Lab 13: Subtractive Color: Filters & Paint

• Experiments with Filters:• Color Samples & Paints (“Pigments”)• Mixing Paint• Spectra of Light Sources

• Additive primary colors: Red, Green and Blue (RGB)• Subtractive primary colors: Cyan, Magenta and Yellow (CMY)

Inte

nsity

400 500 600

Wavelength (nm)700

Graphs are based on RGB

B G R

λ(nm) 400 500 600 700

Absorption of light

Magenta filter absorbs green light

Subtractive material

White LightBGR

MB

R

White Light

Screen

Filter

Additive perception

Filters (Pigments)• Absorb their complementary hue

• Transmit (reflect) their own hue

• Transmittance (reflectance)– Fraction of incident light passed through (reflected back)

B G R

Y M C

Tra

nsm

ittan

ce

1

0

Ideal Magenta filter

Ref

lect

ance

B G R

Y M C1

0

Ideal Yellow pigments

Real FiltersReal filters transmit a fraction of incident light

60 %

B G R

Inte

nsity

100

0

B

Y

G

M

R

C

Tra

nsm

ittan

ce

1

0

B G R

Inte

nsity 100

0

100 %

0.6

0

0.6

TM

R (600 – 700 nm)

G (500 – 600 nm)

B (400 – 500 nm)

Spectral Ranges

Caution

• Probe– Misaligned probe ask your TA to reposition– Polished probe tip do not scratch (it's a $3000 research grade device!)

• Plastic Filters– Wrinkle when heated under the halogen lamp– Set – Freeze – Remove

• Place the filter on the filter holder• Freeze the screen using “snapshot ” feature• Remove the filter

• Brushes– Wash them thoroughly for next section

• Spills?– There paper towels on the sink counter

Paint

B G R

Y M C

1 part

Ref

lect

ance 1

0

+ 2 parts

B G R

Y M C

Ref

lect

ance 1

0

=

B

Y

G

M

R

C

Re

flect

an

ce

1

0

Use ideal paint reflectance,NOT from spectrophotometer!

Now find hue using color triangle