1. 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

2. Radiometry vs. Photometry
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.

3. A general comparison and units

4. 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:

5. Luminous Efficacy
where Iv(λ) is the luminous intensity in candelas, I(λ) 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.

6. Luminous Efficacy Table
Wavelength l (nm)
Photopic Luminous
Efficacy Vl
Photopic Conversion
lm/W
Scotopic Luminous
Scotopic Conversion
380
0.027
1.001
390
0.082
3.755
400
0.270
15.793
410
0.826
59.228
420
2.732
430
7.923
440
15.709
450
25.954
460
40.980
470
62.139
480
94.951
490
500

7. Wavelength l (nm)
Photopic Luminous
Efficacy Vl
Photopic Conversion
lm/W
Scotopic Luminous
Scotopic Conversion
507
510
520
530
540
550
555
560
570
580
590
600
56.355
610
27.081
620
12.529
630
5.670

8. Wavelength l (nm)
Photopic Luminous
Efficacy Vl
Photopic Conversion
lm/W
Scotopic Luminous
Scotopic Conversion
640
2.545
650
73.081
1.151
660
41.663
0.532
670
21.856
0.252
680
11.611
0.122
690
5.607
0.060
700
2.802
0.030
710
1.428
0.016
720
0.715
0.008
730
0.355
0.004
740
0.170
0.002
750
0.082
0.001
760
0.041
0.000
770
0.020

9. 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.

10. Illuminance Ev = Luminous intensity Iv /radius2
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

11. The SI units of Radiometry and Photometry

12. Radiometry Summary Quantityr Abbr. Notes SI radiometry units
Radiant energy J
energy
Radiant flux W
radiant energy per unit time, also called radiant power
Radiant intensity
W·sr−1
power per unit solid angle
Radiance
W·sr−1·m−2
power per unit solid angle per unit projected source area.
Sometimes confusingly called "intensity".
Irradiance
W·m−2
power incident on a surface.
Sometimes confusingly called "intensity".
Radiant emittance / Radiant exitance
W·m−2
power emitted from a surface.
Sometimes confusingly called "intensity".
Spectral radiance
W·sr−1·m−3 or
W·sr−1·m−2·Hz−1
commonly measured in W·sr−1·m−2·nm−1
Spectral irradiance
W·m−3 or W·m−2·Hz−1
commonly measured in W·m−2·nm−1

13. photometry summary Quantity Symbol SI unit Abbr. Notes Luminous energyQv
lumen second
lm·s
units are sometimes called Talbots
Luminous flux F
lumen (= cd·sr) lm
also called luminous power
Luminous intensity Iv
candela (= lm/sr) cd
an SI base unit
Luminance Lv
candela per square metre
cd/m2
units are sometimes called nits
Illuminance Ev
lux (= lm/m2) lx
Used for light incident on a surface
Luminous emittance Mv
Used for light emitted from a surface
Luminous efficacy
lumen per watt
lm/W
ratio of luminous flux to radiant flux;
maximum possible is

14. 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.

15. hue, saturation, and brightness
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.

16. 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.

17. Hue

18. 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".

19. saturation Note that the blue of the sky is more
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.
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.

20. 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.

21. 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.

22. 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.

23. The C.I.E. Color Space Any color on the CIE chromaticity diagram can
Spectral power distribution
reflectance
Color matching function
integration
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 / (X+Y+Z),
y = Y / (X+Y+Z),
z = Z / (X+Y+Z) = 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.

24. The C.I.E. Color Space

25. The C.I.E. Color Space

26. 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.

27. 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.

28. 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.