Machine Vision and Applications (2008) 19:35–42
Characterization of surface deformation with the Edge
of Light™ technique
Z. Liu · M. Genest · A. Marincak · D. S. Forsyth
Received: 28 July 2006 / Accepted: 9 February 2007 / Published online: 20 April 2007 © Springer-Verlag 2007
Abstract The aircraft lap joints are inspected with an
enhanced visual inspection technique named “Edge of
Light”, which is patented by the NRC Institute for Aerospace Research. This technique is applicable for rapid detection of possible hidden corrosion in lap joints. The surface deformation due to hidden corrosion can be characterized by
this optical-based inspection method. In this study, a calibration procedure is developed to quantify the lap joint surface deformation. The effect of surface reﬂectivity is investigated with the solid ﬁlm highlighting technique (SolidHi™), which helps achieve a uniform reﬂectivity during the inspection. The efﬁciency of the technique is demonstrated with the experimental results.
Keywords Edge of Light™ inspection · Aircraft lap
joint · Pillowing deformation · Corrosion quantiﬁcation
The Edge of Light™ (EOL) technique gets its name from
the use of the edge of light region of a full illumination. The intensity of the light changes rapidly in this region. The EOL technique uses such changes to reﬂect the tiny variation in the This work is supported by NRC-IAR New Initiative Research
Z. Liu (B) · M. Genest · A. Marincak
Institute for Aerospace Research, National Research Council
Canada, Montreal Road 1200, Building M-14,
Ottawa, ON K1A0R6, Canada
D. S. Forsyth
Texas Research International Inc., 9063 Bee Caves Rd.,
Austin, TX 78733, USA
surface smoothness [3,5,7]. The technique was patented by
the NRC Institute for Aerospace Research (NRC-IAR) 
and had been licensed for industrial use. The technique can
be used for applications where information on surface topography is needed, for example, nondestructive inspection of surface deformation, forensic, and biometric applications.
The corrosion of aircraft lap joints is mainly caused by the breakdown of sealant or adhesive protection between layers.
Usually, the corroded product is of much higher volume than
the original material  and this will cause an expansion
of the skins between rivets. This phenomenon is known as
“pillowing”. The pillowing deformation is an increase in skin surface height between the rivets [6,14] (see Fig. 1). Siegel et al. proposed using stereoscopic camera to create surface
altitude map of pillowing deformation. To facilitate the identiﬁcation of corresponding points in left and right images, a square grid of 17 × 17 spots was projected on the surface.
However, the study on the accuracy of this system has not
been reported so far. In , Komorowski et al. developed an optical impact detection system based on double pass retroreﬂection, also known as D-Sight™. A method to quantify the result of D-Sight™ inspection was reported in  by comparing with the calibrated images derived from a ﬁnite element model .
Laser-based range measurement can provide a highprecision mapping of deformed surface. However, the laser scanning will take much longer time due to the size of the light beam. In contrast, the EOL system can rapidly search and
locate the pillowing deformation in aircraft lap joint structures and thus it is more efﬁcient and practical. For the lap joints used in our experiment, the laser scan took about 40 min while a typical EOL scan for the same block took only 1 min.1
This also depends on the setup for scanning speed and resolution.
Z. Liu et al.
Fig. 2 The schematic of the Edge of Light™ system
Fig. 1 Pillowing deformation (pointed by the arrow) on aircraft lap joint
Previous studies have demonstrated the relationship
between the pillowing deformation...
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