Inspection of specularly reflective surfaces for shape and defects poses a well-known difficult challenge for machine vision systems. For example, identifying defects on specular surfaces, such as the orange-peel effect on a painted surface or a small chip on the screen of a cell phone can require complex specialized setups and equipment which are obvious barriers for integrators. Phase Measuring Deflectometry (PMD) using Patterned Area Lighting™ (PAL) provides a simple and effective solution for these types of applications.
PHASE Measuring Deflectometry (PMD)
In its simplest form, a typical single camera (monoscopic) setup for PMD uses a patterned light source, such as the Smart Vision Lights’ Patterned Area Lighting™ (PAL) light, and a camera (see Figure 1.1 & 1.2). The PAL light source and camera are oriented at similar but opposite angles (relative to the object) so that the camera captures the reflected image of the light source from the specular surface. In this setup, the specular surface can be flat or curved.
The resulting image captured from the reflection will be distorted by surface shape variations or defects. Distortions are caused by a phase shift in the reflection angle of the pattern image. Based on simple mathematical relations of light reflection and surface shape, software algorithms can be used to determine shape characteristics and locate small defects. The amount of phase shift will be determined by the divergence of the object surface. So for gradual variations in the surface shape, the reflected pattern will have a similar gradual variation. For sharp variations, such as pits or scratches, large variations in phase will occur resulting in an abrupt change in background contrast. For absolute quantitative results, camera and light source positional calibration is required. The monoscopic method can be expanded with an additional camera to achieve a stereoscopic arrangement. For application examples featuring PAL lighting, download our tech note: Locate Flaws on Reflective, Transparent, and Semitransparent surfaces.
On a flat surface, light is reflected at an angle equal (but opposite) to that of the incident light. When a defect is present, light reflects off of the defect at an angle that is phase shifted from the original flat surface reflection angle. By looking at the phase shift in the reflected image, variations and defects can be quantified based on size, direction, and shape.
Transparent Objects and Phase Measuring Deflectometry
PMD can also be used to inspect transparent objects. The principles behind this are the same as with a specular surface inspection. When the light passes through a defect, such as a fold in glass or a small pit in plastic, light is deflected or refracted at an angle out of phase with incident ray angle. Defects in transparent objects can be hard to locate and quantify under normal lighting
Monoscopic Phase Measuring Deflectometry
A monoscopic (single camera) PMD setup uses a single PAL light source and camera. Based on the light source pattern type and orientation, 2D and 3D quantitative/qualitative results can be achieved. Two-dimensional imaging is achieved when a single direction pattern is used, such as in Figure 5 which shows a parallel line pattern. In this case, the lines can be either vertical, horizontal, or diagonal. Three-dimensional results can be achieved with a bi-directional pattern, such as lines placed in a grid pattern; this setup is typically used in a single frame image acquisition application.
Stereoscopic Phase Measuring Deflectometry
Stereoscopic PMD can also be used for 3D and 2D results. Depending on the precision and accuracy required, a quantitative stereo setup can be used to reduce computational error and optimize surface accuracy associated with the various phase reflectometry models. Simple qualitative setups may also be achieved by simply looking at the difference between the camera’s images.
Multi-Angle Phase Measuring Deflectometry
A Multi-Angle PMD setup includes two or more PAL light sources and two or more cameras. Each light source with PAL and camera can be set up in a monoscopic PDM, stereoscopic PDM or any combination thereof, whereby two-dimensional and three-dimensional qualitative image captures can be achieved. Multiple images can be used to look for various defects or recreate the inspected object in 3D. Another practical setup could use an x-axis and y-axis camera/light combo, where the combination of the x- and y-axes images, processed together, can produce 3D information without having to shift the pattern orientation.
PMD has proven to be an effective and simple method for detecting defects and quantifying features on specular surfaces, including applications that run at high-speeds. Using
Jeremy Brodersen is an Optics and Lightning Engineer at Smart Vision Lights.