Chapter 27 Color

Apr 22, 2023 Physics 1 (Garcia) SJSU

Chapter 27Color


Spectrum of Visible Light


Wavelengths & Photons

Red Photon

Blue Photon

Green Photon

Yellow Photon

PrismSpectrum

Particles of light, called photons, each have a wavelength.


Newton’s Color WheelPrism spectrum is a straight line, so why did Isaac Newton describe color using a circular wheel?


Additive Color Wheel

SpectralColors

Not in the spectrum. How do we see these?

R

Y

B G

M

C

RedYellowGreenCyanBlueMagenta


Adding Colored Lights

Stream ofred & green photons looks same as yellow photons(metamerism)

Theatrical lighting

or

YELLOW

Eye toBrain

Notice overlap of red, green, & blue is seen as whitewhite light


Demo: Seeing YellowSodium lamps emit yellow photons

Color monitor can only emit red, green, and blue (RGB); creates other colors by selectively turning RGB pixels on or off.


Trichromatic Theorists

Thomas Young (1773-1829) English physicist

Hermann von Helmholtz (1821-1894)German physicist

James Clerk Maxwell(1831-1879)Scottish physicist


Simple Trichromatic TheoryYellow &

Redphotons

excite me

Yellow, Green &

Cyanphotons

excite me

Cyan &Blue

photons excite me

Inside your eye there are three receptors

MOECURLY

LARRY


Trichromatic: Seeing YellowYellow &

Redphotons

excite me. I’M

EXCITED

Yellow, Green &Cyan photons

excite me.I’M EXCITED

Cyan & Bluephotons excite

me.Yawn.

Yellow seen when Curly and Larry excited, either by yellow photons or red & green photons.

MOE

CURLYCURLY

LARRY


Trichromatic: Seeing MagentaYellow &

Redphotons

excite me. I’M

EXCITED


excite me.Yawn.

Cyan & Bluephotons excite me.

I’M EXCITED

Magenta is seen by eye when Moe and Larry excited, which no single type of photon can achieve.

MOE

CURLYCURLY

LARRY


Demo: Maxwell Color DiskDisk painted half red, half blue looks

magenta when rapidly spinning.


Mixing Blue & Red Paint

Mixing paint or ink is different from adding colors together by light.

Mix of blue and red paint produces a blackish brown


Trichromatic: Seeing WhiteWhiteYellow &

Redphotons

excite me. I’M

EXCITED




I’M EXCITED

White seen when all three very excitedGray seen when all three less excited

MOE

CURLYCURLY

LARRY


Trichromatic: Color BlindnessRed , Yellow,

Green &Cyan photons



Yawn.

MOE

CURLYCURLY

LARRY

Color blindness occurs if the eye is missing one of the three receptors. Other receptors try to compensate but cannot distinguish some colors.

Do I see red or green?


Color Blindness

ClassificationIncidence (%)

Males FemalesAnomalous

Trichromacy 6.3 0.37

Protanomaly(Red-cone weak) 1.3 0.02

Deuteranomaly(Green-cone weak) 5.0 0.35

Tritanomaly(Blue-cone weak) 0.0001 0.0001

Dichromacy 2.4 0.03 Protanopia

(Red-cone absent) 1.3 0.02

Deuteranopia(Green-cone absent) 1.2 0.01

Tritanopia(Blue-cone absent) 0.001 0.03

Rod Monochromacy(no cones) 0.00001 0.00001

29 or 70? 21 or 74?

Weakness or absence of one of the three types of cones is the cause of color blindness, leading to a reduced ability to distinguish colors.


Trichromatic: After-Image

First stare at RED

I’M EXCITED!Yawn.

Yawn.

MOECURLYCURLY LARRY

Me too, but tired.

ME TOO!

I’M EXCITED!

MOECURLYCURLY LARRY

Then stare at WHITE

Moe and Curly are excited so what color is seen?CYAN (light blue-green)


Negative After-image

Stare, unfocused, at the red cross for 10 seconds then look at white wall



Cyan





Cyan Magenta Yellow


Simple Additive Color WheelLARRY

MOE CURLY

Newton’s color wheel is a simple formulation of trichromatic theory.White


Color Vision in the Eye

Three types of cones (color)One type of rod (B/W only)


CIE Hue-Saturation Diagram

Eye is not a perfectoptical instrument.

Color “wheel” isactually distortedcone shape.

Rim is full saturation,center is white

LARRY %

CU

RLY

%

50% Larry50% Curly0% Moe

33% Larry33% Curly33% Moe


Spectral Reflectance Curves

When white light shines on an object, some photons absorbed, others reflected by the object’s surface.

White Red


Name That Color

White Red Dark Brown(Burnt Sienna)

BLUE GREEN RED

0%

100% BLUE GREEN RED BLUE GREEN RED


Name That Color

Bluish Green Bright Yellow Blue(Cobalt)

BLUE GREEN RED

0%

100% BLUE GREEN RED BLUE GREEN RED


Subtractive Color

Absorbing filters or absorbing pigments remove colors from white light.

Check Yourself:Yellow = Red + GreenCyan = Blue + GreenYellow & Cyan filters (or pigments)combine to give … Overlapping color filters


Check Yourself

A soap film absorbs red, green, or blue light, depending on the film’s thickness.

What colors do we see where red is removed?

Where green is removed?

Where blue is removed?

http://upload.wikimedia.org/wikipedia/commons/e/e6/Soap_Bubble_-_foliage_background_-_iridescent_colours_-_Traquair_040801.jpg


Four Color Printing

Magenta Yellow Cyan Black

Magenta+ Yellow+ Cyan

All Four

Green balloon printed with mix of yellow and cyan inks


Check Yourself

Magenta Yellow Cyan Black

Which inks are mixed for the bright red balloon and dark red dress?

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Chapter 27 Color