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

Photographic Image Formation Real world Optics Sensor Dark Room

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

Pinhole Camera

Pinhole Camera

Pinhole Camera

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

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

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

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

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

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

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

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

Sensors Photographic film CCD sensors CMOS chips

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

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

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

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

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

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

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

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

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

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

Foveon Technology Layered sensor Similar structure to photographic film

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

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, …)