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Digital Camera and Image Forensics Katie Bouman

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Digital Camera and Image Forensics Katie Bouman Digital Camera and Image Forensics Katie Bouman In the past twenty years, much emphasis has been placed on technological breakthroughs in consumer electronics. Whether it be the creation CD’s, DVD’s, HDTV’s, or MP3’s, they were all created on the same basic principle; converting conventional analog information into digital information. Since these electronic systems are common today they may seem simple to understand, the conversion of a fluctuating wave into ones and zeros is much more difficult than it may appear. One example of this major shift is in the development of the digital camera. Inspired by the conventional analog camera, which uses chemical and mechanical processes to create an image, the digital camera has instead achieved this through the use of a digital image sensor and a computer (Wilson, et al, 2006). In just seconds a digital camera captures a sample of light that has bounced off a subject and has traveled through a series of lenses, which is then focused on a sensor that records the light electronically by breaking down the light pattern into a series of pixel values, and performs a full-color rendering that includes color filter array interpolation, color calibration, anti-aliasing, infrared rejection, and white point correction. However, a lot goes into doing just that (Adams, et al, 1998). Andreas Vesalius (1514-1564) was one of the first to dissect and examine the human body. As a result of Vesalius’s extensive work, many corrections were made in medicine, and scientists were able to use the enhancement in knowledge to extend their theories (Szaflarski, Diane, September 22, 2004). Although cameras, and even more so digital cameras, were not invented until centuries later, many of the characteristics of cameras and how they work is very similar to the eye, and was essential to their development. In addition, understanding of the eye helped to propel curiosity in understanding of light and its properties (Watson, 2006). The color that is detected by the cone cells in the back of ones eye and in the back of a camera is part of the electromagnetic spectrum. Refer to Figure 1. The area of the spectrum that humans are able to see is called the visible spectra (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 b). The electromagnetic spectrum is an entire range of wavelengths and frequencies that extends from gamma rays, the shortest wavelength, to the long waves, radio waves (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 a). Although colored light is part of the spectrum, there is more to light than meets the eye. Only about 300 nm of the spectrum is visible (400 nm to 700 nm). The rest of the light that is unseen by the naked eye is referred to as the “invisible spectrum” (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 b). There are 7 types of electromagnetic radiation including visible light. Light travels in waves (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 a). In order of length sizes of waves from largest to smallest is: radio waves, microwaves, infrared, the visible spectra, ultraviolet light, X-rays, and gamma rays (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 a). Refer to figure 1. All electromagnetic radiation moves at the same speed through a vacuum, 3.0 times 108 meters/second, however, while moving through matter instead of a vacuum the speed is slowed down slightly. In fact, the denser the material that the light wave is moving through the slower it tends to travel (Davis, et al, 2002). A longstanding debate has persisted as to whether light travel in waves, or is it composed of a stream of particles. Many noteworthy physicists have argued both sides of this question. In fact, light exhibits behaviors of both a stream of particles and a wave. Light travels in a wave composed of photons (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 a; Davis, et al 2002; The Franklin Institute. September 20, 2004). A photon, an uncharged particle that has no mass, is the smallest unit of electromagnetic energy. Each photon contains a certain amount of energy defined by Einstein as E = h*f (Where E is the energy of the emitted electron, h is Planck's constant (6.63 * 10^-34 J*s), and f is the frequency of the given light source (Kudenov, 2003a). The furthur the distance that the waves are 1 from each other the less energy each photon contains (Campbell, et al, 1997; Davis, et al 2002; The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 a). For example, microwaves’ distance between waves (wavelength) ranges from 106 nm to 109 nm, therefore, the amount of energy of each photon contains less energy than a gamma ray, which ranges in wavelength from 10-3 nm to 10-5 nm. However a microwave contains more energy per photon than does a Radio wave, which ranges from 109 nm to 103 meters (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 a; The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 b). Objects and events with more energy, create higher energy radiation than cool objects or events with less energy. Thus, the more heat emitted, the shorter the waves are in the electromagnetic spectrum (Watson, 2006). FigureFigure 2 1 While wavelength λ() is the measure of one period of a wave when moving through space, frequency 1 (v) frequency(v) = is defined as the number of waves that pass a certain point during a certain period(T) period of time (usually a second) and is measured in € cycles/second or a hertz (Hz). Refer to figure 2. Therefore a wave, such as a radio wave, that has a longer wavelength than another wave, such as a gamma ray, will have a lower frequency than the shorter wave (Davis, et al, 2002). The visible spectra ranges from approximately 400 nm to around 700 nm. White light, or light that comes from the sun and many other sources, is composed of all colors, which result from different 2 wavelengths of light. When white light hits a prism, the prism creates an area at which each wavelength bends a different amount, which causes all colors to separate (Levine, 2000; Sekuler, et al. 2002; Szaflarski, September 22, 2004; Davis, et al 2002). These colors are often memorized in the order of their appearance after going through a prism with the longest wave length first as ROY G. BIV, red, orange, yellow, green, blue, indigo, and violet (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 b). Light waves can be transmitted, absorbed or reflected. The object that the white light wave hits determines which wavelengths will be reflected (the energy of the light to be converted to heat), or transmitted (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 c; The Franklin Institute September 20, 2004). In a white object, all the colors are being reflected. A black object, on the other hand, is when all the colors are being absorbed in the substance and no light waves are emitted (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 c; The Franklin Institute. September 20, 2004.). Every atom contains electrons, which can be pictured attached to the nucleus by springs. These springs start to vibrate at specific frequencies. When a light wave of that specific frequency hits the electrons they start to vibrate and the energy from the light turns into vibrational motion. The vibrating electrons then react with the electrons of the atoms surrounding it, creating thermal energy as the light wave is absorbed. This absorbed light wave is not seen. Reflected and the transmitted light is seen. Color occurs when the light causes electrons to vibrate in small spurts. These small spurts of vibration emit this energy as the light wave (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 c). The color of a green leaf is not actually contained within it (Sekuler, Robert, et al. 2002; Levine, 2000; The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 c; The Franklin Institute. September 20, 2004). The green color seen is the light that is reflected because the frequency of the wave doesn’t match the frequency of the electrons in the leaf (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 c). The primary colors of light are red, green, and blue, referred to as RGB. When mixed together these colors combine to create light’s secondary colors magenta (from blue and red), cyan (from blue and green), and yellow (from green and red). When colors of light are mixed together, the color produced becomes closer to the color white than the beginning colors were (The Franklin Institute. September 20, 2004). Color Addition is a method used to produce different colors of light. Computer monitors and televisions both use color addition. Color addition is when the three primary colors, red, blue, and green, are mixed together at varying intensities to create a wide range of colors (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 d; The 3 Franklin Institute. September 20, 2004). Monitors and televisions produce all their colors by mixing phosphors (The Physics Classroom and Mathsoft Engineering & Education, Inc. 2004 d; Brain, 2006.). Phosphors are coated on the inside of screens. They emit visible light when it is struck by an electron beam. In a black and white screen there is only one type of phosphor, which glows white when struck, and black when not. However, in a color screen, three different types of phosphors, arranged as dots or stripes, use color addition to formulate a final color (Brain, 2006).
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