Results of Analysis

9. 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. “Grain” of Film and Paper • Electron Photomicrographs of Emulsion Grains

11. What is Silver Halide? Silver (Ag) Halide group

12. Film base Plastic Structure of a Typical B&W Film • Antihalation backing • Prevents light from reflecting back. • Emulsion • Silver Halide Crystals • Suspended in gelatin, like • fruits in Jell-O™!

13. Exposed AgX Crystals • 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. Silver Halide Process Chain Exposure Processing Develop Stop Fix Visible (Stable) Image Latent Image • A latent image is formed after exposure (invisible to human eye). • After processing, the latent image is turned into a visible, stable image.

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

16. Silver Halide Grains

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

18. What determines how dark film becomes? • THE GRAINS! • Size • Shape • Chemical composition • Distribution

19. “Grain” of Film and Paper • Electron Photomicrographs of Emulsion Grains • (n.b. Measurement Bars indicate scale)

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

22. D Log H D Log H D-Log H Curve and Contrast More contrast Less contrast Image Film response

23. Color Images • In most cases, we also want to capture color information • The way that we capture, store, view, and print color digital images is based on the way that humans perceive color

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

25. Additive Color Mixing Red Green Blue

26. Subtractive Color Mixing Cyan Magenta Yellow

28. Chemical processing Digital processing Traditional vs. Digital Photography Detector: Photographic film Detector: Electronic sensor (CCD)

30. Goal of Charge Coupled Device (CCD) CCD Photons Electronic Signal • Capture electrons formed by interaction of photons with the silicon • Measure the electrons from each picture element as a voltage

31. Charge Coupled Device (CCD) • CCD chip replaces silver halide film • No wet chemistry processing • Image available for immediate feedback

32. Magnified View of a CCD Array Individual pixel element CCD Close-up of a CCD Imaging Array

34. Spatial Sampling Scene • When a scene is imaged onto the CCD by the lens, the continuous image is ‘sampled’ and divided into discrete picture elements, or ‘pixels’ Grid over scene Spatially sampled scene

35. 0 0 0 0 0 0 0 0 0 25 25 0 0 40 40 40 0 0 40 64 40 64 64 25 0 25 0 40 97 97 0 0 40 97 64 64 97 150 64 64 97 40 0 40 0 97 40 97 64 64 97 0 40 0 25 40 25 64 40 64 64 0 0 25 25 0 40 40 40 0 0 0 0 0 0 0 0 0 0 0 0 Quantization • 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 Numerical representation

36. Basic structure of CCD Divided into small elements called pixels (picture elements). Shift Register Image Capture Area Rows Voltageout Columns preamplifier

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

38. Photon/Silicon Interaction e- e- Silicon • Photon knocks off one of the electrons from the silicon matrix. • Electron “wanders around” randomly through the matrix. • Electron gets absorbed into the silicon matrix after some period.

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

40. e- Collection stage Voltage e- e- • Electron formed in the silicon matrix by a photon. • Electron wanders around the matrix. • If the electron wanders into the depletion region, the electron is captured, never recombining with the silicon matrix.

41. Collection Light e- e- e- e- e- e- e- e- e- e- e- • The number of electrons accumulated is proportional to the amount of light that hit the pixel. • There is a maximum number of electron that these “wells” can hold.

42. 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- Bucket Brigade • 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-

43. Bucket Brigade • Charge is marched across the columns into the shift register, then read out 1 pixel at a time. 100 transfers 200 transfers Shift Register 100 pixels 100 transfers 1 transfer 100 pixels

45. Converting Analog Voltages to Digital • Analog voltage is converted to a digital countusing an Analog-to-Digital Converter (ADC) • Also called a digitizer • The input voltage is quantized: • Assigned to one of a set of discrete steps • Steps are labeled by integers • Number of steps determined by the number of available bits • Decimal Integer is converted to a binary number for computation ADC 6.18 volts 01100101 (117)

46. Response of photographic negative Response of CCD Digital Count Density Log H Exposure Response of CCD • 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) • Linearity is critical for scientific uses of CCD

47. 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. CCD Better performance Resolution Sensitivity Signal to noise Hi-end applications CMOS Cheaper Less power required Low-end applications CCD vs. CMOS Historically: Currently: • CMOS is starting to bridge the performance gap

49. RGB Color Images • To capture a color image we record how much red, green, and blue light there is at each pixel. • 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’):