Light, vision, and photography

Waves

• An electro-magnetic (EM) wave is a signal carrying energy through space/time, with a physical value at each space-time point given by the strength of its electrical and magnetic fields. These fields oscillate perpendicular to each other, and perpendicular to the path of the wave (EM waves are transverse).
• EM waves travel through a vacuum (no medium)
• EM waves travel at a fixed speed c = approximately 3 x 10^8 (300 million) meters/second, or 186,000 miles/sec.
• Like periodic sound waves, periodic EM waves are characterized by their:
  • Period: T
  • Frequency: f = 1/T
  • Wavelength: L = c*T = v/f, i.e. c = f*L, f=v/L (higher frequency implies lower wavelength)
  • Amplitude: A
  • Power: proportional to A^2 , where A is the root mean square amplitude.
  • Here is an animation, showing EM wave propagation
• Like sound waves, EM waves can be analyzed as the sum of sine and cosine waves at different frequencies and amplitudes, using the Fourier theorem
• Light comprises a frequency range within the EM spectrum in the visible range: 750 - 390 nanometers (billionths of a meter), i.e. frequencies in the range 400 to 790 terahertz (tera = trillion = 10^12), corresponding to colors ROYGBIV (red-orange-yellow-green-blue-indigo-violet)
  • Waves (transverse, longitudinal)(1D, 2D, 3D).
  • Inverse square law: the spread of wave energy is proportional to the square of the distance from the source. If the power (energy per unit time) is constant, then intensity (power per unit area) is inversely proportional to the square of the distance. What this means: a flashlight's intensity at 8 feet is only a quarter its intensity at 4 feet.
  • Reflection: a wave will bounce off a reflective boundary, partially (glass) or fully (mirror).
  • Diffraction: while light shadows appear relatively sharp (compared to sound shadows), a light wave can travel around an obstacle significantly smaller than its wavelength (see periodic waves, below), and will spread after passing through a hole.
  • Refraction: a wave changes direction when the medium changes.
  • Light waves are tranverse traveling electromagnetic waves through space (no medium) in 3D.

Terms for light and optics

• Watt: unit of power, 1 joule/second
  • a joule is the energy required to exert a force of 1 newton over one meter (recall that a Pascale is a unit of pressure: the force of 1 newton spread over 1 sq meter)
  • a newton is the force required to accelerate a kilogram 1 m/s each second
• Radiant flux: measure of power in EM radiation
• Luminous flux: measure of power of light source, as perceived by the eye (= radiant flux adjusted for wavelength)
• Lumen: unit of luminous flux
• Luminous intensity: measure of power in EM radiation per solid angle unit
• Candela: unit of luminous intensity
• Lux: measure of luminous intensity = one lumen per square meter

Photography

theory

• light
• pinhole camera
  • pinhole
  • film
• SLR camera
  • lens and its focal length
  • f-stop
  • focal plane
  • depth of field
  • shutter speed
• development

Digital photography

Terms

• Sampling and quantization error
• Codecs
• Pixel and megapixel (sampling rate)
• Bit depth (sample size)

Camera types

• point and shoot
• DSLR

File formats

As for audio (mp3, ogg, wav, etc.) there are a multitude of image file formats. These differ according to:

• the codec (the way the image is digitally encoded as pixels
  • color depth per pixel (8 bit pixel = 256 colors; 24 bit pixel = 14 million colors, or "truecolor")
• compression scheme
• whether compression is lossy or lossless
• raster or vector: whether a map of pixels or set of vectors is stored. Normally real-world images are not stored as vectors, which are used for computer graphics

Here are some of the more common ones:

• tiff, Tagged Image File Format: may be uncompressed (~ wav) or compressed, lossy or lossless
• jpg, Joint Photographic Experts Group: compressed, usually lossy (~ mp3)
• PNG, portable network graphics - a lossless, open-source compression scheme
• GIF: graphics interchange format. 8 bit palette provides limited color range (256). Often used also for animations.
• BMP: Windows bitmap format, an uncompressed pixel representation
• SVG: a common and open vector graphics standard

Software

• Adobe Photoshop: commercial
• The Gimp: open source equivalent. Note that you'll need to install and run X11 on Mac OS X before starting the Gimp.