Master Thesis - Fachbereich Informatik

2.2. ILLUMINATION 19
effect of completely smooth objects appearing dark in the image, since the light rays are
not reflected toward the camera, while unevenness leads to brighter image intensities. Due
to this effect, directional lighting is also denoted as dark field illumination in the literature
[18] (See Figure 2.5(c)). Directional lighting mostly qualifies for surface inspection tasks
that consider the surface structure revealing irregularities or bumpiness.
Polarized light In combination with a polarizing filter in front of the camera lens, incident
lighting with polarized light can be used to avoid specular reflections. Such reflections
preserve the polarization of a light ray, thus, with the right choice of filter, only scattered
light rays can pass the filter and reach the camera. A maximal filter effect can be reached if
the polarization of the light source and the filter are perpendicular to each other. Polarized
light is often combined with a ring light setup to avoid both shadows and reflections.
Structured lighting Structured lighting is used to obtain three-dimensional information
of objects. A certain pattern of light (e.g. crisp lines, grids or cycles [18]) is projected
onto the object. Based on deflections of this known pattern in the image, one can infer
the object’s three-dimensional characteristics. For example, in [58], a 3D scanner using
structured lighting is presented that integrates a real-time range scanning pipeline. In
machine vision applications, structured lighting can be used for dimensional measuring
tasks were the contrast between object and background is poor.
Axial illumination In this type of illumination setup (see Figure 2.5(d)), also denoted as
coaxial illumination in the literature, the light rays are directed to run along the optical
axis of the camera [18]. This is achieved using an angled beam splitter or half-silvered
mirror in combination with a diffuse light source. The beam of light has usually the same
size as the camera’s field of view. The main application of axial illumination systems
is to illuminate highly reflective, shiny materials such as plastic, metal or other specular
materials, or for example to inspect the inside of bore holes. Axial illumination is typically
used for inspection of small objects such as electrical connectors or coins.
One potential problem with most incident lighting methods are shadows. Although the
shadow contrast can be lowered using several light sources at different positions around
the object (e.g. ring lights) or axial illumination setups, objects with sharp corners or
concavities might have regions that can not be illuminated and therefore especially regions
close to the object’s boundaries appear darker in the image. Thus, dark objects on a bright
background may appear enlarged [16]. The effect of shadows is less significant for bright
objects on dark background. In applications that require totally shadow-free conditions
for highly accurate measurements of object contours, another lighting setup called back
lighting can be used, as introduced in the following.
2.2.3. Back lighting
The setup were the object is placed between the light source and the camera, compared
to incident lighting, is denoted as back light illumination. In this arrangement, the light
enters the camera directly leading to bright intensity values at non-occluded regions. The
object, on the other hand, casts a shadow on the image plane, thus, leading to darker
intensity values. Non-translucent materials result in a very strong, shadow-free contrast,