photonics tech briefs:

Ultraviolet TechnologyAids Forensic Investigations

by Mark Pontin, Resolve Optics Ltd.,Chesham, UK

A specialized visible/ultraviolet lens recordscomplete crime scenes and high-resolutionclose-up fingerprints for analysis and evidence.

Ultraviolett-Technologie hilft forensischenUntersuchungenEine spezialisierte Linse fUrsiGhtbare/ultraviolette Strahlungzelchnet komplette Tatort-Szenen und hochaufgelosteFingerabdrUckefUrAnalyse und BeweisfUhrungauf.

La technologie de I'ultraviolet a I'aide des enquetesmedico-IegalesUne lendUespecialisee visible/ultraviolette fait I'etude com-plete!'de scenes de crime et produit des agrandissementsd'empreintes digitales a haute resolution pour I'analyseet laconstitution de pieces a conviction.

La tecnologia ultravioletta aiuta le indagini di poliziaUna lente ottimizzata per iIvisibilee per I'ultraviolettopermette di registrare la scena del delitto nella suacompletezza, e rilevare in ripresa vicinaad alta risoluzione leimpronte digitaliper le successive analisi e I'ottenimento delleprove.

Anew technique for detecting and recording fin-gerprints at the scene of a crime makes use of adigital or film camera coupled to an ul-

traviolet image intensifier. When the scene isilluminated with a light source using a "ger-micidallamp" emitting DVe (220 to 290 nm)radiation, the high spectral reflectivity of latentfingerprints in this waveband makes them glow,and they can then be recorded.

The traditional method of dusting for fin-gerprints - lifting them with sticky tape andattaching them to a record card as evidencefor identification - has changed little overthe years. The disadvantage is that any failurein the traceability record of the card couldrender the evidence useless. The camera-based

technique ensures that the original images arealways available to settle any subsequent doubtover the origin of a given fingerprint.

The camera lenses originally used had a num-ber of drawbacks. They were not designed for

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use with broad spectral wavebands, nor for close-upapplications. Resolve Optics Ltd. has developed a high-performance Forensic DV lens specifically for police useand crime scene investigations, with the unique abil-ity to capture visible and ultraviolet versions of the scenequickly and easily. By using a novel telescopic focusingmount, the lens provides a long-focus travel distance ina compact footprint, enabling investigators to image ob-jects from infinity to close-up at high magnification with-out add-on adaptors.

The creation of the Forensic DV lens builds on two

areas of expertise previously developed by Resolve,both of which play a critical role in addressing the fin-gerprint requirements. First, the company has designedand manufactured lenses for research applications inthe UVB (290 to 315 nm) and DVe wavebands. Becauseconventional optical glasses and cements do not trans-mit in either of these regions, the engineers developedsome designs using specialized optical materials thatcontrol chromatic aberrations so that the optical reso-lution is maintained over a broad spectral wavebandwithout the use of doublet groups, which would requireoptical cement.

The second crucial area is in cam mechanisms and op-tical design. In its ongoing production of zoom lenses,the company has gained considerable mechanical and

These graphs show the consistent performance of the lens throughout itsfocus range. Modulation transfer function (MTF) is plotted vertically againstthe spatial frequency in cycles per millimetre for the centre (upper graphs)and the edges (lower graphs) ofthe lens.

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optical expertise, developing devices for precise cam-driven optical components, and optical designs for sys-tems with performance controlled under a variety ofchanging conditions. The earlier lenses used in policeinvestigations relied on separate filters screwed to thefront of the lens to provide waveband selection and sup-plementary lenses for close-up work, and these were li-able to be lost at crime scenes.

Corrects for change in wavebandThe specialized lens enables investigators to iden-

tify and focus on a target in visible light and then quicklyslide an ultraviolet filter across to take images in thisregion without having to refocus. A positively sprungtopside mechanism is incorporated into the camerafitting, and an integral two-position filter slide with clickstops facilitates the change from the ultraviolet to thevisible for general viewing. The thickness of the visi-ble glass filter is set such that the shift in focus be-tween the two wavebands that would otherwise be

apparent is fully corrected. .Another challenge is acquiring high-resolution close-

up images of the fingerprints, both for immediate analy-sis and for long-term archiving. The lens provides truehigh-resolution crime-site macroviewing capability byimaging onto a reflected DV imaging system (RDVIS)manufactured by Horiba Jobin Yvon with a 1:1 lens mag-nification while exactly matching the optical resolution

of both lens and image intensifier. The 1:1 magnifica-tion means that the size of the fingerprint displayedon the scope is exactly the same as the original.

The telescopic focusing mount produces displayed-to-actual-size ratios varying from near-zero to 1.25, asthe field of view is changed from capturing an imageof the whole scene to focusing on a detailed picture ofthe fingerprint magnified to 1.25 times. The long-focus travel distance for macrorecording is achievedwith a cam-driven mechanism such that the lens re-mains fully supported and precisely located through-out the movement. This arrangement ensures that theoverall mount envelope is compact and that the link-age between the fixed aperture control ring and the mov-ing iris diaphragm remains constant.

The lens's wide field of view and high-transmissioncoatings make it considerably more light-efficient thanother longer-focal-Iength lenses offering an identicalview. The field of view is 8.30 at 1:1.25 magnification,and up to 16.60 at long object distances. It achieves su-perior high-resolution macro images for fingerprint-ing and assessing damage to human skin. Furthermore,the lens has a professional look and feel, which is animportant consideration in forensic work. 0

Contact: Mark Pontin, Resolve Optics Ltd.; lax: +441494 775201; e-mail: sales@resolveoptics.com; or circle96.

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