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12. Optical data

12.1 Theory of

RGB additive B B (0.6, 0.2, 1) (0,0,1) (1,0,1) Gray shades (g,g,g) (0,1,1) (1,1,1) (0.6, 1, 0.7) R R Black (0,0,0) (1,0,0) G (0,1,0) (1, 0.7, 0.1) (1,1,0) G The RGB color model converts the additive system into a digital system, in which any color is represented by a point in a color tridimensional space and thus by a three coordinates vector. The coordinates are composed of the respective proportions of the primary light (Red, Green, Blue). The RGB model is often used by as it requires no conversion in order to display colors on a computer screen. 169 CMY model

Y Y Yellow Green (0, 0.3, 0.9) (0,0,1) (1,0,1) Gray shades Red (g,g,g) (0,1,1) Black (1,1,1) C (0.4, 0, 0.3) C White Cyan (0,0,0) (1,0,0) M Magenta (0,1,0) (0.4, 0.8, 0) Blue (1,1,0) M The CMY color model corresponds to the subtractive color mixing system. The coordinates of a point in this are composed of the respective proportions of the primary filter colors (Cyan, Magenta, Yellow). The CMY model is used whenever a colored document is printed. The conversion RGB and CMY models can be written as follows: For the point, for example: 170 HSV color model

V Green Yellow 120° 60° (37°, 0.9, 1) White H 1.0 S Cyan Red 180° 0° V

Blue Magenta 240° 300°

Hue is given in polar coordinate. The Saturation is the radius from the V- H axis and describes the vividness of S Black the color. The Value along the V-axis 0.0 describes the brightness.

Compared to the RGB and CMY, the HSV color model has the advantage that the colors correspond more closely to our subjective description. It is, therefore, easier to make a specific color selection. This color model uses three parameters: , Saturation, and Value. By projecting the RGB unit cube along the diagonal from white (1,1,1) to black (0,0,0), we get the basic hexagon of the HSV pyramid. 171 LAB color model

White +b* L=100 hue

Green Red –a* +a* –a* +a*

Black L=0 –b* LAB color model is commonly used in graphic arts and in the textile industry. Originally proposed by the "Commission Internationale de l'Eclairage" (CIE), it is also referred to CIELAB model. It uses L, a*, b* coordinates. L is the contrast, similar to the brightness Value of the HSV model, except that L values scales from 0 (black) to 100 (white). Hue and Saturation polar coordinates of the HSV model are here converted into cartesian coordinates a*, b*, which values vary from –100 to +100. The color space can then be represented by an ideal sphere. 172 diagram

A convenient way of visually representing all L colors in a two-dimensional space is the use of y "observer" a cut view of the La*b* sphere along an observer plane plane described by L = a* = b*. The obtained x figure is called chromaticity diagram (CIE 1931). –a* This 2-D space has two axes x and y. b* –b* 0.9

0.8 spectral locus 0.7 a* 0.6 T [K] The curved line in this diagram shows where the colours of the spectrum lie 0.5 2,500 y 3,000 and is called spectral locus; the 0.4 4,000 are indicated in nanometres 1,500 6,000 along the curve. The straight line on the 0.3 10,000 ∞ bottom connects the chromaticity co- 0.2 ordinates of extreme red and blue, 0.1 line called purple boundary. The area enclosed by the spectrum locus and the 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 purple boundary cover the domain of all x visible colours. 173 Practical color space and The three primary colors are not available in practice as pure colors, Visible color gamut but are dirtied by a certain pro- "" gamut portion of the other primary colors. RGB color gamut The result is that it is not possible to CMY color gamut create pure white (in the model) or pure black (in the subtractive model). The number of color shades that can be displayed or printed in practice with a three color system is limited to a color gamut smaller than the ideal color space seen by the . Color gamut of a The use of additional primitive colors typical CRT TV screen allow in certain cases to extend the color gamut. Printing uses an additional blacK ink (CMYK quadri- chromy). Textile printing is even more demanding and often employs an additional orange and two more blue and, thus, a total of 6 primitive colors (hexachromy). 174

