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Sin ttulo de diapositiva

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A simple and shorter description of the spectral ... As a general flood light, a laser diode is a poor choice. As a collimated point source, a ... – PowerPoint PPT presentation

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Title: Sin ttulo de diapositiva


1
III. LIGHT SOURCES
  • 1. Color temperature
  • 2. Light measurement units
  • 2. Standard illuminators
  • 3. Light sources
  • incadescent and halogen lamps
  • flourescent lights
  • open arc lamps
  • solid state sources

2
Color temperature
The characterisation of a light source is
complex. It includes the energy content for each
wavelength or relative spectral power. A simple
and shorter description of the spectral content
provided by a light source is color
temperature. Color temperature is the absolute
temperature of black body radiator which provides
a color match between the black body and the
given radiator A black body is a hollow
structure, usually a sphere, whose interior is
black. It has a small opening. If we heat up the
black body, we get radiation from the opening.
The color of radiation changes with the
temperature of the black body. Color temperature
is a measure of the color of light (spectral
power distribution), not actual
temperature. The classification with color
temperature is done also when the curves of the
spectral power distribution deviate more or less
strongly from that of the black body. In this
case, we use the concept of correlated color
temperature (CCT). Correlated color temperature
is the absolute temperature of a black body whose
chromaticity most nearly approximates that of
the light source.
3
Color temperature
A black body is ... black at room temperature
(300 K) red at 800 K yellowish-white at 3000
K white at 5000 K bluish-white at 8000 K blue at
60,000 K
Relative spectral power distribution of the black
body at different temperatures. The curves are
standardized so that the spectral power is 1 for
all temperatures at 560 nm
4
Color temperature
  • The measured/perceived color is a function of
  • spectral light content
  • material surface reflectance
  • spectral sensor response


2800 K
2860 K
2900 K
2960 K
3000 K
3100 K
3060 K
Change of the measured color of a color paper
when illuminated with light of different temperatu
re color. Light source was a pulsed xenon arc
lamp inside a mixing to chamber provide diffuse,
even lighting over the 8 mm diameter measuring
area.
5
Standard illuminators
The International Commission on Illumination
(CIE) has recommended the use of certain
spectral power distributions as standards
illuminants. The standard illuminants A, B, and
C are also called standard sources because they
are based on spectral power distribution of
incandescent light source. Standard
Illuminant A is the relative spectral power
distribution of a tungsten filament
incandescent lamp operating at 2856 K. It matches
very closely a blackbody operating at that
temperature. Therefore, it has a CCT of 2854 K.
Standard Illuminant B is obtained by
combining source A with filters. It is designed
to approximate noon sunlight and has a CCT of
4878 K. Standard Illuminant C is also
obtained by combining source A with filters. It
corresponds to the relative spectral power of
daylight and has a CCT of 6774 K. The standard
illuminants D50, D55, D65, and D75 represent
daylight spectral power distributions with CCTs
of 5000 K, 5500 K, 6500 K, and 75000 K,
respectively.
6
Standard illuminators
Relative spectral power distribution of daylight
Relative spectral power distribution of standard
illuminant A
Relative spectral power distribution of daylight
with a color temperature of 6500K. The spectral
power distribution is called standard illuminant
D65
7
Light measurement units
Photometry vs radiometry Radiometry is the
measurement of radiation in general. Photometry
is the measurement of the properties of
radiation that lies in the visible region of the
spectrum (light). What is more, photometric
units take into account the response of the human
observer to the measurement of radiation while
radiometric units do not.
Pag 8.6 DolanJenner
8
Light measurement units
Flux or luminous flux The rate at which a source
emits light in all directions. Energy is
fluxtime. Photometric units 1 lumen (lm)
0.0758 candela Radiometric units 1 watt 683
lumens at 555 nm 1 joule/s Intensity or
luminous intensity The rate at which a source
emits light in a given direction. Visible flux
per solid angle. Photometric units 1 candela
1 lumen/steradian Radiometric units 1 watt/sr
Illuminance Density of light incident or falling
upon a surface. Visible flux density. Photometric
units 1 lux 1 lumen/m2 Radiometric units 1
watt/steradian 1 watt/m2 at 1 m Luminance The
rate at which the unit area of a source emits
light in a specific direction. Useful when
a source is not a point source but has an
appreciable size (as real sources do). Units 1
lumen/m2/sr
9
Lighting efficiency
  • The interpretation of what light is available is
    often hampered by the fact that lights
  • are rated in output luminous flux, i.e., lumens
    of photopic light. However, most video
  • cameras do not operate as the human eye.
  • The luminous flux data assumes a peak response of
    550 nanometers of green light, falling
  • to less than 1 percent at 400 and 700 nanometers.
    Solid state video cameras respond well
  • out to 800 or 900 nanometers, and are typically
    not good responders in the blue end of the
  • spectrum.
  • To know how much light is available to the camera
    assumes some knowledge of the
  • actual spectral output of the lamp
  • mercury vapor shows about 50 percent of the
    output of high pressure sodium, but
  • the mercury light is made up of discrete spikes
    with about 30 percent of the light
  • being in the blue region
  • high pressure sodium lamp has broad spectral
    content from around 550 to 700 nm,
  • with the majority of the light, close to 75
    percent, being between 580 and 630 nm,
  • a good range for solid state video cameras
    without getting into the near infrared.

