fluorescence imaging

Question 1

visual detection of fingerprints

Answer 1

Need to enhance difference between background and fingerprint using different lighting techniques

Question 2

oblique illumination

Answer 2

fingerprints deposited in dust

Question 3

spectra illumination

Answer 3

detect marks on smooth surfaces, only some diffuse reflections reach image and therefore pale
- fingerprints on reflective surfaces

Question 4

dark field illumination

Answer 4

Used when fingerprints in grease, oil or sweat are on transparent surface
- shows light fingerprint ridges against dark background

Question 5

co axial illumination

Answer 5

used for marks on patterned, reflective background

- fingerprints appear as dark ridges against a light background

Question 6

polarised light

Answer 6

detects marks in contaminant on a reflective background

- images of light ridges against a dark background

Question 7

fluorescence visualisation

Answer 7

1930’s zinc sulphide and anthracene used fluorescing under UV light

1954 Cherrill suggested filters to cut out reflected light from background

1970 Ohki noticed that some components of fingerprints had inherent fluorescence under UV light (also green light)

Menzel and Duff use fluorescent chemicals to enhance latent fingerprints

1979 PSDB - did not need coherent light source, just power and wavelength

1980’s production of the Quaser filtered light source
Herod and Menzel noted that spraying zinc salts on ninhydrin developed latents fluoesced by 488nm line of argon laser

1980’s copper vapour laser and garnet laser (Nd:YAG), the latter being portable

Question 8

fluorescence imaging

Answer 8

Fluorescent dyes developed to enhance latent prints e.g acid yellow 7

(LED) for hand held torches.

Green lasers up to 8W, and semi-conductor yellow lasers up to 5W and blue lasers up to 2 W available

Light in UV, blue, green or yellow are use to excite fluorescence
- Results in light in the yellow, range, red or infra-red region of the spectrum

Question 9

luminescence lifetimes

Answer 9

Emission of light due to:

• Heat (incandescence)
• Chemical reaction (chemiluminescence)
• Prior excitation with light i.e. excitation (photoluminescene)

“Luminescence lifetime” is the decay time for the luminescence intensity one the excitation is cut off

There are two types of photoluminescence:
fluorescence ( short lifetime)
Phosphorescence ( long liftime)