Numerical color processing

In theory it is possible to create every Number of / Number of graduations Number of shade using the RGB or CMY color Red Green Blue colors models. How many colors can be re- 1 2 2 2 8 presented with these models depends 2 4 4 4 64 on the number of different gradua- 3 8 8 8 512 tions possible for each primary color. 4 16 16 16 4,096 5 32 32 32 32,768 To produce >16 Mio different color 6 64 64 64 262,144 shades, as in recent graphics cards, 7 128 128 128 2,097,152 256 graduations of each of the three 8 256 256 256 16,777,216 primary colors are needed. This means 3 x 8 = 24 bits are necessary for each color shade. The number of bits used to represent the color of a single in an image is called the . When the number of available graduations is not sufficient, as in most printers, a halftoning technique has to be used to simulate light color shades, obviously at the detriment of Examples of color halftoning with CMYK spatial resolution. (Cyan, Magenta, Yellow, blacK) separations. 175

!"#$%&'(#)%"*#%*+,-.$/-.(#$01*!203)"3 The Imaging Spectrometer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pectral imaging (/+.%/130D2)&$0+&#$'(%/-$($%&4

0.1')2"&&/&)1'3 A spectral is a combination of scanning4-5%&)4$-".# mirror !!! an imaging spectrograph and an area camera. It works as a line !"#$%&#"'( )*+%,%'- 6,1("'( scan camera, and provides full, !"#$%&'()* dispersing element detectors4$-".# !%-%.-/&# array +&,"-#./light contiguous spectral information for #()0-12 53$/*+%"3&1"+#(+*&)'&%$/&6+,"3 (/,).0%")- /./*/-%,& )'& +-& "*+#"-#& ,7/32 each pixel. Spectral resolution can range 3/..4 %()*/%/(4& & 5)*/& ,/-,)(,& 0,/ *0.%"7./&1/%/3%)(&+((+8,&%)&*/+2 from ca. 2 nm to 20 nm. Spectral ,0(/& $0-1(/1,& )'& -+(()9 imaging typically allows for recording 9+:/./-#%$&;!!! <&6+-1,4 !"#$%& pictures with the equivalent of 20 to Schematic diagram of an imaging spectro- 200 primary colors over the visible meter. Some sensors use multiple detector domain and can even extend to the near arrays to measure hundreds of narrow UV and mid-IR (hyperspectral imaging). bands. Since spectra obtained for an area on a picture can be matched with electronic spectra of chemical compounds (spectral fingerprints), spectral imaging allows for remote chemical analysis in a number of applications, such as for instance astro-nomy, surveillance, mining and geology, ecology, biology, and forensic science. RGB manitol glucose starch 176

12.2 High resolution spectroscopy

Homogeneous vs. inhomogeneous line width

–1 500 cm (12.5 nm) The spectrum of a typical organic molecule in a host matrix at T ≤ 4 K is 5·10–4 cm–1 (1.25·10–5 nm) schematized here on the left. The absorption profile of an individual molecule is fundamentally determined by the uncertainty principle. For a λ fluorescence lifetime of τ = 10 ns, the 500 nm value of the homogeneous width of the (20,000 cm–1) absorption line is expected to be 0.0005 cm–1 or 15 MHz. All molecules in the matrix, however, have slightly different environment, producing an inhomogeneous band broadening. The width of the vibration- less S1←S0 transition is typically 500 cm–1 (15'000 GHz). Thus, ratios of the inhomogeneous/homogeneous 6 width of up to 10 can result. 177 Spectral hole burning

Spectral hole burning is the frequency- N N hν N N H H selective bleaching of the absorption H H spectrum of a material, which leads to an N N N N increased transmission (a "spectral hole") at the selected frequency. Photochemical tautomerization of chlorin dye Two basic requirements, that must be met for the observation of this phenomenon, are: a) The spectrum is inhomogeneously broadened and b) the material undergoes holes upon light absorption a modification which changes its absorption spectrum. Typical materials include dye molecules dissolved in suitable host matrices and the

frequency selective irradiation is usually Absorbance / a. u. realized by a narrow band laser. 30 GHz Up to 106 spectral holes can be "drilled" in a inhomogeneously broadened absorption 620 625 630 635 640 band envelope. This means that each Wavelength / nm adressable pixel of the material (typically a few squared microns) could in principle Spectral hole-burning. Holes burnt into the inhomogeneously broadened band are shown carry the information corresponding to as recorded with low and high wavelength one million colors with a depth of one bit. resolution techniques. 178 12.3 Optical disks

