1. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science

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9. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Results of Analysis Image layer of the First Photograph is not a continuous layer, but rather has a random dot pattern. First Photograph was made using a pewter plate containing a high concentration of tin alloyed with lead, copper and iron. Nondestructive infrared analysis of the image layer of the First Photograph revealed a complex composition of bitumen and oil of lavender. The metal plate of the First Photograph does not have consistent thickness nor are its dimensions uniform. Evaluations of the First Photograph's earlier protective enclosure revealed the urgent need to design and build a new oxygen-free enclosure to protect the artifact.

10. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science “Grain” of Film and Paper u Electron Photomicrographs of Emulsion Grains

11. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science What is Silver Halide? Silver (Ag) Halide group

12. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Structure of a Typical B&W Film u Film base u Plastic u Antihalation backing u Prevents light from reflecting back. u Emulsion Silver Halide Crystals Suspended in gelatin, like fruits in Jell-O™!

13. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Exposed AgX Crystals u When a silver halide crystal is exposed to light, some of the AgX molecules break up into their constituents, one of which is metallic silver (“pure” Ag). Exposure After Exposure

14. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Silver Halide Process Chain u A latent image is formed after exposure (invisible to human eye). u After processing, the latent image is turned into a visible, stable image. ExposureProcessing DevelopStopFix Latent Image Visible (Stable) Image

15. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Processing Photographic Film u Developer “amplifies” the atomic silver to visible silver strands. u Stop Bath stops the development process. u Fix dissolves the unexposed AgX crystals, making the film safe to expose to light. u Wash with water to rinse fix chemicals away.

16. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Silver Halide Grains

17. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Why does processed film look “negative”? u Silver strands formed by exposure of photographic film to light actually appear dark (they are NOT shiny). u So, where light hits the film during exposure, it turns darker.

18. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science What determines how dark film becomes? u THE GRAINS! u Size u Shape u Chemical composition u Distribution

19. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science “Grain” of Film and Paper u Electron Photomicrographs of Emulsion Grains u ( n.b. Measurement Bars indicate scale)

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21. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science What determines how dark film becomes? u Consider the so-called “D-Log H” curve. u Describes how film responds to light: u Density (D) is how dark the film is. u Log H is the exposure (H) in logarithmic scale. Log H D More Exposure Less Exposure Lighter Darker

22. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science D-Log H Curve and Contrast Log H D D More contrast Less contrast Film response Image

23. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Color Images u In most cases, we also want to capture color information u The way that we capture, store, view, and print color digital images is based on the way that humans perceive color

24. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Color Perception u The eyes have three different kinds of color receptors (‘cones’); one type each for blue, green, and red light. u Color perception is based on how much light is detected by each of the three ‘primary’ cone types (red, green, and blue)

25. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Additive Color Mixing Red Green Blue

26. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Subtractive Color Mixing Cyan Magenta Yellow

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28. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Traditional vs. Digital Photography Chemical processing Detector: Photographic film Digital processing Detector: Electronic sensor (CCD)

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30. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Goal of Charge Coupled Device (CCD) u Capture electrons formed by interaction of photons with the silicon u Measure the electrons from each picture element as a voltage CCD Photons Electronic Signal

31. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Charge Coupled Device (CCD) u CCD chip replaces silver halide film u No wet chemistry processing u Image available for immediate feedback

32. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Magnified View of a CCD Array Individual pixel element Close-up of a CCD Imaging Array CCD

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34. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Spatial Sampling u When a scene is imaged onto the CCD by the lens, the continuous image is ‘sampled’ and divided into discrete picture elements, or ‘pixels’ Scene Grid over sceneSpatially sampled scene

35. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Quantization u The spatially sampled image is then converted into an ordered set of integers (0, 1, 2, 3, …) according to how much light fell on each element Spatially sampled scene 0 0 0 0 0 0 0 0 0 0 0 0 0 0 25 40 0 0 25 40 25 40 25 40 25 40 64 97 150 97 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Numerical representation

36. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Basic structure of CCD Divided into small elements called pixels (picture elements). preamplifier ImageCaptureArea Shift Register Voltage out Columns Rows

37. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Basic structure of a pixel in a CCD u Silicon is a semiconductor. u Oxide layer is an insulator. u Metal gates are conductors. u Made with microlithographic process. u One pixel may be made up of two or more metal gates. Silicon base Metal gate Oxide Layer One pixel

38. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Photon/Silicon Interaction u Photon knocks off one of the electrons from the silicon matrix. Silicon e-e- e-e- u Electron “wanders around” randomly through the matrix. u Electron gets absorbed into the silicon matrix after some period.

39. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Collection stage u Voltage applied to the metal gates produces a depletion region in the silicon. (depleted of electrons) u Depletion region is the “light sensitive” area where electrons formed from the photon interacting with the silicon base are collected. Voltage

40. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Collection stage u Electron formed in the silicon matrix by a photon. e-e- u Electron wanders around the matrix. u If the electron wanders into the depletion region, the electron is captured, never recombining with the silicon matrix. e-e- e-e- Voltage

41. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Collection u The number of electrons accumulated is proportional to the amount of light that hit the pixel. u There is a maximum number of electron that these “wells” can hold. e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- Light

42. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Bucket Brigade u By alternating the voltage applied to the metal gates, collected electrons may be moved across the columns. e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e-

43. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Bucket Brigade u Charge is marched across the columns into the shift register, then read out 1 pixel at a time. 100 pixels 1 transfer 100 transfers 200 transfers Shift Register

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45. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Converting Analog Voltages to Digital u Analog voltage is converted to a digital count using an Analog-to-Digital Converter (ADC) u Also called a digitizer u The input voltage is quantized : u Assigned to one of a set of discrete steps u Steps are labeled by integers u Number of steps determined by the number of available bits u Decimal Integer is converted to a binary number for computation ADC 6.18 volts01100101 (117)

46. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Response of CCD u The response of CCD is linear ( i.e., if 1000 captured photons corresponds to a digital count of 4, then 2000 photons captured yields a digital count of 8 ) u Linearity is critical for scientific uses of CCD Log H Density Response of photographic negative Exposure Digital Count Response of CCD

47. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science CMOS Detectors (Complementary Metal Oxide Semiconductor) - Uses same physical principles as CCD’s - Different architecture: - No shift register – each pixel individually addressable - Uses on-chip electronics support - Smaller “fill factor” – area of chip used to sense photons

48. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science CCD vs. CMOS u CCD u Better performance u Resolution u Sensitivity u Signal to noise u Hi-end applications u CMOS u Cheaper u Less power required u Low-end applications Historically: Currently: u CMOS is starting to bridge the performance gap

49. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science RGB Color Images u To capture a color image we record how much red, green, and blue light there is at each pixel. u To view the image, we use a display (monitor or print) to reproduce the color mixture we captured. Q) How many different colors can a display produce? A) It depends on how many bits per pixel we’ve got. For a system with 8 bits/pixel in each of the red, green, and blue (a ‘24-bit image’):

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51. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science RGB Color Images =

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60. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Summary - Detectors u Chemistry-based detectors have been around a long time u Modern films use grains of silver halides to record the intensity of light u Color films use three layers of emulsion, each sensitive to one color (RGB)

61. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Summary - Detectors u Solid state detectors have been around since the early 70’s u Two main classes: CCD and CMOS u Same physics, different architecture u Early cost & performance differences are disappearing u Color images are obtained by filtering the light before it hits the sensor’s silicon base

62. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Summary - Detectors u New technologies are being developed to improve performance and reduce cost

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