SECTION I

Chapter 1
Visible Light

What This Chapter is About

We begin our study of digital image processing with a gentle introduction to light and the human visual system. In order to create high impact images, understanding how light works and how humans perceive the effects of light is useful, if not interesting.

Many books exist that examine the physics of light and the biology of the eye in much greater depth. In this book, we condense this information. The goal is to provide you with a conceptual understanding of the factors that affect image processing techniques. As you will see in later chapters, the quality and impact of your digital images can be attributed to human perception of color. The way in which we see a particular color is a direct result of many phenomena, including the light source and effect of reflective surfaces on the light.

When you have finished this chapter, you will have a primer on the principles of light and color science, the structure of the human eye, and the techniques that the eyes and brain use to process visual information later chapters expand. You may skip those detailed chapters if you desire.

Some Principles of Light

An introduction to the principles of light is the best starting point because, as noted earlier, color perception is a direct result of many phenomena. If you have ever been in a new car showroom, you have experienced the effect of light. A car’s color often fails to look as dazzling once you move your new auto away from the lighting of the showroom. Why is that? This phenomenon is examined later in the chapter, but let’s start with an understanding of what light actually is and how it is measured.

Electromagnetic Energy

Visible light is a form of electromagnetic energy (EM energy). Other EM energy examples include radio waves, radar, microwaves, infrared and ultraviolet rays, X-rays, gamma rays, and cosmic rays. These various energy forms are actually arbitrarily delineated ranges within a continuous electromagnetic spectrum. Visible light occupies a very narrow band between the infrared and ultraviolet ranges.

While not wanting to oversimplify the physical principles of light, visible light behaves simultaneously like waves and particles (photons). In this book, we are most interested in light’s wave nature because it is the primary consideration for image processing and human perception.

Each EM wave has a constant frequency meaning that the wave peaks and dips at constant time intervals. In the vacuum of space, all EM energy travels in a straight line at 299,792,458 meters (approximately 3x10¹⁰ centimeters, or 186,000 miles) per second. This velocity is called the speed of light. Dividing 3x10⁸ meters per second by a wave frequency calculates the wave’s wavelength—the distance required between each peak-to-peak wave position. Wavelength is traditionally expressed in nanometers (abbreviated as [mu] or nm), where one nm equals 10^–9 (one-billionth) meters.

For example, any radio wave has a low frequency and a long wavelength. In contrast, any cosmic ray has a high frequency and very short wavelength. These two energy forces are at opposite sides of the electromagnetic spectrum. In the spectrum, photons with a higher frequency (and a shorter wavelength) contain greater energy than waves with a lower frequency (and a longer wavelength).

Visible light frequency ranges from about 380 nm to 760 nm—a span of about 380 nm. Table 1-1 illustrates the various wavelength ranges we associate with various colors.

Table 1-1: Visible colors and their associated wavelength ranges

Refraction

In 1621, W. Snell developed the Principle of Refraction. Snell found that when light travels through anything except a vacuum, it exhibits a change in direction when it reaches a material through which it must travel and then resumes its straight line course. This process, where the light direction actually bends as the light enters the material, is called refraction. The level of refraction depends on the density of the material and the frequency of the light.