It is sometime difficult to define common concepts. The Color of what is around us and it deeply affects the every day live is a simple concpet, but hard to understand on all its scientific aspects.

However th Color is a very important factor and it influences our sensations: think of the feelings stimulated by an impressionist painting, or the different mood provoked by a beautiful sunny day and a gray autumn day.

We can think of color as a subjective feeling still not completely known in detail (when we talk about color, often actually want to talk about color differences between different objects); its measure still poses several problems related particularly to this aspect of subjectivity.

The color of an object is defined by 3 basic elements:

1. The source (or illuminant, but the two terms are not exactly the same thing)
2. The object

THE ILLUMINANT

An object can be its own light source (the sun, a light bulb, etc.) or be illuminated. In any case our main detector (the human eye) is hit by electromagnetic radiation, and processes, together with our brain, the color sensation.

As already mentioned, terms are not synonymous. With Source we mean any electromagnetic radiation emitting in the visible, while the Illuminating term refers exclusively to sources with spectral distribution of numerically known and defined power.

In particular, we must know, for each wavelength in the visible, exactly how many photons per unit area and time are emitted. Below is reported the spectral distribution of some standard lighting power:

From a numerical point of view, first elemnent of the 'trio' Illuminat-Object-Observer is completely defined by its spectral power distribution:

SOURCE  --->  S(λ)

2. THE OBJECT

The visibile electromagnetic radiation coming from the Source hits the Object, which can:

• reflect
• absorb
• transmit
• diffuse
• re-emanate

Sometimes, all these mechanisms are important, sometimes one is dominant and others can be neglected.

The origin of the color resides in the mechanisms of interaction between electromagnetic radiation and matter, very complex issue that must involve concepts of quantum electrodynamics. We are content here to remember that, once fixed the illuminant, the color of an object can be different depending if the observation is in reflection or in transmission, and depends on the angle of observation and the incidence of light; All these parameters must be specified and known when comparing colors between them.

Follow you will find a scheme of the different ways of an incident radiation:

In this case the incident beam I hits a material layer dx; a part is reflected (specularly and diffusely), a part absorbed and the remainder transmitted. S and K are respectively the scattering coefficients and absorption of the material. K and S are strongly dependent on the wavelength. If performed with spectrophotometers, the color of an object reading provides the reflectance factors (if you are speking of transmittance there are more complications due to the thickness of the object).

The seond element of the Trio is completely characterized by the Reflectance factors R(I):

REFLECTANCE  --->  R(λ)

Reflectance factors are measured at definite values of wavelenghts. Usually 16 or 31 numbers are measured from 400 nm to 700 nm at intervals of 20 nm or 10 nm.

3. THE OBSERVER

It is the last term of the Trio, and may be the most complex, not completely known today in details.

From a numerical point of views, the goal is to simulate as closely as possible the response of the eye to light stimuli. Without going into detail, we remember that the light entering the eye is focused on the retina, where there are fundamental receptors that allow us to see. These receptors are of two types: cones and rods. Rods are more sensitive but do not distinguish colors, the cones are active instead of high light conditions (typically daytime) and are 3 different types: we can think of having three types of color receptors (cones) different, respectively sensitive to blue, red and green.

The description of chromatism of the eye gives 3 spectral sensitivity functions, named x, y e z. The most diffue formulas are the CIE raccomandations (CIE 1964 at 10°).

The Eye is then defined by these 3 functions

OBSERVER --->

At this point, starting form the functions which describes respectively the Source, the Object and the Observer, we can measure a Color with 3 NUMBERS, named Tristimulus Values X, Y and Z, by multiplying the 3 functions and sum for every wevelenght:

COLOR IS THEN DEFINED COMPLETELY BY THE TRISTIMULUS VALUES X, Y e Z.

Since, however, these numerical values ​​do not represent directly the sensations that currently use to describe a color (such as hue, saturation, brightness, etc.), other numbers are derived from these numbers which are closer to common description of a color . One of the most used system is the CIELAB (or CIE 1976), which introduces the variables L *, a * and b * derived from X, Y, Z. L * is a brightness index, a * and b * coordinates that take values ​​from negative to positive and respectively indicate components ranging from green to red and from blue to yellow.

Dr. Maurizio Veronelli

RDLab137 srl - Milano

Last revision: 01/21/2017