By Grégoire Besse
What characterises the colour of an object are its properties to absorb part of the light and re-emit it. And to see a colour, you need at the very least: a light, an object and a sensor instrument (camera, eyes, …).
If we take a closer look at the colour components of an object, we can distinguish between two behaviours. The first is in the volume, the chemical composition of the object will condition the absorption, scattering and refraction of light. The importance of these effects will be conditioned by the shape and thickness of the object. Secondly, the physical state of the object, its surface properties, its roughness, and other properties (film, layers) will also affect the perceived colour.
| Picture 1 |
On the left, a surface sometimes smooth, sometimes rough to see the colorimetric deviation between these two surface states. On the right, our seashell with very variable thicknesses to appreciate the colorimetric variations of this blue glass.
In the left example above, we can see that the gloss of an object can affect its perceived colour, but in this article, we will rather focus on the right example, objects whose colour or appearance varies according to thickness.
These objects are said to be transparent (if they are clear) or translucent (if they are blurry). And they are quite common in everyday life: liquids, plastics, glass, smoke, etc.
The appearance of many transparent/translucent materials can be described with three parameters:
– The refractive index is directly related to the speed of light in that material. While refraction can have an impact on light scattering (the well-known dispersion of light through a prism), it will also directly affect the reflectivity of the surface. For example, glass has an index of refraction around 1.5 and diamond around 2.4, therefore, diamond reflects much more light than glass.
– The absorption of a material defines its property of absorbing part of the light spectrum while allowing part of it to pass through. The part of the light spectrum that is not absorbed gives the perceived colour of a material. Besides, this effect depends on the thickness of the material considered. For example, absorbs a large part of the spectrum except red, so it will often appear dark red like a full glass of wine. But when the glass is empty, that only a few drops remain, it will appear rather light red or even pink.
– The diffusion of a material corresponds to the properties of a material to rediffuse the light it receives. The colour perceived by the diffusion will correspond to the diffusion spectrum. For example, milk diffuses a lot of light compared to its absorption, so it appears white to us. As absorption, the thicker the material is, the stronger the effect will be. But scattering is often mixed with absorption. We will not go into detail here such as the potential anisotropy of the diffusion.
These three physical parameters interact together to give the final appearance of an object. Therefore, it is mandatory to precisely capture and characterise these effects.
To capture the appearance of such materials, we can try to measure the BSDF, with the reflectivity and transmission distribution of a sample. Apart from the technical issues this could pose for liquids or gases, the real challenge is not here.
For manufacturers of materials such as plastics or glass, it is common practice to manufacture reference samples, with a precise thickness, in a specific material. And this approach can quickly become limited or expensive if samples of different thicknesses or even shapes have to be produced. In fact, the BSDF of such a sample would only work for a fixed thickness. So this solution would also reach its limits quite quickly.
To provide a more universal, flexible and versatile solution, it is more interesting to characterise the three parameters described in the previous section: refractive index, absorption and scattering. Because they are the intrinsic properties of a material, these parameters are not related to the shape or thickness of a material.
To avoid producing a large number of physical samples to visualise transparent or translucent materials, virtual prototyping proves to be a quick and comparatively inexpensive solution.
Once the properties described above have been characterised, you can use them on any shape, in any lighting or environment.
Eclat Digital has developed its own method of extracting these optical data from a measurement set, without any specific instrumentation other than an integrating sphere spectrophotometer. Instrumentation frequently present in the measurement laboratories of manufacturers of translucent or translucent materials.
To illustrate this approach, we carried out this study internally. We therefore characterised a set of three samples:
. Clear dark blue, without scattering
. Light blue, strong scattering
. Red, low scattering
The shape of these ‘masterbatch’ type samples has several thicknesses (here 1, 2, 3, 4 and 5 mm).
| Picture 2 |
On this masterbatch sample, we have different zones corresponding to different thicknesses to appreciate the volume effects.
Reflection and transmission measurements were made with an integral sphere on the area of the sample corresponding to 1mm thickness :
| Picture 3 |
The solid line curves represent the measured total reflectance of the samples: dark blue, light blue and red. The dashed curves represent the total measured transmittance of the samples: dark blue, light blue and red.
If we look at each sample:
– Dark blue :
. We can see that the reflectivity is very monotonous, since it is a sample without diffusion, it is only related to the refractive index.
. The transmission characterizes the blue colour.
– Light blue :
. The reflectivity here is much more consequent and not monotonous, it is the signature of a rather important colour diffusion.
. The transmission corresponds again to blue, more balanced, therefore less saturated than the previous sample.
– Red :
. The reflectivity is less pronounced and not monotonous. So, there is some red diffusion here.
. Transmission is quite high, characteristic of a fairly translucent sample.
Once these measurements have been made, we will extract the volume properties of each material to get rid of the thickness of the measured sample. In fact, we can apply these properties to any shape or thickness.
We can first re-simulate the appearance of the sample under controlled conditions, such as a light booth, to make a first appearance control rendering by comparing to reality.
| Picture 4 |
From left to right: transparent dark blue, highly diffusing translucent light blue, slightly diffusing translucent red.
Starting from the desired model, here it will be water bottles, you can apply these properties to the 3D model and run a simulation to see how it looks.
| Picture 5 |
On the left, the CAD view of the three bottles. On the right the rendering of these bottles in a lightbooth (Illuminant D65). From then on, you can see the final aspect of the materials on the chosen shape.
The different aspects of translucent and transparent materials were discussed. Eclat Digital has developed its own process to characterize these materials in order to render them under different conditions. This process really illustrates that virtual prototyping can be a faster and more economical solution than the manufacture of physical samples. Moreover, these virtual prototypes can be taken further in-situ, by integrating a realistic context (shop shelves, photo studio) as well as additional elements: liquids, labels, etc., to visualize the interactions between all these materials.
Here we have only looked at the optical volume properties of a translucent/transparent material. Obviously, surface effects can be added and interact together: coatings, roughnesses and even textures.