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Colour

UNDERSTANDING COLOR

To help you clearly understand how color is measured, we should first study the fundamentals of color’s physical and physiological properties.

Color results from an interaction between light, object, and the viewer. It is light that has been modified by an object in such a manner that the viewer—such as the human visual system—perceives the modified light as a distinct color. All three elements must be present for color as we know it to exist. Let’s examine color’s origins in more detail by first studying light.

Light—Wavelengths and the Visible Spectrum

Light is the visible part of the electromagnetic spectrum. Light is often described as consisting of waves. Each wave is described by its wavelength – the length from wave crest to adjacent wave crest. Wavelengths are measured in nanometers (nm). A nanometer is one-millionth of a millimeter.
The region of the electromagnetic spectrum visible to the human eye ranges from about 400 to 700 nanometers. This amounts to a mere slice of the massive electromagnetic spectrum. Although we can’t see them, we use many of the invisible waves beyond the visible spectrum in other ways—from short-wavelength x-rays to the broad wavelengths that are picked up by our radios and televisions

Our eyes have light sensors that are sensitive to the visible spectrum’s wavelengths. When light waves strike these sensors, the sensors, the senors send signals to the brain. Then, these signals are perceived by the brain as a particular color. Exactly which color is perceived depends on the composition of wavelengths in the light waves. For example, if the sensors detect all visible wavelengths at once, the brain perceives white light. If no wavelengths are detected, there is no light present and the brain perceives black.

Now we know how our eyes and brain respond to the presence of all visible wavelengths or no wavelengths. Next, let’s examine how our vision system responds to each individual wavelength.

Passing a beam of white light through a prism disperses the light so that we can see how our eyes respond to each individual wavelength. This experiment demonstrates that different wavelengths cause us to see different colors.We can recognize the visible spectrum’s dominant regions of red, orange, yellow, green, blue, indigo, and violet; and the “rainbow” of other colors blending seamlessly in between.

When our visual system detects a wavelength around 700nm, we see “red;” when a wavelength around 450-500nm is detected, we see “blues;” a 400nm wavelength gives us “violet;” and so on. These responses are the basis for the billions of different colors that our vision system detects every day. However, we rarely see all wavelengths at once (pure white light), or just one wavelength at once. Our world of color is more complex than that. You see, color is not simply a part of light—it is light.When we see color, we are seeing light that has been modified into a new composition of many wavelengths. For example, when we see a red object, we are detecting light that contains mostly “red” wavelengths. This is how all objects get their color—by modifying light.We see a world full of colorful objects because each object sends to our eyes a unique composition of wavelengths. Next, let’s examine how objects affect light.…cont soon

Objects—Manipulating Wavelengths

When light waves strike an object, the object’s surface absorbs some of the spectrum’s energy, while other parts of the spectrum are reflected back from the object. The modified light that is reflected from the object has an entirely new composition of wavelengths. Different surfaces containing various pigments, dyes, and inks generate different, unique wavelength compositions.

Light can be modified by striking a reflective object such as paper; or by passing through a transmissive object such as film or a transparency. The light sources themselves – emissive objects such as artificial lighting or a computer monitor – also have their own unique wavelength composition



Reflected, transmitted, or emitted light is, in the purest of terms, “the color of the object.” There are as many different colors as there are different object surfaces— each object affects light in its own unique way. The pattern of wavelengths that leaves an object is the object’s spectral data, which is often called the color’s “fingerprint.” Spectral data results from a close examination—or measurement—of each wavelength. This examination determines the percentage of the wavelength that is reflected back to the viewer—its reflectance intensity.    …cont soon

Viewer—Sensing Wavelengths as “Color”

For our visual palette of colors to exist, all three elements of color—light, object, and
viewer—must be present.Without light there would be no wavelengths; without
objects there would be only white, unmodified light; and without the viewer there
would be no sensory response that would recognize or register the wavelengths as a
unique “color.”
There is a well-known riddle that asks: “If a tree falls in the woods and no one is
there to hear it, does it make a sound?” Actually, a similar question can be asked in
regards to color: “If a red rose is not seen, does it have color?” The answer—which
may surprise you—is no. Technically, color is there in the form of wavelengths (the
spectral data). However, the color we know as “red” only happens in our minds, after
our visual sensory system has responded to those wavelengths.

The basis for human vision is the network of light sensors in our eyes. These sensors
respond to different wavelengths by sending unique patterns of electrical signals to
the brain. In the brain, these signals are processed into the sensation of sight—of
light and of color. As our memory system recognizes distinct colors, we then
associate a name with the color.
So, do our brains also examine discrete wavelength information and plot curves for
every color we see? Not exactly. The human visual system must work far too quickly
to do all that, given the deluge of new wavelength information that it receives every
second. Instead, this system’s miraculous design uses a more efficient method for
“mass-processing” wavelengths. It breaks the visible spectrum down into its most
dominant regions of red, green, and blue, then concentrates on these colors to
calculate color information.

