COLOR

Visible Light

Color Science – Light & Spectra

� Light is an electromagnetic wave� It’s color is characterized by it’s wavelength

� Most light sources produce contributions over many wavelengths, contributions fall in the visible wavelength can be seen

� Light from 400 to 700 nanometer (10-9

meter)

Color Science – Light & Spectra

Light, Color, and Vision

Light Absorption, Reflection, and Transmission

ROYGBIV: red (R), orange (O), yellow (Y), green (G), blue (B), and violet (V).

Single Frequency Light

� When it strikes an object, it can be:� absorbed by the object, changes to

heat

� reflected by the object

� transmitted by the object

Vibrating Electrons Match

� Electrons of a material tend to vibrate at a natural frequency (like a tuning fork)

� Light frequency matches the electron frequency, resonance occurs

� Electrons absorb the energy, convert it to vibrational motion

� Interact, convert to thermal energy

Absorption:

� electrons vibrate for brief periods of time with small amplitudes of vibration

� the energy is reemitted as a light wave

� If transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object

Vibrating Electrons Not Match

� If object is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material.

� the electrons of atoms on the material's surface vibrate for short periods of time and then reemit the energy as a reflected light wave.

Vibrating Electrons Not Matchcont’d

Reflected Light

� The color of an object is not actually within the object itself

� the color is in the light which shines upon it and is reflected or transmitted to our eyes.

� So if an object absorbs all of the frequencies of visible light except for the frequency associated with green light, then the object will appear green in the presence of ROYGBIV

Reflected Light

Transparent Materials

Key Point:

Electromagnetic Spectrum

� The range of all possible electromagnetic radiation

� EM spectrum ranges from frequencies below those used in modern radio (wavelengths over 1000 km) to gamma radiation (wavelengths smaller than atoms).

EM Spectroscopy

� When electrons change to a higher energy level they must absorb energy, and when they move to a lower energy level they give off energy.� Ground state vs excited state

� The use of a spectroscope to observe this EM radiation is called EM spectroscopy.

� This EM radiation can be classified as either an absorption spectra or emission spectra

Absorption Spectra

� “Dark line spectra”

� Electrons can jump to a higher energy level, but the atom must absorb energy in order to reach this excited state

Absorption spectra are different Absorption spectra are different for each elementfor each element

Hydrogen

Sodium

Emission SpectraEmission Spectra

�� ““Bright line spectraBright line spectra””

�� When electrons leave the excited state When electrons leave the excited state and return to the ground state, they and return to the ground state, they emit energyemit energy

Emission spectra are different for Emission spectra are different for

each elementeach element

Iron

Hydrogen

WhatWhat’’s the point?s the point?

�� Each element gives off unique bright Each element gives off unique bright line and dark line spectraline and dark line spectra�� Can be used to determine the composition Can be used to determine the composition of stars, comets, and planets by analyzing of stars, comets, and planets by analyzing the received lightthe received light

�� Can be used to determine the concentration Can be used to determine the concentration of chemical compounds in a sampleof chemical compounds in a sample

�� Reports of its use in differentiating malign Reports of its use in differentiating malign tumors from benign. tumors from benign.

Color Theory

Primary colors

• Irreducible components of color• Combinations of the 3 primaries

produce entire (infinite) spectrum of color

ADDITIVE primary

colors of LIGHT Green

Red Blue

Yellow

Magenta

Cyan

RGB color of computer

monitors, television, and (approximately) human

vision

A computer monitor pixel is composed of 3 subpixels (each a tiny transistor) with red, green, and blue filters. Through the careful control and variation of the voltage applied, the intensity of each subpixel can range over 256 levels (black =0, white=255). Combining the subpixels produces a possible palette of 16.8 million colors (256 shades of red x 256 shades of green x 256 shades of blue).

SUBTRACTIVE primary

colors of pigments

Yellow

Magenta Cyan

Red

Blue

Green

SUBTRACTIVE primary

colors of TRANSPARENT PIGMENTS/INKS

CMYK color of printing

ADDITIVE mixture involves the addition of spectral components (light)

SUBTRACTIVE mixture involves the absorption (or subtraction) of spectral components (pigments and dyes)

Color Models for Image Color Models for Image –– RGB Vs CMY RGB Vs CMY [2][2]

Color Models for Image Color Models for Image –– CMYKCMYK

�� Eliminating amounts of Eliminating amounts of yellowyellow, , magentamagenta, and , and cyancyan that that would have added to a dark neutral (black) and would have added to a dark neutral (black) and replacing them with black inkreplacing them with black ink

�� FFourour--color printingcolor printing uses black ink(K) in addition to the uses black ink(K) in addition to the subtractive primaries subtractive primaries yellowyellow, , magentamagenta, and , and cyancyan..

�� Reasons for Black addition includesReasons for Black addition includes

�� CMY Mixture rarely produces pure blackCMY Mixture rarely produces pure black

�� Text is typically printed in black and includes fine detailText is typically printed in black and includes fine detail

�� Cost saving : Unit amount of black ink rather than three unit Cost saving : Unit amount of black ink rather than three unit amounts of CMYamounts of CMY

Color Models for Image Color Models for Image –– CMYKCMYK[2][2]

� Used especially in the printing of images

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