Times are not always the same; the grass is not always green.”
Let there be the colors
LET THERE be THE COLORS!
Times are not always the same; the grass is not always green.”
one of the biggest challenges of our project has been to make our stylus match the colors of the Note Air. Some will say that it suffices to take a picture of the tablet and then use the color picker functionality of a graphic editor to identify the desired color. Unfortunately, color perception varies widely and is affected by the surrounding illumination, the size of the object, the surrounding colors, and the angle of observation. Thus, identifying the color of a sample object is not an easy task.
First things first: How we see colors
The human eye possesses photoreceptors —specialized cells that are sensitive to light and transmit information about light to our brain. There are two types of photoreceptors: cones, which are sensitive to colors; and, rods, which are sensitive to intensity. We are able to see objects because the light that strikes them is reflected back into our eyes. We can only perceive visible light, a narrow band of the electromagnetic spectrum, which also includes non-visible radio-waves, infrared light, ultraviolet light, X-rays, and Gamma rays. Different wavelengths of light are perceived as colors.
The cones in our eyes are sensitive to visible wavelengths of light that correspond to short-wavelengths (blue color), medium-wavelengths (green color) and longer-wavelengths (red color). All other colors are seen as mixtures of these primaries. Interestingly, color is not an inherent property of objects. Rather, the surface of an object reflects some wavelengths and absorb others. We perceive only the reflected wavelengths. For instance, the surface of a “red” apple is reflecting the wavelengths we see as red and absorbing all the rest. An object appears white when it reflects all wavelengths and black when it absorbs them all.
In colorimetry, there are two basic color models: Additive (light) color primaries and subtractive (pigment) color primaries.
In the Additive model red, green, and blue are the primary colors of light. They are combined in different proportions to make all other colors. Colors are then represented in a coordinate system known as RGB. This additive color system is used by light sources, such as televisions and computer monitors.
The Subtractive model employs another set of primary colors with which you might be more familiar: cyan, magenta, and yellow (commonly simplified as blue, red, yellow). These are colors of pigment, which are used when producing colors from reflected light; for example, when using a color printer. Pigments are chemicals that absorb selective wavelengths. For instance, cyan pigments absorb red light but reflects blue and green light; yellow pigments absorb blue light but reflects red and green light. If a cyan pigment is mixed with a yellow pigment, then you will see a green color because both red and blue light are absorbed, and only green light is reflected.
When all CMY colors are added together we obtain a near black. To achieve a true black, pure black must be added, thus giving the CMYK model (K=black). This is the standard color model for most printing systems.
The primaries in the CMYK model are ultimately derived from the RGB model as secondary colors. Thus, from a color identified in the RGB system, we can know its equivalent in the CMYK.
Now that we understand how we perceive colors and the way they are represented, it comes the first challenge in matching the Note Air’s color: Color identification.
We need first to determine which sample colors we want to identify. The Note Air has two blue background colors. The first is a metalized Navy blue that covers the aluminum case. The second blue decorates the left side of the bezel. In addition, two stripes and the Boox logo ornament the tablet with an orange color. We needed to choose between the two blue colors, and we decided to go for the second one. Although this blue color is less predominant than the first, it is the more visible of the two, as it is in the front of the tablet. On the other hand, for us it is impossible to obtain the metallic effect characterizing the case of the tablet. Thus, even if we were able to identify the same blue color, the perception of this color in a matte tone would be very different from that observed with the metallic effect.
In order to identify the blue color on the bezel, we have used a colorimeter. A colorimeter is a device that performs accurate measures of sample colors, eliminating issues related to color subjectivity. In particular, by blocking out all ambient light and providing its own controlled light source, colorimeters are able to ignore bright light and dark shadows and perform an accurate reading, every time. Colorimeters employ three photocells as receptors to see color in much the same way as the human eye.
Without any intention to make advertisement, and for those who may be interested, we have used the Nix Sensor Tool, which scans any surface (like painted walls) to find color codes in the RGB and/or CMYK systems (or find its paint match).
Combination of Pigments
If we were printing on a sheet of paper, it would have been sufficient to identify the desired color code. We would have simply loaded our print job, and the printer would have made the correct mixture of CMYK inks to obtain the desired color. Unfortunately, resin 3D printing doesn’t use conventional ink, but photosensitive resins cured by a light source. Moreover, unlike a traditional printer, a 3D printer does not perform the pigment mixing required to obtain a certain color. It is the user’s task to provide the resin with the right color.
Because the CMYK colors are obtained via reflection of light, a pure white ground resin is required as the base to which pigments are added. Pigments modify the chemical, thermal, and mechanical properties of the base resin, which may cause failures during printing, deformation of printed objects, and poor tensile strength. Consequently, not all pigment mixtures are viable, which means that some colors cannot be achieved.
On the other hand, pigment mixtures in the proportions specified by a CMYK code do not always result in the desired color. This is so because pigments do not correspond to the theoretical colors specified by the CMYK model. That is, a cyan pigment, for instance, does not absorb exactly the same wavelengths light as the theoretical cyan color.
Despite all the above, we can approximate as much as possible to our target color. For that, we compare the resulting color from a particular pigment mixture with our sample color. To evaluate how different are two colors we use the Delta-E distance. This is a metric that allows to quantify distances between points in the color space. The average human eye cannot detect any color difference with a Delta-E of 3 or less. To give you an idea of how different Delta-E values appear in real-life look at the following sets of color comparisons (please notice that the quality of your monitor or smartphone might affect your perception of colors).
Following an iterative back-and-forth process of pigment combinations, Delta-E distance evaluation, and pigment mixture adjustment, we were able to achieve a Delta-E difference of about 10 for both colors. Our blue is a little brighter than the target blue over the bezel of the tablet. However, despite this noticeable difference, we are very satisfied with the final result. You can judge for yourself below.
We hope that you liked learning a bit about light, and how we perceive and analyze colors. If you did enjoy reading this article, you may consider signing up to our mailing list. You can also follow us on our Facebook page where we’ll share additional material.
Thank you very much for your interest in this adventure, we love to have you on board! We are looking forward to sharing more of this journey with you here on the eTrament project.
Audrey & Andrés
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Audrey & Andrés
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