Light Interactions with Materials  Light – has wavelength matched to an energy (short wavelengths are higher energy)

Can be absorbed or transmitted Can be reflected or refracted

These interactions have BIG effect on appearance of painting   Varnish layer: more internal reflectances, more color absorption, more intense hues. Glazes: subtractive color mixing AND depth / rich colors (as above) Light penetrating deeper produces more saturated hues

All because of Refraction of Light Refraction through calcite crystal

  

Air, nD ~ 1.00

crystal, nD > 1.00

Quartz 1.55ave malachite 1.81ave

Air, nD ~ 1.00- Higher index of refraction in crystal,- Slower light velocity through crystal,- Causes ray to “bend”,- Emerges into air with original direction

Light Reflection and Refraction: How it affects the painting appearance

Refracted ray

Reflected ray

Original ray if no refraction occurred

Light through oil with red pigment particles on white ground : some light is reflected back out

Light through oil with red pigment particles on black ground : all light is absorbed. None is reflected back out

How the ground makes a difference

reflected specular light --> Eye sees White

reflected diffuse light without absorption--> Eye sees White

reflected light off pigment with aabsorption --> Eye sees Red (color)

Actually, it’s more complicated than that...

reflected ray: specular reflection

refracted ray: diffuse reflection

Large difference in index of refraction, n = 1.5causes mainly diffuse reflections from surface and little penetration of light into paint. Produces less saturated colors.

The Effect of a Varnish Layer

Air, nD ~ 1.00

Paint layer, nD ~ 2.5

Smaller difference in index of refraction, n = 1.0causes increased light penetration into paint and more refraction and absorption by pigment.Produces deep, rich colors.

Air, nD ~ 1.00

Paint layer, nD ~ 2.5

Varnish, nD ~ 1.50

Varnish layer,

Paint layer

Multiple reflections within varnish layer,Allow light to be absorbed by pigmentCauses more saturated colors.

Reflected rays after absorption by pigment

Internal Reflections in Varnish Layer

Oil with red pigment particles

How greater penetration of lightinto oil layer with pigment particlesincreases saturation.

1

4 5 62 3

400 500 600 700

Rays 5 & 6: absorbed by two particles;strong red reflection:red

Rays 3 & 4: absorbed by one particle; weak red reflection --> pink

Rays 1 & 2: no absorption by pigment; reflects all light -->white

Internal Light interactions,Up-close

Thick layer of paint covered by thin varnish lets

little light through, looks flat

Combination of internal reflections in varnish and glaze layers gives depth.

Van Eyck

How Van Eyck differs from the others

varnish

second glaze

first glaze

thin underpaintingchalk ground

oak panel

It’s all about the Index of Refraction

Cubic crystal Tetragonalcrystal

Rhombiccrystal

Non-symmetric in all directionsMore than one nD value

Anisotropic

Symmetric in all directionsnD same in all directions

Isotropic

Iso = sameTropic = rotate

a

b

c

a

b

c

a = b = c

a = b ≠ c

a ≠ b ≠ c

c

a

b

c

n , ntwo values

n , n, nthree values

Isotropic Anisotropic

Light Refraction: How it reveals a pigment’s identity

polarized light

non- polarized light

Check out result of polarizers……

It’s all about the Index of Refraction and Polarized Light

Cubic crystal

Symmetric in all directionsnD same in all directions

a = b = c

Isotropic

YourEyes seeblack!

No lightemerges fromanalyzer

polarizer analyzer

withparallelpolarizers,you see -->

withcrossedpolarizers,you see -->

entering light emerging light

It’s all about the Index of Refraction and Polarized Light

Tetragonal or rhombic crystal

nD different in at least two directions

Anisotropic

You seeglowingcrystalsagainstblack!

polarizer analyzer

c

a

b

c

withparallelpolarizers,you see -->

withcrossedpolarizers,you see -->

entering light emerging light

Part of pigment table from Stout and Gettens: note info