RGB—Color’s Additive Primaries

By mixing these dominant colors—called the additive primaries—in different combinations
at varying levels of intensity, the full range of colors in nature can be very
closely simulated. If the reflected light contains a mix of pure red, green, and blue
light, the eye perceives white; if no light is present, black is perceived. Combining two
pure additive primaries produces a subtractive primary. The subtractive primaries of
cyan, magenta, and yellow are the opposing colors to red, green, and blue.

The human eye’s three-value color system has been imitated
and exploited—by inventors of color scanners, monitors, and
printers. The color rendering methods used by these devices
are based directly on our response to stimuli of red, green,
and blue light.

Like the human eye, these devices must also process a large amount of color information
at once—on screen or on paper. In logical fashion, these devices imitate the eye’s
response to the additive primaries to create a colorful illusion: For example, a monitor
blends varying intensities of red, green, and blue light at each of its tiny pixels. These
pixels are so small and tightly packed that the eye’s RGB response is “fooled” into the
perception of many different colors when really there are only three.

CMY and CMYK—The Subtractive Primaries……..will continue

CMY and CMYK—The SubtractivePrimaries

Monitors and scanners can employ the additive color system because they are emissive devices—they can directly add red, green, and light to darkness. Printers, on the other hand, must render colors on paper and other substrates, so they must work with reflected light. To do this, printers employ the opposing subtractive primaries of cyan, magenta, and yellow.

In the visible spectrum, cyan is directly opposed to red; magenta is the opposite of green; and yellow is the opposite of blue.When cyan, magenta, and yellow pigments are deposited on a white, reflective substrate, each completely absorbs—or subtracts—its opposing counterpart from the oncoming white light. For this reason, the printing process uses cyan, magenta, and yellow inks to control the amount of red, green, and blue light that is reflected from white paper.

These colors are printed on paper as separate layers of halftone dot patterns. The illusion of different colors and tones is created by varying the size, balance, and angle of the dots. The effects of varying dot sizes is similar to the varying intensities of a monitor’s red, green, and blue phosphors.
This diagram demonstrates how the subtractive primaries remove their additive counterpart from light to produce the appearance of a color:

HSL—The Three Dimensions of Color…will continue soon

25-05-12 Update

HSL—The Three Dimensions of Color

So far, we’ve learned that color consists of complex wavelength information, and
that the human eye, monitors, and printers, convert this complex information into
three-value systems of primary colors in order to simplify processing and rendering
of that information. Another way to simplify color description is to describe its three
attributes or “dimensions:”
• Hue—its basic color, such as red, pink, blue, or orange.
• Saturation—its vividness or dullness.
• Lightness—its brightness or darkness.

Light waves also have three attributes that directly affect the attributes of hue,
saturation, and lightness. Of course, wavelength determines the color’s hue; wave
purity determines saturation; and wave amplitude (height) determines lightness.

Spectral curves demonstrate the relationship between wave attributes and the way
we perceive these attributes.
Vibrant, colorful objects reflect a distinct part of the spectrum at high intensity;
objects that are near-white or light gray reflect most of the spectrum uniformly and
at high intensity; dark gray, dark brown, and black objects absorb most of the spectrum’s
energy; and so on.

Color Space—Mapping Color’s Dimensions

Hue, saturation, and lightness demonstrate that visible color is three-dimensional.
These attributes provide three coordinates that can be used to “map” visible color in
a color space. The early-20th Century artist A.H.Munsell—creator of the Munsell
Color Charts—is credited as a pioneer of intuitive three-dimensional color spacedescriptions. There are many different types of color spaces that are based on or
resemble Munsell’s designs.
Basically, a color space based on hue, saturation (or chroma), and lightness (or value)
uses cylindrical coordinates. Lightness is the center vertical axis and saturation is the
horizontal axis that extends from the lightness axis. Hue is the angle at which the saturation
axis extends from the lightness axis.
We can apply the relationship between wave attributes and color attributes to a three-dimensional color space.Wave amplitude determines a color’s position on the
lightness axis; wave purity determines its location on the saturation axis; and wavelength
determines hue angle. Around the “equator” lie vibrant, pure hues. As the hues
blend together toward the center, they become less pure and lose saturation. On the
vertical axis, colors of different hue and chroma become lighter or darker. The lightness
extremes of white and black lie at the “poles.” And of course, at the center of it
all lies neutral gray—where white, black, and all hues meet and blend together.

Tristimulus Data…. comming soon 25-05-12


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