### PowerPoint Presentation - Physics 1230: Light and Color

```Hue, Saturation, and Brightness (HSB)
• What they mean in terms of
intensity distribution curves?
• Hue is specified by the dominant
wavelength color in the intensitydistribution curve
• Saturation is the purity of a color
(absence of other wavelengths).
• The pure spectral colors are the
most saturated
• Brightness refers to the sensation
of overall intensity of a color
Brightness
Bright white
Grey
Black
Hue
Saturation
Desaturatated
orange=saturated
Brown (same) orange + white
Orange
Blue
Blue
Hue, Saturation and Brightness (HSB):
One way to use 3 numbers to specify a color
instead of using an intensity-distribution curve
• Color tree (e.g. Fig. 9.5 in book)
• Photoshop: uses H, S and B
hue
saturation
lightness
• Moving up the tree increases the
lightness of a color
• Moving around a circle of given
radius changes the hue of a
color
• Moving along a radius of a
circle changes the saturation
(vividness) of a color
• These three coordinates can be
described in terms of three
numbers
Red, green and blue (RGB):
RGB is another way to use 3 numbers to specify a color
instead of using an intensity-distribution curve or HSB
•
•
In addition to using Hue, Saturation and
Brightness (HSB);
Many (but not all) colors can be described
in terms of the relative intensities of a light
mixture of a certain wavelength red,
wavelength green and wavelength blue
lights
•
•
•
•
•
650-nm red
530-nm green
460-nm blue
These are called the additive primaries
The mixing of the additive primaries is
•
Additive mixing is usually done by mixing
primary color lights with different
intensities but there are other ways to be
discussed later
•
Demonstrate with Physics 2000
2000/tv/colortv.html
530-nm green
cyan
yellow
650-nm red
magenta
460-nm blue
•
Definition of complementary color (for
•
•
The complement of a color is a second
color.
When the second color is additively
mixed to the first, the result is white.
•
•
•
•
•
Blue & yellow are complementary
B + Y = W.
Green & magenta are complementary
G+M=W
Cyan and red are complementary
C+R=W
Magenta is not a wavelength color— it
is not in the rainbow
There is at most one wavelength
complementary color for each
wavelength color (Fig 9.9)
yellow
green
white
cyan
magenta
blue
red
primaries
green = cyan.
red = yellow.
blue = magenta.
Complementary colored lights
Blue (primary)
and yellow.
Green (primary)
and magenta.
Red (primary)
and cyan.
Chromaticity diagrams: Yet another way to represent
colors by (3) numbers
•
The chromaticity diagram is in many
ways similar to a color tree
•
•
•
A chromaticity diagram has a fixed
brightness or lightness for all colors
Wavelength colors are on the horseshoe
rim but non-wavelength colors like
magenta are on the flat part of the rim
Inside are the less saturated colors,
including white at the interior
less saturated colors
saturated
wavelength
colors
saturated
non-wavelength
colors
Using the chromaticity diagram to identify colors
•
•
The numbers that we use to identify a
color are its x-value and y-value inside
the diagram and a z-value to indicate its
brightness or lightness
x and y specify the chromaticity of a
color
•
•
•
•
Example: Apple pickers are told around
the country that certain apples are best
picked when they are a certaim red (see
black dot)
Since the chromaticity diagram is a world
standard the company can tell its
employees to pick when the apples have
chromaticity
• x = 0.57
• y = 0.28
The "purest" white is at x = 0.33 and y =
0.33
Chromaticity diagram can be related to
colors in Photoshop
Using the chromaticity diagram to understand the
result of additive mixing of colors
• An additive mixture of two wavelength
colors lies along the line joining them
• Example: The colors seen by mixing
700 nm red and 500 nm green lie along
the line shown
• Where along the line is the color of the
mixture?
• Answer depends on the relative
intensities of the 700 nm red and the
500 nm green.
• Here is what you get when the green is
much more intense than the red (a
green)
• Here is what you get when the red is
much more intense than the green (a
red)
• Here is what you get when the red is
slightly more intense than the green (a
yellow)
Note — this works for adding
two colors in middle also!
