Color Fundmentals

• Color involves three fundamental elements: a light source, an object and an observer.
• Color is the human perception of light reflected by the object from the light source.
• The color is determined by the spectral composition of the reflected light.
• Visible light is the range of about 400 to 700 nanometers.

What is White Light? : Color Temperature

• A heated object emits electromagnetic radiation over a range of frequencies.
• The distribution of frequencies (hence the color) shifts with the temperature.
• Light can be characterized in terms of its color temperature.
• Note that hotter temperatures corresponds to cooler (more bluish) light.

1500 K		Candlelight
2680 K		40 W incandescent lamp
3200 K		Sunrise/sunset
3400 K		Tungsten lamp
3400 K		1 hour from dusk/dawn
5500 K		Sunny daylight around noon
5500-5600 K		Electronic photo flash
6500-7500 K		Overcast sky
9000-12000 K		Blue sky

The effect of light source temperture on color perception

Sun behind cloud f/4; 1/10 s; ISO 400	Full sun (reflected) f/4; 1/90 s; ISO 400

Metamers

Solid line: Spectral composition from a red Cortland apple illuminated by a flourscent light source. Dotted line: Spectral composition from a red Cortland apple illuminated by a tungsten light source.

• Light is electromagnetic waves with wave lengths in the range of roughly 400 to 700 nanometers.
• Spectral composition is the distribution of power over that spectrum.
• It is possible for two different spectral compositions to be perceived as the same color, these are called a metameric pair, or metamers.

Solid line: Spectral composition for a real object and light source. Dotted line: Metameric spectral composition produced by a CRT monitor. The colors will be perceived as the same.

• Metamerism occurs because the human visual system includes only three types of color sensors, roughly corresponding to red, green and blue.
• This makes it possible to produce a broad gamut of colors by mixing red, green, and blue light.

Color Perception and Chromatic Adaptation

Additive vs. Subtractive Color

Additive Color (monitor) Subtractive Color (ink) Adding a component increases the spectral composition and results in a brighter combination. Adding a component decreases the spectral composition and results in a darker combination. All components full produces white. All components full produces black. All components zero produces black. All components zero produces white (paper color). Complements: Red light Cyan ink absorbs red light, reflecting blue and green. Blue light Yellow ink absorbs blue light, reflecting red and green Green light Magenta ink absorbs green light, reflecting red and blue.

In practice, subtractive CMY systems also include black ink (denoted K) because CMY don't produce a good black, and printers typically need black ink anyway for text.

Color Gamut

[Kodak 2000]	[Kodak 2000]
The CIE system is able to represent all possible colors that a human can perceive	A gamut defines the range of colors that can be produced by a particular color production method. a CMYK printers generally have a more limited gamut than an RGB montors. Print gamut can be increase by using more inks.

Color Spaces and Channels