Figure 4: Comparison of ray paths using bright eld illumination (left)
and in-line illumination (right).
Figure 5: Comparison of wooden object with bright eld illumination
(left) and in-line illumination (right).
imaging resource guide imaging lenses filters microscopy cameras illumination targets
When Not to Use In-Line Illumination
Due to its multiple advantages, it is often believed that in-line illumination
is always the superior choice for space-constrained systems. Unfortunately,
it is not the best solution for objects that are optically di use or
for objects requiring a large eld of view. When used with di use objects,
in-line illumination produces a hotspot on the image caused by the Lambertian
(a nearly constant bidirectional re ectance distribution function)
tendencies of the object, which is detrimental to any inspection system.
Figure 5 shows an image of a di use object of wooden material both with
(right image) and without (left image) in-line illumination.
When the primarily Lambertian object is in-line illuminated, the image
has a well-de ned hotspot in the center of the eld of view. This hotspot
e ectively washes out the desired contrast, yielding a contrast of about
70% for the bright eld image, and about 8% for the in-line illuminated
image, with both contrast values taken at the center of the image.
There are, of course, other situations where in-line illumination is
not the ideal option. When a large eld of view is required, the étendue
of the illumination system becomes a problem, in that spreading
out the ux of the light over a large eld inherently leads to a much
less dense bundle of photons, and therefore has a negative impact on
the throughput of the system as a whole. Imperfect light sources also
signi cantly and negatively impact the performance of in-line illumination
systems with large elds of view, as the small imperfections are
bolstered over the large projection in the object plane.
Section 12.3: Using Structured Illumination
Illumination is a critical component of any machine vision system,
and can often be the di erence between a good imaging system and
a great one. Not only does the illumination position and wavelength
need to be uniquely considered for each application, but certain systems
require structured illumination to maximize system performance.
Structured illumination utilizes speci c patterns of light to determine
the geometric shape and depth of objects. An e ective 3D
system can be constructed by illuminating objects with di erent patterns,
such as grids, dots, or lines, while minimizing cost, components,
Since a well-calibrated system increases measurement accuracy, it
is important to understand that structured illumination is not universal,
and certain structures should be used to obtain certain measurements.
For example, a dot grid pattern may su ce to inspect a few
points on an object, but a line or multiple line pattern is required to
measure an object’s three dimensional pro le.
Table 2 demonstrates common structured illumination patterns and
their ideal applications.
Structured Illumination Method of Determination Purpose
dimensions of most
objects while the
object is scanned
dimensions of refractive
objects while the
object is scanned
Determining the depth
information at multiple
discrete points in
a single exposure
Table 2: Common Structured Illumination Patterns.
(Continued from page 160)
If the CCD cover glass were to be inspected for digs or chips, in-line illumination
would also be the more advantageous choice since the overall
image has a much more even contrast. The dark chips shown using
in-line illumination (a result of the light being scattered as shown in
Figure 4) would appear at a much higher contrast to the busy CCD
background than the chips shown in the high contrast image formed
using a bright eld system, as demonstrated in Figure 3.
Go to www.edmundoptics.eu/imaging-lab
to learn more about Illumination.