Fundamentals of Colorimetry

I. Color Temperature

1. Concept of Color Space

A color space, simply put, is a collection of all possible colors.

The more colors it contains, the larger the color space is, and the broader the color gamut it can represent.

2. Lighting Effects

Lighting is closely related to color.

Without illumination, colors would be invisible regardless of the color of the light source.

The way colors appear depends on the properties of the light source. For example, the same object may appear differently under sunlight and under fluorescent light because their spectral distributions (the amount of each monochromatic component) differ.

In the comparison between natural daylight and fluorescent light:

You can see that fluorescent light contains more blue components and fewer red components.

Therefore, objects under fluorescent light tend to appear “whiter” than under sunlight.

3. Color Temperature (Correlated Color Temperature)

“Color temperature” is a technical term used to describe the type of light source.

It is based on the temperature of an idealized object called a blackbody, which can completely absorb all wavelengths of light.

The unit is Kelvin (K). (0 K = –273.15 °C)

When a blackbody is heated, its color changes in the sequence:

Red → Orange → Yellow → White → Blue-White

Stars behave similarly to blackbodies; thus, blue stars have higher temperatures than red stars.

Although we describe light sources such as fluorescent lamps and halogen lamps using color temperature values, their 5000K color temperature does not correspond to an actual blackbody color, because these light sources are not blackbodies.

Color temperatures assigned to non-blackbody sources are called correlated color temperature (CCT).

4. Standard Light Sources

Definition: Standard illuminants are defined by the CIE (International Commission on Illumination).

Standard Light Source	Color Temperature	Meaning
A	2856K	Incandescent lamp (tungsten filament lamp)
D65	6504K	Synthetic daylight (daylight containing ultraviolet)

Common color temperatures:

Object	Color Temperature
Standard Light Box	D65 (for viewing reflective samples)
Xenon Headlights	5000K
Ordinary Light Bulb	3000K
High Altitude Clear Sky	10000K

Auxiliary standard illuminants:

Light Source	Color Temperature
D50	5003K (For viewing transmitted samples)
D55	5503K
D75	7504K
C	6774K

II. Visual Attributes of Color

Color has three attributes (except black, white, and gray):

Hue, Saturation, Brightness

Hue

The basic characteristic of a color, used to distinguish colors such as red or blue.

Saturation

The purity of a color.

Highly saturated red appears vivid, while low-saturation red appears grayish.

Brightness (Lightness)

Brightness indicates the lightness or darkness of a color based on how much light the object reflects.

III. Device-Dependent Color Systems

* Color content depends on the device generating the color.

* Device-dependent systems cannot reliably transfer color across different devices.

Common device-dependent color systems:

1. RGB Color Model

* RGB is the primary color model for light.

* Mixing increases brightness (becomes lighter).

* Used primarily for monitors and scanners.

2. CMYK Color Model

* CMYK is the primary model for pigments/inks.

* Mixing decreases brightness (becomes darker).

* Used mainly for printing.

3. HSB (Hue, Saturation, Brightness)

* Developed by Alvy Ray Smith in 1978, based on artistic terminology.

* Used in CorelDRAW, Photoshop, and Macintosh color pickers.

4. HSL (Hue, Saturation, Lightness)

* Developed in the late 1970s.

* Commonly used in Microsoft Office applications.

IV. Device-Independent Color Systems

* Designed to describe colors accurately and consistently across devices.

* Suitable for cross-device color communication.

Examples:

CIE XYZ, CIE Luv, CIE Lab

CIE = Commission Internationale de l’Eclairage (International Commission on Illumination)

1. Uniform Color Spaces

Uniform color spaces match human visual sensitivity.

The chromaticity diagram represents color perception, while the luminance curve represents brightness sensitivity.

Together, they form a 3D uniform color space.

In 1976, the CIE released two major uniform color spaces:

1. CIE 1976 L*u*v*

2. CIE 1976 L*a*b*

2. Differences Between Luv and Lab

• CIE L*u*v*: Widely used in industries involving light, such as television.

• CIE L*a*b*: Widely used in digital printing and output industries.

3. CIE L*a*b* Color Space

L*a*b* is composed of:

* L*: Lightness

* a*: Green–Red axis

* b*: Blue–Yellow axis

The center point is achromatic (neutral).

The difference between two colors can be expressed using ΔE, which is the distance between two points in the L*a*b* space.

ΔE Range	Color Difference Level
0.0 - 0.2	None to Slight
0.2 - 1.0	Very Slight
1.0 - 3.0	Slight
3.0 - 6.0	Noticeable
More than 6.0	Large
L: Lightness a: Green–Red axis (Positive = Red, Negative = Green) b: Yellow–Blue axis (Positive = Yellow, Negative = Blue)