Using the chromaticity diagram to understand
complementary colors
• The complement to any
wavelength color on the edge of
the chromaticity diagram is
obtained by drawing a straight
line from that color through
white to the other edge of the
diagram
• Example: The complement to
700 nm red is 490 nm cyan
• Example: The complement to
green is magenta - a nonwavelength color
Using the chromaticity diagram to find the dominant
hue of a color in the interior of the diagram
• To find the dominant hue of the
color indicated by the black dot
• Draw st. line from white through
the point to get dominant
wavelength, and hence, hue
(547 nm green)
of white with a fully-saturated
(wavelength) color gives the
desaturated color of the original
point
What is partitive
mixing?
• Partitive mixing is another
but not achieved by
superimposing colored
lights!
• Instead, it works by putting
small patches of colors next
to each other.
• From a distance these
colors mix just as though
they were colored lights
superimposed on each
other
• Examples:
• Seurat pointillism
• Color TV and computer
screens (Physics 2000)
• Photoshop example
A colored filter subtracts
colors by absorption.
=
Incident white light
Cyan
filter subtracts
red
Yellow
filter subtracts
blue
Only green
gets
through
A colored filter subtracts certain
colors by absorption and transmits
the rest
=
Incident white light
Magenta
filter subtracts
green
Cyan
filter subtracts
red
Only blue
gets
through
A colored filter subtracts
colors by absorption.
=
Incident white light
Magenta
filter subtracts
green
Yellow
filter subtracts
blue
Only red
gets
through
What is the effect of combining (sandwiching)
different colored filters together?
• Rules for combining the
subtractive primaries,
cyan, yellow and magenta:
• White light passed through
a cyan filter plus a magenta
filter appears blue
• White light passed through
a yellow filter plus a
magenta filter appears red
• White light passed through
a yellow filter plus a cyan
filter appears green
• Why?
cyan
yellow
magenta
Colored surfaces subtract certain
colors by absorbing them, while
reflecting others
White in
Magenta out
Magenta surface
absorbs (subtracts)
green.
White in
Green out
Green surface
absorbs (subtracts)
red and blue (magenta).
Green light on a
magenta surface
appears colorless
because green is
absorbed
Green in
Magenta surface
absorbs (subtracts)
green.
Magenta light on a
green surface
appears colorless
because magenta is
absorbed
Magenta in
Green surface
absorbs (subtracts)
red and blue (magenta).
When looking at a colored object in a colored light
source what is the resulting color?
• Rule: Multiply the
intensity-distribution of the
light source by the
reflectance of the colored
object to get the intensity
distribution of the the
illuminated object
• Example: Look at a
magenta shirt in reflected
light from a Cool White
fluorescent tube.
• It appears grey (colorless)
Confirm by multiplying the
intensity distribution curve
by the reflectance curve to
get the new intensity
distribution curve for the
reflected light
Cool white fluorescent bulb
This number times
Magenta shirt
this number
How the shirt
appears in this light
equals this number
You multiply the two y-values
at each x to get the new curve
Halftone
• Left: Halftone dots.
• Right: How the human eye
would see this sort of
arrangement from a
sufficient distance or when
they are small.
• Resolution: measured in lines
per inch (lpi) or dots per
inch (dpi); for example,
Laser Printer (600dpi)
Color halftoning
Printer's
ink
Paper beneath
Three examples of color halftoning with CMYK separations. From left to right: The
cyan separation, the magenta separation, the yellow separation, the black separation, the
combined halftone pattern and finally how the human eye would observe the combined
halftone pattern from a sufficient distance.
Demonstration
Color Liquid Crystal Displays (LCDs)
Chapter 10: We have three different kinds of cones whose
responses are mainly at short, intermediate and long
wavelengths
• s-cones absorb short wavelength light best,
with peak response at 450 nm (blue)
• L-cones absorb long wavelength light best,
with peak response at 580 nm (red)
• i-cones absorb intermediate wavelengths
best, with peak response at 540 nm (green)
• Light at any wavelength in the visual
spectrum from 400 to 700 nm will excite
these 3 types of cones to a degree
depending on the intensity at each
wavelength.
• Our perception of which color we are
seeing (color sensation) is determined by
how much S, i and L resonse occurs to
light of a particular intensity distribution.
