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

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

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

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)

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.

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.

Internal Light interactions, Up-close 1 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

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

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

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

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 -->

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 -->

Part of pigment table from Stout and Gettens: note info