colour & masterbatch | Understanding colour
using the same pigment. This is because the two materials have completely different refractive indexes. Diffuse refl ection, on the other hand, dictates how an
Figure 2: When a green pigment is added to an object it will only refl ect the green colour, while other colours are absorbed
We’ll begin by looking at the four elements of colour: light, object, human eye (observer), and human brain. Light: Light is a continuum of electromagnetic
energy and can be broken into two categories. The fi rst is “source”, a device serving to illuminate, while the second is “illuminant”, a theoretical source of visible light with documented attributes that are used as a basis for comparing colours. Illuminants can represent average incandescent light, direct sunlight, daylight, and more. For example, Illuminant D65 is the standard daylight illuminant that represents average illumination conditions throughout the world. The ‘D’ stands for daylight and 65 is the temperature, which happens to be 6,500 degrees Kelvin. Illuminant D65, and its counter- parts, are represented by a set of tables that can be used to simulate conditions and to view objects, and colour, in controlled lighting. Further, Illuminant F2 represents average illumination conditions for cool white fl uorescent light (See Figure 1). Object: Now, we’ll move on to what objects do to
Figure 3: Colour
perception is an extremely complex process
involving the interaction of the eye, optic nerves and brain
light. For starters, all objects interact with light in the same ways: Specular refl ection – refractive index, which is responsible for gloss; Diffuse refl ection – scattering of light, which is responsible for opacity and hiding; Absorption of light – mainly responsible for colour; Transmission – how light passes through the object. Every material reacts with light in these ways. But
it’s to what extent light refl ects, diffuses, absorbs and transmits with the object that gives it its appearance. Light absorption has a great impact on the colour of a
plastic object. For example, when a green pigment is added to an object it will only refl ect the green colour and all other colours are absorbed by the object (Figure 2). Specular refl ection is to what extent the light
scatters. Imagine you have an object made of polypro- pylene and an object made of polyester and you use the exact same pigment in both. You will likely fi nd that the two objects are completely different colours, despite
34 INJECTION WORLD | April 2014
www.injectionworld.com
object selectively absorbs (or hides) some colour. Start with a white pigmented fi lm and gradually add colour. For a while the fi lm will not change in appearance, effectively absorbing the colour. The moment in which the colour begins to be visible is known as the percola- tion point. For polymer engineers, this is an important moment. Slight changes in the object’s make-up or the pigment can change the percolation point, creating a teeter- totter effect when a product is produced. One time the colour is there, the next time it isn’t. This is caused by the scattering of light.
Light absorption is what truly provides visible colour, which is what is refl ected back from an object when it is exposed to a light source – therefore, the type of light source directly impacts the appearance of the object. Human Eye: Perception of colour is the result of an
extremely complex process. While we still do not fully understand how it works, we do know that it involves the interaction of three separate areas: the eye, the optic nerves and the brain. There are two kinds of light-sensitive cells in the eye
– rods and cones. Rods control sensitivity to light and are responsible for night vision. They are so sensitive that during daylight they turn off. This is why you might feel blind when you enter a movie theatre from a brightly lit street. When the light level falls below their overload threshold, they slowly regain their sensitivity, a process that takes about 30 minutes to complete. Where rods are colour blind, cones step in and are
responsible for distinguishing colour. Approximately 7 million cones are packed into the area of the retina called the fovea. They are not nearly as sensitive to light as rods, but are able to distinguish between colour wavelengths.
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