Incandescent Light Sources

Incandescent Light Sources

• Thermal sources emit photons with a wide range of energies (broad spectrum)
• Incandescent source color relates to temperature
• Hotter sources emit more blue light
• Cooler sources emit more red light
• About 75 percent of the radiation from incandescent bulbs is infrared.
• Scientists compare real incandescent lights to a theoretical ideal called a black body to study their properties.
• Black body emission spectrum depends on temperature, not material

Lighting

• Illumination uses devices converting electrical energy into light
• Common electric lighting devices: incandescent lamps, fluorescent lamps, arc lamps, electric-discharge vapor lamps, and LEDs

Sources of Light

• Light sources differ by how energy is provided to charged particles
• Incandescent sources create light from heat
• Luminescent sources create light from other forms of energy (chemical, electric)

Incandescent Light

• Hot atoms collide in incandescent sources
• Collisions transfer energy to electrons, boosting them to higher energy levels
• As electrons release energy, they emit photons
• Varying collision strength excites electrons to different levels, emitting different photons
• Candle light is incandescent, from excited soot atoms in the flame
• Incandescent bulbs produce light from excited atoms in a heated filament

Sunlight

• The sun's heat comes from nuclear fusion.
• Atoms interact and combine, releasing energy
• This energy travels to the surface where the temperature is about 6000°C (11,000°F)
• Different stars have different frequencies and colors depending on mass and age

Luminescence

• A luminescent light absorbs energy in a non-heat form, making it cooler than incandescent
• Fluorescent lights are luminescent using phosphors
• Fluorescent tubes contain mercury vapor and phosphors
• Electricity excites mercury atoms, emitting blue, green, violet, and UV light
• Phosphor atoms absorb UV, release heat, and then emit visible light at lower frequencies.
• Phosphor compounds convert electron energy to light in TV picture tubes
• Electrons collide with phosphor atoms on the screen, exciting them
• Electrons drop back to their original levels, emitting heat and visible light
• The light from all dots forms the picture
• Some phosphor compounds remain excited for a time before radiating light
• Phosphorescent light sources have a delay between absorption and emission longer than 1 second
• Phosphorescent materials glow in the dark after exposure to strong light

Lasers

• Lasers produce tightly focused, regular light waves
• Laser stands for Light Amplification by Stimulated Emission of Radiation
• Nonlaser light sources have radiating charges that produce different light waves
• Laser sources have atoms emitting in sync
• Laser light is polarized, monochromatic, and coherent
• Waves remain in step over long distances
• Laser applications: medicine, research, military, and communications
• Lasers are focused, powerful, and controllable energy sources
• Lasers can drill holes in diamonds and make microelectronic components
• Lasers allow precise surgery
• Lasers are useful for space communication because they can carry information over long distances

Behavior of Light

• Light behavior is divided into how it interacts with matter and how it travels
• Light propagation is similar to sound and water waves

Interaction with Material

• Light's interaction with material depends on the light's frequency and the material's atomic structure
• In transparent materials, electrons oscillate
• Oscillation takes energy and returns it, slowing the light wave without losing energy
• White light separates into a spectrum of colors when passing through a prism
• The prism refracts light of different colors at different angles
• Red light bends the least, and violet light bends the most
• Denser materials slow light more
• Effect also depends on frequency/wavelength of the light
• Nontransparent materials absorb or reflect light
• Dark cloth absorbs light energy and increases motion making the material heat up
• Reflective materials re-radiate light
• Study of light behavior in materials is called optics
• All materials exhibit absorption, refraction, and reflection.

Refraction

• Refraction is the bending of a light ray passing from one substance to another
• Light travels at different speeds in materials, so it changes speed at boundaries
• A beam of light at an angle bends as one side slows down or speeds up
• The refractive index is the ratio of light's speed in a vacuum to its speed in the material
• Different frequencies travel at different speeds, so the refractive index varies
• Different colors are bent at different angles
• Snell's Law relates bending angle to refractive indexes and is used to design lenses

Reflection

• Reflection occurs when light hits the boundary between two materials
• Some light is reflected back into the first material
• Light at an angle is reflected at the same angle
• Light reflects off of flat boundaries like a mirror image
• Light reflected from a curved surface may be focused to a point

Scattering

• Occurs when a transparent material's atoms are not smoothly distributed
• Molecules and particles are bunched up over distances longer than the light wave
• Air molecules scatter sunlight
• Higher frequencies / shorter wavelengths scatter more
• The atmosphere scatters violet light the most

