1. 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

2. Light Reflection and Refraction: How it affects the painting appearance
Reflected ray
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
Refracted ray
Original ray if no refraction occurred

3. How the ground makes a difference
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

4. Actually, it’s more complicated than that...
reflected ray: specular reflection
refracted ray: diffuse reflection
reflected specular light --> Eye sees White
reflected diffuse light without absorption--> Eye sees White
reflected light off pigment with aabsorption --> Eye sees Red (color)

5. The Effect of a Varnish Layer
Air, nD ~ 1.00
Air, nD ~ 1.00
Varnish, nD ~ 1.50
Paint layer, nD ~ 2.5
Paint layer, nD ~ 2.5
Large difference in index of refraction, Dn = 1.5
causes mainly diffuse reflections from surface and little penetration of light into paint.
Produces less saturated colors.
Smaller difference in index of refraction, Dn = 1.0
causes increased light penetration into paint and more refraction and absorption by pigment.
Produces deep, rich colors.

6. Internal Reflections in Varnish Layer
Reflected rays after absorption by pigment
Varnish layer,
Paint layer
Multiple reflections within varnish layer,
Allow light to be absorbed by pigment
Causes more saturated colors.

7. Internal Light interactions, Up-close1
Internal Light interactions,
Up-close 2 5 6 3 4
Rays 1 & 2: no absorption by pigment;
reflects all light -->white
Oil with red pigment particles
Rays 3 & 4: absorbed by one particle;
weak red reflection --> pink
How greater penetration of light
into oil layer with pigment particles
increases saturation.
Rays 5 & 6: absorbed by two particles;
strong red reflection:red
400
500
600
700

8. How Van Eyck differs from the others
Thick layer of paint covered by thin varnish lets
little light through, looks flat
Van Eyck
Combination of internal reflections in varnish and glaze layers gives depth.
varnish
second glaze
first glaze
thin underpainting
chalk ground
oak panel

9. It’s all about the Index of Refraction
Light Refraction: How it reveals a pigment’s identity
It’s all about the Index of Refraction
Isotropic
Anisotropic
Cubic crystal
Tetragonal
crystal
Rhombic
crystal a a c a b b b c c
a = b = c
a ≠ b ≠ c
a = b ≠ c
Symmetric in all directions
nD same in all directions
Isotropic
Iso = same
Tropic = rotate
na , nb , ng
three values
ne , nw
two values
Non-symmetric in all directions
More than one nD value
Anisotropic

10. polarized light
non- polarized light
Check out result of polarizers……

11. It’s all about the Index of Refraction and Polarized Light
Isotropic
Cubic crystal
a = b = c
Symmetric in all directions
nD same in all directions
Your
Eyes see
black!
No light
emerges
from
analyzer
entering light
emerging light
polarizer
analyzer
with
parallel
polarizers,
you see -->
with
crossed
polarizers,
you see -->

12. It’s all about the Index of Refraction and Polarized Light
Anisotropic
Tetragonal or rhombic crystal
nD different in at least two directions c a b
You see
glowing
crystals
against
black!
entering light
emerging light
polarizer
analyzer
with
parallel
polarizers,
you see -->
with
crossed
polarizers,
you see -->

13. Part of pigment table from Stout and Gettens: note info