Rule: To get the overall response of each type of
cone, multiply the intensity of the light at each
wavelength by the response of the cone at that
wavelength and then add together all of the
products for all of the wavenumbers in the
intensity distribution
L-cones
i-cones
s-cones
Spectral response of cones in typical human eye
Light color
Brightness
460 nm blue
1
575 nm yellow
1.66
Mixture ( perceived as white)
S-cone response
60
0
60 + 0 = 60
I-cone response
5
1.66 x 33
5+1.66 x 33 = 60
L-cone response
2
1.66 x 35
2+1.66 x 35 = 60
Examples of two different ways we see white
• Our sensation of color depends on how much total s, i
& L cone response occurs due to a light intensitydistribution
• Multiply the intensity distribution curve by each
response curve to determine how much total S, i,
and L response occurs
• We experience the sensation white when we have
equal total s, i & L responses
• There are many ways this can occur!!
• E.g., when broadband light enters our eye
• Another way to experience white is by viewing a
mixture of blue and yellow
• E.g., 460 nm blue of intensity 1 and 575 nm
yellow of intensity 1.66
• The blue excites mainly s-cones but also a
bit of i-cones and a bit of L-cones
• The yellow excites i-cones and (slightly
more) L-cones but no s-cones
• The result is an equal response of s-cones, icones and L-cones (details)
Spectral response of cones in typical human eye
1.66
1
0
460 nm blue of
intensity 1
575 nm yellow
of intensity 1.66
Light color
Brightness
S-cone response
I-cone response
L-cone response
530 nm green
1
negligible
41
How does a normal
person
see yellow
when only 28red
650 nm red
2.15
negligible
2.15 x 2
2.15 x 9
and asgreen
are superimposed?
Mixture
(perceived
yellow ) lights
negligible
41 +2.15 x 2 =45
28 +2.15 x 9 =47
575 nm yellow
1.35
negligible
• Our sensation of yellow depends on a special s, i &
L cone response
• We experience the sensation yellow when 575 nm
light reaches our eyes
• What really gives us the sensation of yellow is
the almost equal response of i and L cones
together with no s-cones!!
• Another way to experience yellow is by seeing
overlapping red & green lights
• E.g., 530 nm green of intensity 1 and 650
nm red of intensity 2.15
• The green excites mainly i-cones but also
L-cones, while the red excites mainly Lcones but also i-cones
• The total respone of s & i-cones due to the
spectral green and red is the same as the
total response due to spectral yellow
• In general need 3 wavelength lights to mix to
any color
1.35 response
x 33 = 45of cones
1.35
x 35 = 47
Spectral
in typical
human eye
650 nm red
575 nm yellow of intensity
of intensity 1.35 2.15
530 nm green
1 of intensity 1
2.15
0
We can verify color naming of hues in terms of the
psychological primaries on the chromaticity diagram
All of the hues can be named
qualitatively by how much green, red,
blue or yellow is "in" them
• We don't need orange, purple or pink:
• orange can be thought of as yellow-red
• purple can be thought of as red-blue
• pink has the same hue as red but differs
only in lightness
We can break up the diagram into 4
different regions by drawing two lines
whose endpoints are the psychological
primary hues
• The endpoints of the yellow line are 580
nm "unique" yellow and 475 nm
"unique" blue
• One endpoint of the red line is 500 nm
"unique" green and the other is "red"
(not unique or spectral - really more like
magenta)
Greenness &
yellowness
Redness &
yellowness
What is meant by the opponent nature of red vs green
(r-g) perception and of yellow vs blue (y-b) perception.
• Viewing a progression of colors in
the direction of the yellow line from
475 nm blue towards 580 nm yellow,
we see more yellowness of each color
and less blueness.
• We call this perception our y-b
channel
• Yellow & blue are opponents
• Moving parallel to the red line from
500 nm green towards nonspectral
red we see more redness in each color
and less greenness.
• We call this perception our r-g
channel
• Red and green are opponents
• The lines cross at white, where both
y-b & r-g are neutralized
Greenness &
yellowness
Redness &
yellowness
How might the three types of cones be "wired" to neural cells to
account for our perception of hues in terms of two opponent
pairs of psychological primaries r-g and y-b?