Light waves

• With a slit close to light's wavelength, light diffracts/spreads
• With two slits, waves interfere
• Constructive interference is when crests coincide, forming a larger wave
• Destructive interference is when a crest and trough coincide, canceling each other
• Huygens suggested that light wave peaks form surfaces like layers of an onion.
• Surfaces advance through space at the speed of light
• Each point on a wave surface acts like a new source of spherical waves
• The envelope of all wavelets is a wave surface
• Diffraction
• Diffraction is the spreading of light waves passing through a small opening/around a boundary
• Young's principle of interference and Huygens's explanation explain fringe patterns
• Light from a slit has overlapping wavelets, interference leads to light/dark patterns

Measuring Light

• Accounts for the eye's varying response to different colors of light
• Lumen
• Measures the total amount of energy radiated in all directions
• Candela
• Measures the amount radiated in a specific direction
• Flux
• The rate at which light energy is falling on a surface
• Illumination or Luminance
• Measured in lumens
• Luminance of one square foot is defined to be one foot-candle
• Candela was based on the light produced by a standard candle
• Defined as energy flow in a yellow-green light with a frequency of 540 x 1012 Hz and radiant intensity of 1/683 watt
• Lumen Definition
• Source radiating one candela uniformly in all directions
• Illumination can be calculated using the inverse square law
• Light per square foot decreases as the inverse square of the distance from the source
• One square foot of a surface at 1 foot away as an illumination of 1 foot-candle
• One square foot of a surface that is 4 feet away an illumination of 1/16 foot-candle
• Metric system unit: lux

Home Lighting

• Hallway:
• Creates the first impression for guests
• Light should be friendly, bright, and welcoming.
• Functional and economical lighting
• High illuminance save time searching
• Living Room
• Place to rest and relax
• The lighting should be bright, glare-free, and flexible
• Living rooms need different light sources
• Bedroom
• Lighting should meet varied needs
• Soft, not too bright light needed
• Recessed furniture light turns shelving and cabinets into real eye-catchers
• Luminaires offer a small pool of bright light
• Wardrobes and dressing tables can be lit with warm or cool LED light
• Savings
• Reflector lamps direct light precisely
• Motion detectors save money
• Using halogen lamps enjoy 15% more light
• Multiple light sources can create pools of light and give a room structure
• Indirect lighting creates height
• Incandescent can be replaced in energy-saving lamps
• Bathroom
• Time each morning to start the day calmly
• Water and electricity require safety measures
• Dining Room
• Lighting should be subtle, relaxed and atmospheric
• Dimmable lighting system is ideal
• The lighting over the table must be bright and glarefree
• Avoid beam angles that are excessively narrow so people around the table are not in shadow
• Energy saver technology saves up to 60%
• Avoid antiquated unappetizing lighting
• Warm light colors create a soft and pleasant light
• Kid's Bedrooms
• Lighting has different phases for different functions
• Light sources must by safe
• Small children lamps out of reach
• Energy saving background lighting provides savings up to 80%
• Indirect lighting creates a pleasant glare-free atmosphere

Three Dimensions of Light

• 1st Dimension
• theory of optimum vision
• Artificial light sources
• Based on scientific findings
• 2nd Dimension
• Improves human perception
• Design and construction
• Lighting systems
• 3rd Dimension
• Biological effects of light
• Improves well-being, greater powers of concentration and better quality of life
  • Lights control of our internal clock

Control of Internal Clock

• Biorhythms
• Dictate when we sleep
• dictates when we wake up
• Determine our body temperature
• The control center of the internal clock is in our brain
• The "Circadian Rhythm" is determined by our genetic makeup
• A third photoreceptor was found to be the way to have a more direct impact on the control center in the brain.
• discovered only a few years ago
• effect is greatest when a larger surface area is used
• Light acts on the control center
• High Blue Component
• Indirect lighting
• White wall light surface has a stronger impact
  • Light from a spotlight that illuminates only a small area
• Internal Clock
• Clock synchronizes internal clock with the external day
• The biological clock is the internal clock
• Progress is genetically predetermined
• Biological or physiological clock
  • Influenced significantly by light
• The most important "zeitgeber" for the internal clock
• Status of the internal clocks with other words the phase of the biological clock
  • Body's circadian rhythms
  • Can be deduced from the melatonin level curve