We are able to see all that is around us in all its colors and vividness due to the interplay between photons and matter. As light travels through space, the photons interact with particles in its path which can cause the photon to change its course or convert to another form of energy. How light interacts with objects depends on the chemical composition of the material, and can be described by three properties – specularity, absorptivity and transmissivity. The magnitude of these properties varies within a material as a function of the wavelength of light.
Mirrors, chrome and window glass are examples of highly specular materials. What these materials have in common is that their surface texture is very smooth and fine textured and are often described, somewhat unprecise, as reflective. Rubber, wood, leaves and chalk have low specularity. What these objects have in common is that though they are easy to see, they do not cast a reflection. You would not be able to see a reflection in a mirror made with chalk. This is because the material surface has a very diffuse texture, which causes incident light to scatter off almost uniformly in all directions.
Transmissivity describes a materials ability to pass light through. Window glass, clear water, certain plastics and epoxies are examples of materials with high transmissivity. Even though the light may change direction, be diffracted or refracted, it passes through the material rather than being bounced back where it came from. A material with low transmissivity will therefore reflect light instead, and you will not be able to see through it.
Absorptivity describes a materials ability to absorb an incident photon and convert it to another form of energy, such as heat or electricity. Black objects like rubber tires, coal, TV and smartphone displays have high absorptivity of visible light and typically get quite hot when left out in the sun. Objects with low absorptivity will either reflect the light or pass it through, depending on the level of transmissivity as mentioned above. Materials with low absorptivity can therefore be window glass and still water, but also white paper, wood, aluminum and white chalk has a low level of absorptivity. In addition, comes materials that are partially absorptive – colored objects. Red and green apples are highly absorptive of all wavelengths of visible light, except red (700 nm) and green (530nm), respectively. For this reason, some structured light scanners like those using a red laser source can get very good results on red objects, but very poor on blue objects, even though the objects have the same shapes.
Typical effects of specularity and reflectivy
When dealing with materials that are highly specular and reflective, there are a handful of optical effects that take place.
Highlights from direct reflection either caused by ambient or an intended light source.
Interreflections between objects:
Reflections from surrounding objects onto the object.
Reflections from the object onto surrounding objects.
Lowlights caused by light being scattered off angular planes.
The image below shows an example of how some of these effects may take place.
An overview of the various optical properties of material described above are depicted in the image below. The two figures to the left illustrates different levels of specularity, while figures three and four from the left illustrates different levels of absorptivity. The rightmost figure illustrates transmissivity.
The sum of these optical properties of a material determines how easy or difficult it is to see it, and thus how applicable it is for a 3D scanner. Taking images of objects and scenes that have low specularity, absorptivity and transmissivity are preferable for most optical instruments, while increasing degree of any of the three properties is the fundamental challenges as to why many 3D scanners today are struggling to produce satisfactory results on what is typically referred to as transparent and shiny objects. An example of a challenging scene that contains both a combination of reflective, specular and absorptive materials is depicted below.