Comparison of various data storage systems

Storage density Bit size Storage capacity Access time Storage system [ Mb cm–2 ] [ µm ] [ MB ] [ ms ]

Printed page 4.5 ·10–5 2 ·103 0.02 1 ·104

Solid state device (NAND) 2 ·106 7 ·10–3 2.6 ·105 1- 4 ·10–2

AgX color 24x36 mm 10 3 11

Floppy disk 0.2 - 10 5 1.44 - 200 10 - 100

Magnetic hard disk 7 ·104 0.28 106 / plate 8

Magneto-optic disk (MOD) 7 ·102 0.4 650 - 9.2 ·103 10

CD-ROM 58 1.6 650 110 - 170

DVD, DVD+RW 400 0.49 4.7 ·103 / side 80 - 220

Blu-Ray® Disk (BD) 3 ·103 0.18 5.6 ·104 / side 100 - 250

Holographic versatile disk (HVD) 3.3 ·105 10–2 3.9 ·106 2 179 Magneto-Optical Disk (MOD)

heating laser lens All commercially available MO disks are beam based on an amorphous terbium-iron- cobalt [Tb2(Fe9Co)8] magnetic alloy. This magnetization ferromagnetic material is sealed beneath a plastic coating. During recording, an IR diode laser beam is focused on the surface fast moving of the alloy and heats the material up to medium S magnetic the Curie point (typically TC = 180°C), at electro- field lines which the material becomes paramagnetic. magnet N This allows an electromagnet positioned on the opposite side of the disk to change the local magnetic polarization. The latter is retained when temperature drops. magnetic field During reading, the laser power is decreased to a level unable to heat the light polari- material significantly. According to the zation magnetic state of the surface, the reflected laser light varies due to the magneto-optic Kerr effect (MOKE) that causes the rotation of the direction of linear polarization of light upon reflection from the surface of a magnetic medium. magneto-optical Kerr (Faraday) effect 180 label Compact Disk (CD-ROM) acrylic aluminium

125 nm 1.2 mm polycarbonate base photo- diode 1

laser 0.5 µm 0.8 µm 1.6 µm half- 0.5 µm mirror reflective layer Compact disks are made of a 1.2 mm-thick disk of polycarbonate, with a thin layer of aluminium as a reflective surface. The most common size of CD-ROM disc is photo- 120 mm in diameter. Data is stored on the disc as a series diode 0 of microscopic indentations. A NIR laser (λ= 780 nm) is shone onto the reflective surface to read the pattern of laser pits and lands. Because the depth of the pits is approximately λ/4 to λ/6, the reflected beam's phase is half- shifted in relation to the incoming beam, causing mirror reflective destructive interference and reducing the reflected beam's layer intensity. This pattern of changing intensity of the reflected beam is converted into binary data. 181 Digital Versatile Disks (DVD, Blu-ray® BD)

Feature CD-ROM DVD Blu-ray® DVD uses 650 nm wavelength laser diode light as opposed to Diameter / thickness (mm) 120 / 1.2 120 / 1.2 120 / 1.2 780 nm for CD. This permits a Sides 1 1 or 2 1 smaller pit to be etched on the Layers per side 1 1 or 2 2 media surface compared to Capacity (GB) 0.7 4.7 - 17 50 CDs (0.74 µm for DVD versus Track pitch (µm) 1.6 0.74 0.32 1.6 µm for CD), allowing in Minimum pit length (µm) 0.83 0.44 0.14 part for a DVD's increased storage capacity. Linear scan velocity (m/s) 1.3 3.5 4 A dual-layer disk employs a Laser wavelength (nm) 780 635 or 650 405 second physical layer within the disc itself. The drive with dual- layer capability accesses the dual layer disc reflective second layer by shining the layer

0.6 0.6 mm laser through the first semi- semi-reflec- transparent layer. tive layer In comparison, Blu-ray® disk, 0.6 0.6 mm the successor to the DVD format, uses a wavelength of mobile lens 405 nm, and one dual-layer disc laser beam has a 50 GB storage capacity. 182 dvd4-7gb3.qxd 3/4/00 12:11 pm Page 5

