6. Interaction of Light & Object
- Wilhelm Scheermann
Analytics
Light has the characteristics of waves and particles (wave-particle duality), which affect how it interacts with a workpiece and the resulting inspection image. Understanding what interactions occur with a particular object is key to optimizing illumination for machine vision.
6.1 Reflection
Light has the characteristics of waves and particles (wave-particle duality), which affect how it interacts with a workpiece and the resulting inspection image. Understanding what interactions occur with a particular object is key to optimizing illumination for machine vision.
6.2 Absorption
All material, organic or inorganic, absorbs radiation at specific wavelengths from the incident white light and reflects light that has not been absorbed (or otherwise interacted with the object). An object's color corresponds to the spectral composition of this reflected light. The wavelengths of the light absorbed cause energy changes within the atomic and molecular structure of the material and are specific to that material. Therefore, any given object will absorb the same wavelengths no matter what.
However, different illumination sources have different spectral distributions, which changes the composition of the reflected light, meaning that the color of an object can appear to vary depending on the light source used.
6.3 Transmission
Materials can also transmit light depending on wavelength, making it possible to obtain different information from an object. For example, many opaque plastic packaging materials allow the transmission of NIR wavelengths but not white light. Thus white light can image the packaging itself while NIR wavelengths car show the contents of the package.
6.4 Flourescence
Some materials can absorb light in the UV wavelength range, which results in higher internal energy transitions.
It is then possible for some of this light to be re-radiated but with less energy in a process known as fluorescence.
The longer wavelength emitted falls in the visible region of the spectrum and can be imaged. Manufacturers who want to minimize visible markings on their products may use UV-sensitive dyes, which do not absorb in the visible range of the spectrum (and are therefore transparent).
6.5 Refraction
Refraction is a change in the direction of light rays when they move from one medium to another and underpins the operation of lenses. The angle of refraction depends on the material the light passes through, the angle of incidence, and the wavelength of light, which can cause aberrations such as spherical and chromatic aberrations in lenses.
6.6 Optical Rotation (Polarization)
As mentioned in Section 3, the electric and magnetic wave components of electromagnetic light oscillate at 90° to each other in the direction they travel. These two components have the same wavelength and are usually in phase with each other. Polarization refers to the orientation of the electric field of an electromagnetic light wave with respect to the direction of travel. Most light sources emit unpolarized light where the waves oscillate in all directions around the direction of travel.
Using a polarizing filter in front of the light source allows transmission of light on a single plane according to the filter's orientation, known as line (or plane) polarization.
Elliptical polarization occurs if the phase of the electric and magnetic waves is not the same, which forms an ellipse around the direction of travel. If the two waves are out of phase by 90° (114 wavelength), the resulting electric field traces a circle around the axis, and thus the light is circularly polarized.
When imaging highly reflective surfaces in machine vision, specular reflectance often creates a glare that interferes with imaging the features of interest. Cross-polarization resolves this using polarized light to illuminate the sample and another polarizer on the camera lens. The polarizer on the camera does not transmit the specular reflection of polarized light from the workpiece. Only scattered light can pass the polarization filter on the camera, so no reflections appear in the image.
The surface characteristics of some materials, such as coatings, scratches, and depressions can also affect the angle of polarization (optical rotation). If the light is polarized before it illuminates the object and is then observed through another polarizing filter, previously invisible features become visible. Transmission imaging of polarized light can also detect particles in liquids. Some polarizing filters are wavelength-independent in the visible range for polarization in color imaging. In addition to the qualitative effect of reducing glare and reflections, it is also possible to make quantitative measurements of polarization using the Stokes vector. When imaging photoelastic materials, polarization imaging can provide information on properties such as stress inside transparent objects.
