Fundamentals of Imaging

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Fundamentals of Imaging
Tao Zhou
Chapter 1 Light Source
1.1 Different kind of light sources and their
mechanisms
1.2 Intensity and color of light sources
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1.2.1 Light Characterization
Intensity and Power
Radiometry vs. Photometry
Radiometry
radiant flux or radiant power is the measure of
the total power of electromagnetic radiation
(including visible light).
Photometry
luminous flux or luminous power is the measure of
the perceived power of light by human eye.
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A general comparison and units
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Radiant vs. Luminous Power (Flux)
The radiant power is the total radiated power in
watts, also called radiant flux. This power must
be factored by the sensitivity of the human eye
to determine luminous flux in lumens. The
standard definition is as follows
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Luminous Efficacy
where Iv(?) is the luminous intensity in
candelas, I(?) is the radiant intensity in W/sr
and y(l) is the standard luminosity function
The curves represent the spectral luminous
efficacy for human vision. The lumen is defined
such that the peak of the photopic vision curve
has a luminous efficacy of 683 lumens/watt.
This value for the scotopic peak makes the
efficacy the same as the photopic value at 555
nm. The scotopic vision is primarily rod vision,
and the photopic vision includes the cones. The
response curve of the eye along with the
spectral power distribution of a luminous object
determine the perceived color of the object.
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Luminous Efficacy Table
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Power Per Unit Solid Angle
The power (flux) per unit solid angle (sometimes
called pointance) is the nearest precise
terminology to the common term intensity. It
expresses the directionality of the radiated
energy and is appropriate for the description of
point sources. In the case of radiant power, it
is expressed in watts per steradian. In
photometry it is expressed in lumens per
steradian candela.
The candela is the foundation unit for the
measurement of visible light. It is one of the
seven foundation SI units. It's formal
definition is The candela is the luminous
intensity, in a given direction. of a source
that emits monochromatic radiation of frequency
540 x 1012 hertz and that has a radiant
intensity in that direction of 1/683 watt per
steradian. The candela is abbreviated cd and its
Standard symbol is Iv. The candela is then used
to define the lumen and other quantities used in
the measurement of visible light.
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Power Per Unit Area Per Unit Solid Angle
The power per unit area per unit solid angle is
sometimes called sterance. In the radiant case
it is measured in watts/m2 steradian and is also
called radiance. In the luminous case it is
measured in lumens/m2 steradian which is
equivalent to candela/m2 nit. This quantity is
also called luminance.
Power Per Unit Area of Surface
The power per unit area on an illuminated
surface, sometimes called areance, is
distinguished from the similar quantity for the
source. In radiometry the surface areance may be
called irradiance and luminous areance may be
called illuminance. This is the quantity of
practical importance in judging whether an area
is lighted well enough for reading or other
activities. The illuminance is measured in lux.
The lux is defined as a lumen per square meter
and is a unit of illuminance. An equivalent term
is luminous flux density.
Illuminance Ev Luminous intensity Iv /radius2
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The SI units of Radiometry and Photometry
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Radiometry Summary
Notes
Abbr.
Quantityr
SI radiometry units
energy
J
Radiant energy
radiant energy per unit time, also called radiant
power
W
Radiant flux
power per unit solid angle
Wsr-1
Radiant intensity
power per unit solid angle per unit projected
source area. Sometimes confusingly called
"intensity".
Wsr-1m-2
Radiance
power incident on a surface. Sometimes
confusingly called "intensity".
Wm-2
Irradiance
power emitted from a surface. Sometimes
confusingly called "intensity".
Wm-2
Radiant emittance / Radiant exitance
commonly measured in Wsr-1m-2nm-1
Wsr-1m-3or Wsr-1m-2Hz-1
Spectral radiance
commonly measured in Wm-2nm-1
Wm-3orWm-2Hz-1
Spectral irradiance
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photometry summary
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1.2.2 The color of light source
A spectral color is composed of a single
wavelength and can be correlated with wavelength
as shown in the chart above.
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The characteristics of color hue,
saturation, and brightness
It is common practice to define pure colors in
terms of the wavelengths of light as shown. This
works well for spectral colors but it is found
that many different combinations of light
wavelengths can produce the same perception of
color.
The inherently distinguishable characteristics of
color are hue, saturation, and brightness. Color
measurement systems characterize colors in
various parameters which relate to hue,
saturation, and brightness. They include the
subjective Munsell and Ostwald systems and the
quantitative CIE color system.
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Hue
The Newton Color Circle
The terms "red" and "blue primarily describe hue
hue is related to wavelength for spectral
colors. It is convenient to arrange the
saturated hues around a Newton Color Circle.
Starting from red and proceeding clockwise around
the circle below to blue proceeds from long to
shorter wavelengths. However it shows that not
all hues can be represented by spectral colors
since there is no single wavelength of light
which has the magenta hue - it may be produced by
an equal mixture of red and blue.
Newton's color circle is a convenient way to
summarize the additive mixing properties of
colors. R,G,B are thought of as the additive
primary colors, and their complementary colors
are placed across from them on the circle. The
colors then fall on the circle in the order of
the wavelengths of the corresponding spectral
colors. Magenta is not a spectra color.