Laser structuring: CD-R, CD-RW, DVD-R, DVD+RW

Figure 1. Data writing, erasing and re-writing can be achievedCreating amorphousby photo-thermal regions phaseCreating change polycrystalline of an regions absorbing medium. In its original state, the recordingtemp layer of a recordabletemp disk is o o polycrystalline. During writing, a focused laser beam~600 C (~10 mW)melting selectively point ~600 Cheats areas meltingof point tcooling the phase-change material above the melting temperature~200o C (500-700°C).Then,crystallisation ~200o C tpass if cooledcrystallisation temperature temperature sufficiently quickly, the liquid state is quenched and an amorphoustime state is obtained. If thetime phase-change layer is annealed below the melting temperaturetcrystal but above the crystallizationtcrystal temperature (200°C) for a sufficient time (~5 mW laser beam), the atoms revert back to an ordered state, i.e. the crystalline state. Figure 2. Single sided disc The amorphous and crystalline states (Not to scale) have different refractive indexes, and can therefore be optically distinguished (read Label Polycarbonate laser beam <1 mW). In the DVD+RW 2P resin system, the amorphous state has a lower Reflective layer Upper dielectric layer reflectance than the crystalline state. Recording layer (Ag-In-Sb-Te) The phase-change medium consists of a Lower dielectric layer grooved polycarbonate substrate, onto Polycarbonated disc substrate which a stack (usually four layers) is sputtered. The phase-change (recording) Laser beam layer is sandwiched between dielectric- or Groove absorbing dye layers. A commonly used Fig.2.Section through a single-sided 4-layer DVD+RW disc (4.7 GB). phase-change material is Ag-In-Sb-Te alloy. By to the heat from a laser,the recording layer can be changed from a polycrystalline 183(more reflective) state to an amorphous (less reflective) state,and vice versa.The layers are deposited onto a polycarbonate substrate;the latter molded with a spiral groove for servo guidance,address information and other data. DVD+RW discs are supplied ready-for-use in the polycrystalline state.They can be written at between 1x and 2.4x DVD- data rates,i.e.11-26 Mbit/s,allowing CAV operation (Constant Angular Velocity).

5 Digital holographic data storage

A. Recording Holographic recording tech- nology records in storage medium the form of laser interference reference beam data to fringes in the volume of a be stored storage medium, rather than on hologram its surface. By varying the angle interference between both inter-ferring laser spatial light modulator beams, up to 106 holograms can be created in the same piece of signal beam polymer, enabling holographic data pages versatile disks (HVD) the same size as today's DVDs to store as detector array much as one terabyte of data reference beam (200 times the capacity of a single layer DVD), with a storage medium transfer speed of one gigabyte per second (40 times the speed of DVD). Associated with high-reso- recovered holograms lution spectral hole burning, holographic recording is seen as B. Reading the future of data mass storage. 183 !"##$%&'()*"#"+(',-$.)/&0$')!"##$%&'()*"#"+(',-$.)/&0$') 1%'#23&()4256&7)849:;<=)1%'#23&()4256&7)849:;<=)Holographic versatile disk (HVD)

ƒ (&."(0$%+)7&0$') HVD is an optical disk technology developed &>'#?'6$"%)5256&7) recently that can store the same amount of 849:;<=)&7,#"2$%+) information as 20-200 Blu-ray® disks. It 6-& ."##$%&'()5256&7 employs collinear within a photopolymer recording layer rather than a dual-beam system. Reading is achieved with a blue and a red laser beams. The blue laser reads data encoded as laser interference fringes from the recording layer. A red laser is used as the reference beam to read servo information from a regular CD-style aluminium layer near the bottom. Servo information is used to monitor the position of the read head over the disc, similar

Blue or Red laser to the head, track, and sector information on green laser light a conventional hard disk drive. On a CD or DVD this servo information is interspersed photo- hologram polymer amongst the data.

dichroic Although HVD standards were published in mirror layer 2007, no company has released an HVD and Al layer the related drive in the public yet. 185