Notes on Color

Why we can perceive color

There are two classes of retinal photo-receptors in human eyes: rods and cones. They are named by their prototypical shapes, rods tend to be long and slender while cones are conical (Though this distinction can be misleading as foveal cones are also long and slender since their high density). The real difference between them lies in their functions: rods serve vision at low luminance levels, allowing us to see in dim in dim environments. Cones, on the other hand, serve vision at higher luminance levels.

There are three types of cones while only one type of rods. The three types of cones are responsible for color vision, as they are sensitive to three different wavelength ranges of wavelength (roughly speaking): long (red), middle (green), and short (blue). Human eyes combine the signals from the three cone receptors to perceive color, hence the trichromacy. Color blindness is normally due to the lack of long-wavelength-sensitive cones or middle-wavelength-sensitive cones. We have three types of cone photo-receptors is the reason why all the color spaces are three dimensional. Some animals, like chickens, have as many as 12 color sensitive cells.

Color

“Color is an attribute of visual sensation”(page 58, Color Apperance Models, Third Edition).

An object’s color is determined by three aspects:

1. The light source (A source of visible electromagnetic energy),
2. The physical and chemical properties of the object, which modulate the electromagnetic energy (e.g., What and how much light does the object reflect, refract, and absorb?),
3. The human visual system, which detects the modulated electromagnetic energy through photoreceptors and processes by neural mechanisms.

Colorimetry

The measurement of color.

Luminance, brightness, and lightness

The following definitions are from Information Visualization: Perception for Design, Third Edition, page 80.

Luminance refers to the measured amount of light coming from some region of space. It is measured in units such as candelas per square meter.

Brightness refers to the perceived amount of light coming from a source.

Lightness refers to the perceived reflectance of a surface.

These definitions are more specific to the book than to a general definition. In Color Appearance Models-ibid., Brightness is attribute of a visual perception according to what an area appears to emit, or reflect, more or less light. While Lightness is the brightness of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transimitting.

To sum up, luminance emphasize the physical amount of light, brightness the perceived amount of light, and lightness the comparsion to white.