10
Light sources
  • The current stable of popular light sources for
    use in machine vision leans heavily on
  • home consumer types of lighting
  • incandescent lamps
  • fluorescent lights
  • quartz halogen lamps
  • A variety of high efficiency lights have emerged
    for use in scientific applications beyond
  • machine vision, which may now be adapted to use
    with vision applications
  • open arc (xenon, mercury) lamps
  • solid state LEDs and laser diodes

11
Light sources incandescent
  • Visible radiant energy emitted by any heated
    material is called incandescence. They are
  • lamps with a tungsten filament that has a gas
    sealed within the bulb.
  • very inexpensive as a bright source
  • produce a great deal of heat along with the
    light
  • two types standard and halogen (longer
    lifetime)
  • The color temperature of any incandescent lamp is
    predicated on specific filament design
  • criteria the diameter of the wire and its
    length.
  • The correlated color temperature (CCT) of an
    incandescent lamp is related to its luminous
  • efficacy (lumens per watt).

12
Light sources incandescent
Tungsten standard It is a lamp with a tungsten
filament that has gas (nitrogen or argon) sealed
within the bulb. Has a relative short life time
(1500 h) but degrades causing illumination
fading and color change, therefore being
unreliable.
13
Light sources incandescent
Tungsten Halogen It is a lamp with a tungsten
filament that contains not only argon or
nitrogen, but also one of the regenerative
elements (bromine, iodine, chlorine, fluorine, or
astatine) sealed within the bulb. Thus, the
filaments can operate at higher temperatures,
increasing efficacy (lumes per watt) and
producing "whiter" light (higher color
temperature) than tungsten standard lamp.
  • Advantages over standard incandescent lamps
  • smaller dimensions
  • uniform bright light
  • greater economy
  • longer lamp life as they get older,
    conventional incandescent lamps get dimmer
  • because vaporised tungsten from the filament
    is deposited as a dark coating on the inside
  • of the glass bulb. In halogen lamps. The
    halogens added to the filler gas ensure that the
  • vaporised tungsten is returned to the
    filament.


-
tungsten
halide 250o
14
Light sources incandescent
  • Typically made for slide projectors of similar
    applications.
  • Lamps come in a wide range of sizes, from a few
    watts to many hundreds of watt.
  • The most popular for machine vision
    applications have been the bulbs with built in
  • reflectors in the 150 to 300 watt range.
    These bulbs are used either directly or as input
  • to fiber optic sources.
  • Close to the bulb the light is very hot but
    fiber optic light guides deliver cold light.
  • Lifetimes for these bulbs can be up to 2000
    hours
  • Light can be very uneven due to the filament
    structure.
  • Common for very localized illumination.

