1. Photographic Image Formation I
Szymon Rusinkiewicz, Tim Weyrich: Technology in Art and Cultural Heritage. Princeton Freshman Seminar 2006
Acknowledgment: some figures by B. Curless, E. Hecht, W.J. Smith, B.K.P. Horn, and A. Theuwissen

2. Photographic Image Formation
Real world
Optics
Sensor
Dark Room

3. Pinhole Camera Acquiring images using a pinhole camera?
Use light-sensitive film at image plane
Image plane
Object
Image
Pinhole
Pinhole camera

4. Pinhole Camera

5. Pinhole Camera

6. Pinhole Camera

7. Pinhole Camera Limitations depending on size of aperture:
Aperture much too small: diffraction through pinhole  blurry image
Image plane
Object
Image
Pinhole
Pinhole camera

8. Pinhole Camera Limitations
Can we have sharp images, no diffraction, and enough light at the same time?

9. Pinhole Camera Limitations
Can we have sharp images, no diffraction, and enough light at the same time?
Optical lenses do the trick!

10. Lenses
Focus a bundle of rays from a scene point onto a single point on the imager
Increases aperture without loss of sharpness

11. Ideal “Thin” Lens Law
Relationship between focal distance and focal length of lens:
1/do + 1/di = 1/f

12. Camera Adjustments Focus? Iris? Zoom / wide-angle? Changes di
Changes aperture
Zoom / wide-angle?
Changes f and sometimes di
Changes field of view

13. Focus and Depth of Field
For a given di, “perfect” focus at only one do
In practice, OK for some range of depths
Circle of confusion smaller than a pixel
Better depth of field with smaller apertures
Better approximation to pinhole camera

14. Field of View Depends on Focal length of lens Size of imager
Object distance?

17. Sensors
Photographic film
CCD sensors
CMOS chips

18. Photographic Film Until recently the most common imager
Silver salts or dyes darken under light exposure
After fix step, image prints from negative
Multiple film layers with filters: color images

19. MOS Capacitors MOS = Metal Oxide Semiconductor Gate (wire)
SiO2 (insulator)
p-type silicon

20. MOS Capacitors
Voltage applied to gate repels positive “holes” in the semiconductor
+10V
Depletion region
(electron “bucket”)

21. MOS Capacitors Photon striking the material creates electron-hole pair
+10V        +

22. Charge Transfer
Charge has to be transported off the chip to digitizing circuits
Charge-coupled devices (CCD) build bucket-chains:

23. CMOS Imagers
Recently, can manufacture chips that combine photosensitive elements and processing elements
Benefits:
Partial readout
Signal processing
Eliminate some supporting chips  low cost

24. Where do all the colors come from?
Electrons don’t have a color…

25. Where do all the colors come from?
Electrons don’t have a color…
We can separate images into red, green, and blue:

26. 3-Chip Cameras
Prisms separate incoming light into red, green, and blue wavelengths
One detector chip for each color

27. Single-Chip Cameras A single detector chip
Small color filters in front of each pixel
Images have to be processed for per- pixel RGB
Bayer mosaic

28. Foveon Technology Layered sensor
Similar structure to photographic film

29. Development Process Classical film requires development in dark room
What about digital images?

30. Development Process Classical film requires development in dark room
Digital images require Digital Darkroom
Mapping of sensor data to pixel values
Mapping defined by
Contrast & intensity (dynamic range)
Gamma
Simulated film (grain, solarizatoin, …)