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Hue
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Hue
Additive color mixing
Tristimulus Values
Any color which can be produced by the primary
colors blue, green, and red can be written
where B,G,R can be considered to be "unit
values" for blue, green, and red and B,G,R are
the magnitudes or relative intensities of those
primaries and are called "tristimulus values".
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saturation
Note that the blue of the sky is more saturated
when you look further from the sun. The almost
white scattering near the sun can be attributed
to Mie scattering, which is not very wavelength
dependent. The mixture of white light with the
blue gives a less saturated blue.
Pink may be thought of as having the same hue as
red but being less saturated. A fully saturated
color is one with no mixture of white. A
spectral color consisting of only one wavelength
is fully saturated, but one can have a fully
saturated magenta which is not a spectral color.
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Brightness
The brightness of a colored surface depends upon
the illuminance and upon its reflectivity. Since
the perceived brightness is not linearly
proportional to the reflectivity, a scale from 0
to 10 is used to represent perceived brightness
in color measure- ment systems like the Munsell
system. It is found that equal surfaces with
differing spectral characte- ristics but which
emit the same number of lumens will be perceived
to be equally bright.
If one surface emits more lumens, it will be
perceived to be brighter in a logarithmic
relationship which yields a constant increase in
brightness of about 1.5 units with each
doubling of brightness.
The images on the left from the commercially
available Color Wheel depict 10 levels of
bright- ness for gray or achromatic light.
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Color Space
The three essential parameters hue, saturation,
and brightness can be thought of as defining a
color space. in analogy with three spatial
dimensions. Three color "coordinates" would
specify a color.
Space can be described by different coordinate
systems, and the three most widely used color
systems, Munsell, Ostwald, and CIE, describe the
color space with different parameters. The
Munsell system uses hue, value, and chroma and
the Ostwald system uses dominant wavelength,
purity, and luminance. The more precise CIE
system uses a parameter Y to measure brightness
and parameters x and y to specify the
chromaticity which covers the properties hue and
saturation on a two dimensional chromaticity
diagram.
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The MUNSELL system is a collection of color
samples for comparison, with adjacent samples
based upon equal perceived differences in color.
Some features of the Munsell system are used in
commercially available paint and pigment mixing
guides like the Color Wheel.
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The C.I.E. Color Space
Color matching function
integration
Spectral power distribution
reflectance
Any color on the CIE chromaticity diagram can be
considered to be a mixture of the three CIE
primaries, X,Y,Z. That mixture may be specified
by three numbers X,Y,Z called tristimulus
values.
x,y,z is normalized according to X,Y,Z. x and y
are used as the coordinates in the CIE color
space. Color C xX yY zZ x X / (XYZ),
y Y / (XYZ), z Z / (XYZ) 1- x- y
X,Y,Z uniquely represent a perceivable hue, and
different combinations of light wavelengths
which gives the same set of tristimulus values
will be indistinguishable in chromaticity to the
human eye.
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The C.I.E. Color Space
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The C.I.E. Color Space
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Color Temperature
An incandescent light is very close to being a
black-body radiator. Of all colors based on the
black body blue is the "hotter" color, while red
is actually the "cooler" color. This is the
opposite of the associations both colors have
taken on, with "red" as "hot", and "blue" as
"cold". A proof of this is that while
incandescent bulbs glow a reddish to yellowish
color throughout their lifetimes, when one blows
out, the flash of light is noticeably bluish. The
filament is hotter when it burns out.
5000 K and 6500 K are also called respectively
D50 (US standard) and D65 (Europe standard) in
all professions working with light
(photographers, publishers, etc). It is a
temperature similar to the black body temperature
but not strictly identical.
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Correlated Color Temperature
The Kelvin system for lamp description works
well for an incandescent light bulb. Since these
lamps are very nearly black body radiators,
their chromaticity coordinates land directly on
the Planckian locus in the CIExy color space.
(In Color theory, the Planckian locus is
generally the path that the color of a black
body would take in a particular color space as
the blackbody temperature changes. See the black
line on The left graph.)
However, many other light sources, such as
fluorescent lamps, do not emit radiation in the
form of a black-body curve, and are assigned
what is known as a correlated color temperature
(CCT), which is the color temperature of a black
body which most closely matches the lamp's
perceived color.
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Correlated Color Temperature
Fluorescent lighting is not incandescent and
presents a new challenge. Fluorescent lamps are
made using myriad combinations of phos- phors and
gases. The illumination that they produce is
almost never described by a point in color space
which lies on the Planckian locus. The left plot
shows lines crossing the Planckian locus for
which the correlated color temperature is the
same. Nevertheless, the colors are not the same,
and the method gives only an approximate
specification of a particular color. Due to this
shortcoming, the rated CCT of any fluorescent
tube does not completely specify its color. To
be more precise A number of color spaces have
been developed in which the difference between
two colors
may be estimated by the distance between them on
a chromaticity diagram. On a chromaticity diagram
for which distances specify color distances, the
best estimate of the color temperature of any
point will be the color temperature of the point
on the Planckian locus closest to that point.