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The Tech Challenge 101 initiative is designed to help marketing and corporate executives expand their knowledge of CMOS image sensor technology and cell phone design through a quick-read series of focused insights and articles. Please check back periodically for new tech topics.

Featured Article: "Color Theory Behind the Scene: Color Calibration of a CMOS Image Sensor" --By John Lin, Senior Design Engineering Manager, Toshiba America Electronic Components, Inc.

Highlights:

• How the human eye perceives color plays an important role in developing good image quality with accurate color representation.
• Light is considered a form of electromagnetic radiation. The visible wavelength is from 380 to 780nm. The color of the light depends on the distribution over the spectrum. Different wavelengths appear as different colors.
• The color of an object is defined by two spectra: the surface reflectance of the object and the light source shining on it. The product of these two spectra is the light that enters the eye and stimulates the cones.

Accurate color representation is one of the most important aspects in evaluating the image quality of an image sensor. To fully understand color representation, one must first be knowledgeable in the age-old science of color theory and how it plays a critical role behind the scene in converting the real world into electronic images using advanced image sensor technology.

To explain, in a simple fashion, how color is reproduced and calibrated in the CMOS image sensor from Toshiba using a software-based system, it is important to first cover the topic of color vision. How the human eye perceives color plays an important role in developing good image quality with accurate color representation.

This article discusses color theory and how it is used in CMOS image sensor color reproduction from sensor to printer and display.

Color Perception

Human vision starts with light striking the retina. The photoreceptor cells in the retina encode light into signals that are interpreted by the brain to generate the perception of color. There are two basic types of cells-cone and rod. Cones generate the color perception information and rods are responsible for night vision. There are three type of cones that are sensitive to different wavelengths of light-long, medium and short wavelength cones or informally red, green and blue cones.

Fig. 1: Spectral Distribution

Light is considered a form of electromagnetic radiation. The visible wavelength is from 380 to 780nm. The color of the light depends on the distribution over the spectrum. Different wave-lengths appear as different colors. Fig. 1 shows 10 types of spectral distribution of 10 daylight sources. The area beneath the curve is the density of the energy, which is a measure of brightness to human vision.

The cone and rod cells show the following absorption characteristics.
Normalized absorption spectra of human cone

Color of an Object

The color of an object is defined by two spectra: the surface reflectance of the object and the light source shining on it. The product of these two spectra is the light that enters the eye and stimulates the cones.

Color vision theory

The signal sent to the brain is the product of input stimulate and the cone response. This is the foundation of the color vision theory.

The encoding created by the cones means that every spectrum is represented by exactly three signals. This is the principle of trichromacy. Digital colors are encoded as three primary colors due to trichromacy. The principle of metamerism states that different spectra that produce the same encoded signals look like the same color to the human eye. Color is defined by the product of cone response multiplied by input spectrum, not the spectrum alone. This means that if two light sources have the same apparent color, then they will have the same tristimulus values, no matter what different mixtures of light were used to produce them.

The principle of metamerism underlies all color reproduction technologies. Instead of reproducing the original spectrum distribution of the color, it is possible to create an equivalent response or metameric match by mixing the three colors.

Trichromacy and metamerism are used to create instruments that measure color. They allow colored material to be described quantitatively and create metrics that define when colors match.

Colorimetry is the science of color measurement. It is based on the study of human matching colors. Colorimetry is used to create color-matching functions that can be used to convert any spectrum into a standard encoding.

As mentioned, cone response and trichromacy are the foundation of color theory. In order to measure color with three numbers, the concept of color space is used to associate and visualize a color with three numbers.

One of the first mathematically defined color spaces was the CIE XYZ color space (also known as CIE 1931 color space), created by the International Commission on Illumination (CIE) in 1931. However, the CIE XYZ color space is unique, because it is based on direct measurements of the human eye, and serves as the basis from which many other color spaces are defined. In the CIE XYZ color space, the tristimulus values are not the S, M, and L stimuli of the human eye, but rather a set of tristimulus values called X, Y, and Z, which are also roughly red, green and blue, respectively.

In the 1920s, W. David Wright and John Guild independently conducted a series of experiments on human sight, which laid the foundation for the specification of the CIE XYZ color space.

The experiments were conducted by using a circular split screen two degrees in size, which is the angular size of the human fovea. On one side of the field a test color was projected and on the other side, an observer-adjustable color was projected. The adjustable color was a mixture of three primary colors, each with fixed chromaticity, but with adjustable brightness. The observer would alter the brightness of each of the three primary beams until a match to the test color was observed. Not all test colors could be matched using this technique.

When this was the case, a variable amount of one of the primaries could be added to the test color, and a match with the remaining two primaries was carried out with the variable color spot. For these cases, the amount of the primary added to the test color was considered to be a negative value. In this way, the entire range of human color perception could be covered. When the test colors were monochromatic, a plot could be made of the amount of each primary used as a function of the wavelength of the test color. These three functions are called the color matching functions for that particular experiment.

Based on the result, in 1931, CIE standardized the color matching function (CMF) by mathematical transformation (into x,y,z) in which there is no longer any negative value. It is an abstract curve now. No physical light can generate the function.

The CIE 1931 Standard Colorimetric Observer XYZ functions is the CMF now widely used. The XYZ color space is calculated using the formula shown left.

Color Matching   Colormetric Observer

1) Maureen C. Stone, "A Field Guide to Digital Color", A.K. Peters, Ltd, 2003
2) www.wikipedia.org