Calendar

Public Event: Light, and us, part I: mostly about color

When: Saturday, October 1, 2011, 1:30-3:30 PM

How do we perceive light, use light, create light? A brief overview, before we focus on color

Intensity, and variation
Color
Patterns, including lines, contrast, diagonals, motion - with an eye that focuses images
Why are these processings useful? Evolved for effective and safe omnivory, detection of threats from predators, toxic animals, fall hazards

Are these fixed perceptions?

Depend on observer- including:
- Color-blindness
- Acclimation, especially to intensity
  - Including afterimages, as a transient; some interesting “illusions”
Depends on the environment:
- Light quality – color confusion under filtered light
  - A bit about white balance
- Surrounds – Mach bands and more
Depends on species! Bees can see UV
- Also, some species see polarization
  - Polarized light with crossed polarizers (and, for fun, birefringence in stressed plastic)

What confers color?

Emission – hot objects…including the sun: the solar spectrum
Reflection – surfaces that reflect various wavelengths differently / what is wavelength?
- Special case: diffraction, or interference colors – butterfly wings, oil slicks on water
Transmission, with selective absorption – filters, and multiple filters
Re-emission – fluorescence and phosphorescence

How do we create color?--> Subtractive color– pigments

Simple mixing of primaries (or any colors)
- Unmixing by chromatography, or spectrophotometry: measuring color ?

Additive color – active elements, such as computer monitors

Creating light, first
- Incandescence
- Emission line sources
  - Electronically excited – elements, as in mercury or sodium lights; semiconductors, …
  - Coherent light – lasers
Now adjusting it in each primary color
- Broad light source and filters – e.g., an LCD projector
- Narrow-band sources mixed together: LEDs , or even two simple projectors
- The computer monitor, with color adjusted by commands to the video card
- The Engineer Guy tears down a monitor
- Let’s photograph a computer monitor and enlarge it
- Earlier method: pointillism; Georges Seurat and colleagues

How do we specify color?

Some cases where we have to do this precisely: color printing, TV, …
Color systems – HSV (incomplete), RGB, CMYK, CIE, color matching (Munsell color chips, …)
We need to ask: How do we perceive color? Why do primary additives or subtractives work?
- 3 types of cones
- Tristimulus theory…but also Land’s retinex theory
- Pure colors – single wavelengths (rainbows/spectra) – not the full range of colors!
Ergo, additive color – R, G, B
- Any pure color can be mimicked, in effects on the 3 receptors, by a mix of 3 primaries
- Oops: not a complete system – need some negatives
Subtractive color – as fractional reflectance at all wavelengths

Other aspects of light perception – we’ll cover these later

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