Section 12.2: In-line Illumination
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Figure 2a Figure 2b
Figure 2: Chrome on glass USAF 1951 resolution target with bright-
eld illumination (Figure 2a), and in-line illumination (Figure 2b).
In-line illumination is a unique style of lighting that incorporates the
illumination into the optical train of the machine vision lens, usually
by means of a ber optic light guide or LED light source and a beam
splitting optic. Although in-line illumination is not as bulky as di use
axial illumination, and may be easier to integrate into a system that
has tight space constraints, there are important optical di erences a
system designer must consider. In-line illumination is much more directional
when compared to di use axial lighting, which, due to the
chief rays being nearly parallel to the optical axis in object space, is
one of the reasons that in-line illumination is widely incorporated with
telecentric lenses (Figure 1). In comparison, di use axial lighting will
project the illumination at a multitude of di erent angles, displaying
di erent properties on the object and image planes.
Vastly di erent results will also come from comparing in-line illumination
to bright eld illumination. Figure 2 shows a chrome on glass,
positive USAF 1951 contrast target illuminated both with bright eld
illumination and with in-line illumination.
The most immediate di erence between the two types of illumination
is the complete contrast reversal between the two images. Additionally,
the defects in the target are more readily apparent in the bright eld image,
which, depending upon the application, can be either a positive or
a negative e ect. Interestingly, the highly re ective nature of the target
yields about 10% better contrast for the in-line image when compared to
the bright eld image – the reason for this is explained below.
Figure 1: Diagram of in-line illumination within a telecentric lens.
When to Use In-Line Illumination
When considering using in-line illumination, it is important to understand
exactly where it is applicable and where it is not. In-line illumination
is ideal for the inspection of specular or semi-specular objects,
such as semiconductor wafers or CCDs, due to the nature of the rays
on the illumination path. Using two di erent telecentric lenses, one
with in-line illumination and one without, images of the same CCD
demonstrate the di erences between bright eld illumination (using a
ring light) and in-line illumination. The images are shown in Figure 3.
In-line illumination would be a better choice to inspect the wires
along the edge of the CCD due to the higher, more even contrast
between the wires and the rest of the CCD. As shown in Figure 4, the
reason that the wires in Figure 3 appear bright using bright eld illumination
and dark using in-line illumination is due to the ray paths of the
lighting. With bright eld illumination, the rays are scattered into the
lens, and with in-line illumination, the rays are scattered away from
With bright eld illumination, the rays originating from the ring
light are re ected by the object into the lens. The re ections will vary
based on the angle of the individual sources in the ring light as well
as the angle of the wires themselves with respect to the CCD surface
and the solder material at the tips, which is why the re ections have
non-uniform pixel values along the length of the wire. Using in-line
illumination, all of the rays are re ected by the object and scattered
away from the lens such that none of the light that hits the wires is
re ected back into the lens and onto the sensor. The more even contrast
of the background, along with the stronger contrast of the wires,
makes in-line illumination a better choice for the inspection of the
wires than bright eld illumination.
Figure 3: Comparison of bright eld illumination (left) with in-line
Brightfi eld Illumination In-line Illumination
Low Contrast Wires High Contrast Wires
Bright Chip on Faceplate Dark Chip on Faceplate
High Contrast Image Even Illumination with Consistent Contrast between Features (red square)
Table 1: Comparison of Bright eld and In-Line Illumination.