15
Light sources fluorescent
Fluorescent lamps are tubes filled with mercury
gas at a low internal pressure. The inner wail
of the tube is painted with phosphors. An
electrical field is produced between two
electrodes in a gas-filled tube. This electrical
field causes mercury atoms to radiate. Phosphors
applied to the inside of the glass convert this
radiation into visible light. Thus, fluorescent
lamps are low-pressure discharge lamps.
  • Popular for general diffuse, even illumination.
  • Cost of fluorescent tubes is low, but often a
    high frequency ballast is needed
  • to increase the 120 hertz oscillation to 20
    kilohertz or so.
  • The output level is on the order of 2000 lumens
    for 40 watt electrical ratings.

16
Light sources fluorescent
  • Large variety of shapes and sizes
  • Have a variety of spectral outputs which range
    from high green content to pink.
  • CIE illuminants F1 through F12 show the
    relative spectral power distribution of 12
  • different fluorescent lamps from 380 to 780 nm.

17
Light sources fluorescent
The CCT of the fluorescent lamps is related with
the "atmospheric effect" radiated by the
phosphor
Thus, the symbols used by the lamp companies to
designate fluorescent lamps mean the following
CW Standard cool white WW Standard warm white
CWX Deluxe cool white WWX Deluxe warm white ES
Energy saving HO High output
18
Light sources open arc or high-pressure
discharge lamps
This light is generated in a similar fashion to
lightning of an arc welder. A discharge of
electric energy is released producing
wavelengths of energy in the visible portion of
the spectrum. Unlike lightning however, the arc
discharge occurs 120 times per second (once
every half cycle on 60hz systems). The process
creates a plasma (arc) that is so hot, that it
continues to emit infrared radiation and visible
light during the off times. The arc tube is a
hermetically sealed chamber which typically
contains some form of starting gas (argon,
xenon, krypton), mercury (99.999 pure). High
voltage forces the starting gas to become
charged and breaks down the electrical resistance
between the main and starter electrode.
19
Light sources open arc lamps or high-pressure
discharge
  • Incandescent lamps light generation are
    extremely inefficient as only about 7 of the
  • energy consumed ends up as visible light, with
    the majority of consumed energy being
  • transmitted as heat.
  • With HID typically 30 40 of the energy
    consumed is transmitted as visible light.
  • Very bright and localized source.
  • Spectral content is well characterized. Xenon
    lamps have a fairly broad, sunlight type
  • spectrum but also put out ultraviolet which can
    actually degrade glass optics. Mercury
  • lamps have very specific spectral spikes, which
    if selected out, can provide a very good,
  • single color source.
  • Very high powers of 7000 to 10,000 watts
    electrical are available in short arc lamps.
    They
  • can produce upward of 300,000 lumens of output
    flux. If all the light is put in a square
  • meter, this would be on the order of 100 times
    brighter than outside sunlight.
  • Typically exhibit very long lifetimes, on the
    order to 20,000 hours
  • Wide range of configurations, ranging from a
    standard incandescent bulb shape to a long,

20
Light sources open arc lamps or high-pressure
discharge
  • Three types mercury, metal halide and high
    pressure sodium lamps
  • High pressure sodium offers a very broad spectral
    content, peaking in the reddish region
  • where solid state cameras are most sensitive.
  • The mercury lamps look very white to people, but
    actually put out specific spikes in the
  • yellow and blue region of the spectrum.

spectral output of a HgXe lamp
21
Solid state sources LEDs and laser diodes
  • As a general flood light, a laser diode is a
    poor choice. As a collimated point source, a
  • laser diode can produce higher irradiance over
    a small area than would be practical
  • from any of the bigger sources, having small
    divergence.
  • Both leds and laser diodes have
  • long lifetimes
  • good mechanical endurance
  • narrow color spectrums.
  • Total output of a few milliwatts is now
    available from LEDs, but this would typically
  • not be considered high light output unless
    only used on a very small area at a time.
  • With the wider use of LEDs and laser diodes in
    consumer goods prices are expected
  • to decrease (a laser diode can be had for
    under 20 these days).

22
Light sources
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