that will be more realistic to what a latent examiner does on a daily basis. I know in Ashbaugh's book he talked about the average number of ridges/cm, so I could just divide one cm by this number to create an approximate distance, but I don't know if that would work. I also understand that distortion plays a factor in determining this, but any help that you can give me would be great.


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