• The 3 kinds of cones are related to r-g and y-b by
the way they are connected to neural cells (such as
ganglion cells)
• Cones of each kind are attached to 3 different
neural cells which control the two chromatic
channels, y-b and r-g, and the white vs black
channel called the achromatic channel (lightness)
• "wiring" is the following:
• When light falls on the L-cones they tell all 3
neural cells to increase the electrical signal they
send to the brain
• When light falls on the i-cones they tell the r-g
channel cell to decrease (inhibit) its signal but tell
the other cells to increase their signal
• When light falls on the s-cones they tell the y-b
channel cell to decrease (inhibit) its signal but tell
the other cells to increse their signal
s-cone

++
neural cell
for y-b
chromatic
channel
i-cone
L-cone
+  +
+ ++
neural cell
for r-g
chromatic
channel
Electrical signal to brain
neural cell
for w-blk
achromatic
channel
How can this "wiring" work to produce the chromatic
channels?
• The neural cell for the y-b chromatic
channel has its signal
s-cone
i-cone
L-cone
• inhibited when (bluE) light excites the
s-cone
INTERPRETED AS BLUE
• enhanced when light excites the i & L
cones
INTERPRETED AS YELLOW
• The neural cell for the r-g chromatic
channel has its signal
•
• inhibited when (green) light falls on the
 ++ +  +
i-cone
INTERPRETED AS GREEN
neural cell
neural cell
• enhanced when light excites the s and
for y-b
for r-g
L cone
chromatic
chromatic
INTERPRETED AS MAGENTA
channel
channel
(Psychological red)
The neural cell for the achromatic channel
has its signal enhanced when light excites Electrical signal to brain
any of the cones
+ ++
neural cell
for w-blk
achromatic
channel
Systematic description of
color-blindness (no need to
memorize terminology)
• Monochromacy (can match any colored light
with any 1 spectral light by adjusting
•
intensity)
• Either has no cones (rod monochromat)
or has only 1 of the 3 types of cones
working (cone monochromat).
• Sees ony whites, greys, blacks, no hues
• Dichromacy (can match any colored light
with 2 spectral lights of different intensities of
(rather than the normal 3)
• L-cone function lacking = protanopia
• i-cone function lacking = deuteranopia
• s-cone function lacking = tritanopia
• no y-b channel but all 3 cones OK =
tetartanopia
Anomalous trichromacy (can match
any colored light with 3 spectral lights
of different intensities as in normal
vision, but still have color perception
problems)
• Protanomaly
• Shifted L-cone response curve
• Deuteranomaly (most common)
• Shifted i-cone response curve
• Confusion between red and green.
• Tritanomaly
• Yellow-blue problems: probably
defective s-cones
• Neuteranomaly
• ineffective r-g channel
Receptive field of a double-opponent
cell of the r-g type
• 2 different ways to INCREASE the
signal the ganglion cell sends to brain
• Red light falling on cones in center
of receptive field attached to
ganglion cell
• Green light on surround
• 2 different ways to decrease the
signal the ganglion cell sends to the
brain
• Red light on surround
• Green light on center
• Electrical signal to brain from ganglion
cell is at ambient level when no light is
on center or surround
• When signal to brain is
INCREASEDwe interpret that as red
• When signal to brain is decreased we
interpret that as green
signal to brain
We can summarize this by just showing the center &
surround of the receptive field and indicating the effect of
red (R) and green (G) on each
• A double-opponent cell differs
from a single opponent cell
• In both of them R in the center
increases the signal
• In a single-opponent cell G in
surround would inhibit signal,
whereas in double-opponent cell
G enhances
• In a double-opponent cell
• R in center enhances signal
(ganglion cell signals red)
• G in surround enhances signal
(ganglion cell signals red)
• R in surround inhibits signal
(ganglion cell signals green)
• G in center inhibits signal
(ganglion cell signals green)
Fictional cell
real cell
Here is an illustration of the effect of red or green light
falling in various combinations on the center or
surround of a double-opponent r-g cell
Strongest
signal
(interpreted
as red)
Weakest
signal
(interpreted
as green)
Note, you would
still "see" red if
the center were
grey!
Note, you would
still "see" green
if the center
were grey!
No change in
signal (color
not noticed)
No change in
signal (color
not noticed)
y-b double-opponent receptive fields and cells work the
same way
Strongest
signal
(interpreted
as yellow)
Weakest
signal
(interpreted
as blue)
Note, you would
still "see" yellow
if the center
were grey!
Note, you would
still "see" blue if
the center were
grey!
