Heathcote, Regional Sales Manager at Phoseon Technology (Hillsboro, OR, USA; www.
phoseon.com). However, these UVC and UVB
LEDs are currently low power, relatively inefficient, short lifetime devices that cost hundreds
of times more than UVA devices.
Indeed, according to Matt Pinter, Director of Engineering, Smart Vision Lights, UV
illumination may be an expensive proposition for some applications. To highlight the
glue on envelopes, for example, may require
a 280nm UV light. Because the glue absorbs
this wavelength of light, it will appear black in
the reflected image (Figure 4). Because such
applications may require hundreds of these
types of LEDs, they may currently be cost-prohibitive for LED lighting and more suitable
for mercury-arc lamp illumination.
This, however, is not the case in the UVA
spectrum as Vincent Forte, Chief Technology Officer at Marktech Optoelectronics (Latham, NY; www.marktechopto.com)
points out in “Understanding Ultraviolet LED
Applications and Precautions” ( http://bit.ly/
VSD-UV-LED). Here, such illumination is
used in applications that include counterfeit
detection of banknotes forensics and printed
circuit board (PCB) inspection.
One such application, developed by Industrial Vision Systems (IVS; Didcot, Oxfordshire, UK; www.industrialvision.co.uk) has
been designed to inspect PCBs using UV fluorescence (Figure 5). Because of the property
of the UV traces added to the conformal coating used, IVS developed custom lighting to
illuminate both sides of the PCB and captured
images with two cameras mounted above and
below (see “Machine vision inspects protective
PCB coatings,” http://bit.ly/VSD-IVS).
IR LED lighting products can also be used
in applications to either capture reflected IR
illumination or images of products or materials that fluorescence under IR light. In the
IR spectrum wavelengths range from 750-
1400nm near infrared (NIR), 1400-3000nm
short wave infrared (SWIR), 3000-8000nm
medium wave infrared (MWIR) to 8000-
15000nm long wave infrared (LWIR). NIR
applications that use these wavelengths
include medical applications that image
deoxygenated blood within veins.
As Rihards Fuksis and his colleagues point
out in their paper “Infrared Imaging System
for Analysis of Blood Vessel Structure”; (http://
bit.ly/VSD-IR-VEINS), choosing the correct IR wavelength is important. In developing commercial products based on reflected
IR imaging techniques, Evena Medical (
Roseville, CA, USA; http://evenamed.com), for
example, contracted ProPhotonix (Salem,
NH, USA; www.prophotonix.com) to build
a number of chip-on-board near IR LEDs to
identify the best wavelength for their application ( http://bit.ly/VSD-NIR).
Such NIR wavelengths in the 700
–1000nm spectral band can also be used in
fluorescence imaging to view deep tissue
after NIR fluorophores are introduced since
the NIR light can penetrate several centime-
ters into tissue. Such techniques are used,
for example by the Biospace Lab (Nesles la
Vallée, France; www.biospacelab.com) in its
PhotonIMAGER system to study live animal
tissue structures. In its design, the Photon-
IMAGER incorporates an intensified CCD
camera to increase the sensitivity of the cap-
tured fluorescence images, highlighting the
need to properly match the correct illumina-
tion with the correct camera.
Choosing the correct camera to capture
images of reflected or fluorescent light is as
important as the wavelength of light used to
illuminate the part. As an example of this, the
quantum efficiency of the Elite- 5 Mono camera
from Micropix (Midhurst, West Sussex, UK;
www.micropix.co.uk), that employs an IMX250
2/3-in CMOS imager from Sony (Tokyo, Japan;
www.sony-semicon.co.jp/products), has a spectral response from approximately 300-1000nm
( http://bit.ly/VSD-MPIX). Although cameras
based on such sensors may be useful in some
UV fluorescence, visible and IR applications,
their lack of high sensitivity in UVC, UVB,
MWIR and LWIR demands that more specialized cameras must be used (see “Spectral
Response of Silicon Image Sensors”; http://bit.
Figure 5: In a UV fluorescence application designed to inspect printed circuit board traces,
Industrial Vision Systems (IVS) developed custom lighting to illuminate both sides of the PCB and
captured images with two cameras mounted above and below.