PowerPoint Presentation - Physics 1230: Light and Color Chapter 1

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Our Plans

• Today, Dec 8 Review of material for the exam
  (chapters 9,10,13)
• Dec. 10 Exam 3 (exam scores preliminary
  grades will be posted on Dec. 14)
• Dec. 18 Final grades

• Exam
• Multiple choice questions
• Problems (2-3 per chapter)
• Information/preparation
• http//www.colorado.edu/physics/phys1230/phys1230_
  fa08/Exams.htm
• Practicing problems
• Reading Material
• Solutions will be posted on the web page soon
  after the exam

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Chapter 9 How we characterize colors Hue,
Saturation, and Brightness (HSB)

• What they mean in terms of intensity distribution
  curves?
• Hue is specified by the dominant wavelength color
  in the intensity-distribution 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
Hue
Saturation
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The same color sensation can often be produced by
2 or more different intensity distribution curves

• Here is an intensity distribution curve which
  gives us the sensation of yellow
• Here is a different intensity distribution curve
  which also gives us the same sensation of yellow
• The two colors described by the two different
  intenstiy curves are called metamers

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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)
• 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
• Photoshop uses H, S and B

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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 called
  additive mixing
• 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

http//www.colorado.edu/physics/2000/tv/colortv.ht
ml
yellow
650-nm red
530-nm green
magenta
cyan
460-nm blue
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Complementary additive colors

• Definition of complementary color (for additive
  mixtures)
• 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)

white
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Additive mixing of colored light primaries
Blue added to green cyan.
Green added to red yellow.
Red added to blue magenta.
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Complementary colored lights(additive mixing)
Blue (primary) and yellow.
Green (primary) and magenta.
Red (primary) and cyan.
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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

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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

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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 addingtwo colors in middle
also!
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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
  non-wavelength color

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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)
• Works because additive mixture of white with a
  fully-saturated (wavelength) color gives the
  desaturated color of the original point

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What is partitive mixing?

• Partitive mixing is another kind of additive
  color mixing 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

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A colored filter subtracts colors by absorption.

Incident white light
Only green gets through
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A colored filter subtracts certain colors by
absorption and transmits the rest
Incident white light
Only blue gets through
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A colored filter subtracts colors by absorption.

Incident white light
Only red gets through
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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
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Colored surfaces subtract certain colors by
absorbing them, while reflecting others
White in
White in
Green out
Magenta out
Magenta surface absorbs (subtracts) green.
Green surface absorbs (subtracts) red and blue
(magenta).
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Green light on a magenta surface appears
colorless because green is absorbed
Magenta light on a green surface appears
colorless because magenta is absorbed
Magenta in
Green in
No color
No color
Magenta surface absorbs (subtracts) green.
Green surface absorbs (subtracts) red and blue
(magenta).
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When looking at a colored object in a colored
light source what is the resulting color?
Cool white fluorescent bulb

• 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

Magenta shirt
this number
This number times
How the shirtappears in this light
equals this number
You multiply the two y-valuesat each x to get
the new curve
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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)

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Color halftoning
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.
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Demonstration

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Color Liquid Crystal Displays (LCDs)
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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

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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
  intensity-distribution
• 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,
  i-cones and L-cones (details)

1.66
460 nm blue of intensity 1
1
0
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How does a normal person see yellow when only red
and green lights are superimposed?
Light color Brightness S-cone response I-cone response L-cone response
530 nm green 1 negligible 41 28
650 nm red 2.15 negligible 2.15 x 2 2.15 x 9
Mixture (perceived as yellow ) negligible 41 2.15 x 2 45 28 2.15 x 9 47
575 nm yellow 1.35 negligible 1.35 x 33 45 1.35 x 35 47

• 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 L-cones 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

650 nm red of intensity 2.15
2.15
575 nm yellow of intensity 1.35
530 nm green of intensity 1
1
0
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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
Greenness blueness
Redness yellowness
Redness blueness
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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
r-g
Greenness blueness
Redness yellowness
y-b
Redness blueness
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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
i-cone
L-cone
neural cellfor y-b chromaticchannel
neural cellfor r-g chromaticchannel
Electrical signal to brain
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How can this "wiring" work to produce the
chromatic channels?

• The neural cell for the y-b chromatic channel
  has its signal
• 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
• enhanced when light excites the s and L
  coneINTERPRETED AS MAGENTA (Psychological red)
• The neural cell for the achromatic channel has
  its signal enhanced when light excites any of the
  cones

s-cone
i-cone
L-cone



?
?
neural cellfor y-b chromaticchannel
neural cellfor r-g chromaticchannel
neural cellfor w-blkachromaticchannel
Electrical signal to brain
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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

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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
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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
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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)
No change in signal (color not noticed)
No change in signal (color not noticed)
Note, you would still "see" green if the center
were grey!
Note, you would still "see" red if the center
were grey!
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y-b double-opponent receptive fields and cells
work the same way
Note, you would still "see" yellow if the center
were grey!
by-
Note, you would still "see" blue if the center
were grey!
yb-
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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)

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Color constancy depends on double-opponent
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 recognize colorful
  things in broad daylight, late afternoon, and
  early evening

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Illustration of how the three opponency channels
work in your perception of the design below

• Here are the enhanced edges resulting from your
  y-b chromatic channel
• Note the edges that separate a yellowish from a
  bluish color are enhanced the most
• Here are the enhanced edges resulting from your
  r-g chromatic channel
• Note the edges that separate a reddish from a
  greenish color are enhanced the most
• Here are the enhanced edges resulting from your
  wt-blk achromatic channel
• Compare with the way a photocopy machine would
  see the design

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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

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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

For same reason sun looks yellow (red green)
More atmosphere allows next shortest wavelengths
(green) to scatter so sunset looks red
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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 z-direction
  and polarized in the y-direction
• Here is a light ray traveling in the same
  direction but polarized in the x-direction
• 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

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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
• The many crossed double sided arrows are the
  symbol for unpolarized light
• See Physics 2000

electric force arrows jump around while remaining
perpen-dicular to the ray
http//www.colorado.edu/physics/2000/index.pl
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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 polariza-tions
• 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

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Some material from Chapter 8
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How do 3D movies use polaroid filters?