No change in
signal (color
not noticed)
No change in
signal (color
not noticed)
b+yy+b-
Here is an optical illusion which can be explained by
double-opponent retinal fields and cells
• Look at the grey squares in your
peripheral vision
• Does the grey square
surrounded by yellow appear to
take on a tint?
• What color is it?
• Repeat for the grey squares
surrounded by
• Blue
• Green
• Red (pink)
Color constancy depends on doubleopponent processing
• Color constancy means we see the
proper colors of a picture or scene or
object relatively correctly even though
the overall illumination may change its
color
• This is because our double-opponent
receptiive fields compare neighboring
colors and are not very sensitive to an
overall change in color
• Color constancy developed in the
evolution of mankind so that we could
daylight, late afternoon, and early
evening
No change in
signal (color
not noticed)
No change in
signal (color
not noticed)
Illustration of how the three opponency channels work
in your perception of the design below
• Here are the enhanced edges
channel
• Note the edges that separate a
yellowish from a bluish color are
enhanced the most
• Here are the enhanced edges
channel
• Note the edges that separate a
reddish from a greenish color are
enhanced the most
• Here are the enhanced edges
achromatic channel
• Compare with the way a photocopy
machine would see the design
Chapter 13: What can a light wave do when it
encounters matter?
• Be TRANSMITTED
 laser aimed at water or glass
• Be REFLECTED
 specular reflection of light by a
mirror
 diffuse reflection of the light in
this room off all the other
students
 reflection is re-radiation of light
by the electrons in the reflecting
material
• Be ABSORBED
 Cyan light shining on a red apple
is absorbed by electrons in the
apple
• A light wave shining on
molecules in the air or plastic or
other “transparent” materials
can be
• SCATTERED
 Light ray moves over to the side in
all directions rather than forward,
backward or being absorbed.
 Intensity of the scattered light can
depend on wavelength
What is Rayleigh scattering?
(or why is the sky blue)
• The shorter the wavelength, the more
light is scattered
 blue is scattered more than red.
 this is why the sky is blue and
sunsets are red. (Fig. 13.1)
 Dust or smoke enhances red look of
the sun by providing more scattering
• Larger particles scatter red as well as
blue and hence look white.
 Clouds;
 Milk;
 Colloidal suspensions
Think of white light
from sun as a mixture
of R, G and B
Blue is scattered the
most so sky looks
blue when we look
away from the sun
For same reason sun
looks yellow (red +
green)
More atmosphere
allows next shortest
wavelengths (green)
to scatter so sunset
looks red
What is polarized light?
• Light is polarized if the waveform and
electric force field arrows remains in
the same plane
 The (green) electric force arrows must
always be perpendicular to the ray
• This is a light ray traveling in the zdirection and polarized in the ydirection
• Here is a light ray traveling in the same
direction but polarized in the xdirection
• We will visualize the polarization in
the x-y plane, looking at rays head-on
 The green force arrows point up and
down or left and right, stacked up
behind one-another.
 Here is the convention for visualizing
vertical and horizontal polarization
green arrows up & down
y
y
z
z
x
x
y
x
What is unpolarized light?
• For unpolarized light the
plane of polarization keeps
jumping around
 But the electric force field
arrows remain perpendicular to
the ray (direction of travel of
the wave)
 We visualize this in the x-y
plane (looking into the ray) as
shown at right
y
electric force arrows
jump around while
remaining perpendicular to the ray
wave travels in
z-direction
z
x
• The many crossed double
sided arrows are the symbol
for unpolarized light
• See Physics 2000
y
x
When unpolarized light reflects off a horizontal surface
(such as water or beach) near a special angle, the reflected
light is polarized in the horizontal direction
 The special angle of incidence is
where the refracted ray and reflected
ray are perpendicular to each other
 This is called Brewster's angle
 To understand, imagine the electric
force arrows of the incident
unpolarized light to be decomposed
into two perpendicular polarizations
• the first polarization is horizontal
(force arrows are parallel to the flat
reflecting horizontal surface and
perpendicular to the ray)
• in the 2nd (Fig. 13.5), the arrows are
perpendicular to both the ray and the
horizontal force arrows
– The second polarization cannot be
sustained in the reflected ray because
the force arrows would be parallel to
that ray (impossible for a light ray)
– Hence, only the horizontal polarization
survives in the reflected ray
Some material from Chapter #8
How do 3D
movies use
polaroid filters?
```