From light to Enlightenment

From light to Enlightenment The physical layer • origin and nature of light • light as particles • light as waves • light as energy • the illumination equation • absorption and scattering; color perspective • the spatial behavior of light • refraction and lenses • the perspective projection • summary Kees van Overveld -1-

electron in excited state electron falls back to lower energy state; energy surplus E is emitted as light with wavelength =hc/E available states occupied states From light to Enlightenment – the physic of light origin and nature of light • Radiation of 'black(=non-reflecting)' body: a continuous distribution of energy over , only depending on the temperature of the body • Energy transitions in atoms between discrete electron states cause light-quanta (photons) with distinct  Kees van Overveld -2-

v optical axis l b retina plane u pupil plane From light to Enlightenment – the physic of light the illumination equation d2R P ri i d2S Kees van Overveld -3-

From light to Enlightenment – the physic of light the illumination equation d2Eretina=P cos i cos4l AP d2R / 32 3ri2b2 cos u d2R v P optical axis l ri b retina plane u pupil plane i d2S Kees van Overveld -4-

From light to Enlightenment – the physic of light properties of the illumination equation perceived light intensity: proportional to cos4l : at 45 degrees, mere 25% left interpretation of : difference between dull and shiny (among other things) proportional toi : plasticity  interpreted as relief the case where u =i : full moon proportional to 1/cos u : bright silhouettes no v-dependency r – dependency: clair obscur or impressionism the role of the pupil size,AP no symmetry between source and detector Kees van Overveld -5-

From light to Enlightenment – the physic of light proportional to cos4l difficult to get uniform sensitivity for wide viewing angles (fish-eye lenses; endoscopes) Kees van Overveld -6-

From light to Enlightenment – the physic of light interpretation of  simple empiric bahavior of  (Phong-shading: computer graphics (1973)) = • =cos(angle between normal vector and halfway-direction) halfway-direction = direction between incoming and reflected ray • =1: i=u (condition for symmetrical reflection)  <1: iu (condition for symmetrical condition doesn't hold)  = 0: dull (Lambert surface)  = : perfect (Snellius) mirror Kees van Overveld -7-

local variations in shininess are caused by the behavior of  in dependence of reflection angles color differences in a surface are often caused by varying spectral dependencies of   may not only depend on the angle between incoming and outging light ray and the surface normal, but also on their directions: anisotropic reflection From light to Enlightenment – the physic of light interpretation of  demo Kees van Overveld -8-

From light to Enlightenment – the physic of light proportional toi if possible, the HVS gives an interpretation to brightness differences in terms of variations of i , and hence as relief (height modulations) Kees van Overveld -9-

From light to Enlightenment – the physic of light the case where u =i in every point of the full moon, the viewing direction and the direction of the incoming rays are (almost) equal. A Lambertian-surface then gives uniform brightness. Kees van Overveld -10-

From light to Enlightenment – the physic of light proportional to 1/cos u there are brighter zones near silhouettes of shiny surfaces. Difficult to perceive: • high shinyness: reflection of the surrounding world interferes • low shinyness: (u) is close to zero near the silhouette Kees van Overveld -11-

A single point source: light field is dominated by 1/r2 behavior: dramatic clair-obscur, characteristic of 17th centure indoor scenes (Rembrandt, Caravaggio). Homogenous distribution of point sources (e.g. due to atmospheric scattering): outdoor light gives no clair obscur. Characteristic of many impressionistic landscapes. From light to Enlightenment – the physic of light dependency of 1/r2 Kees van Overveld -12-

if the illumination equation would behave symmetrically in r and v, remote surfaces would appear darker remote and nearby surfaces with equal  and equal orientation w.r.t. light source, however, apear equally bright From light to Enlightenment – the physic of light no dependency of 1/v2 Kees van Overveld -13-

small pupil: • lower intensity • higher aquity • sharp over large depth range large pupil: • higher intensity • lower aquity • sharpness drops sharply over limited depth range From light to Enlightenment – the physic of light the size of the pupil Kees van Overveld -14-

From light to Enlightenment – the physic of light absorption and scattering Absorption: if a layer of thickness h absorbs a fraction K (K=K(); K<1) of the light intensity, intensity becomes a function of propagation distance x: L (x)= L 0 exp (-Kx/h) Scattering: Einstein gave a derivation of the empirical Tyndall formula:: Lscatterred () = L 0 -4 Kees van Overveld -15-

From light to Enlightenment – the physic of light absorption and scattering Kees van Overveld -16-

From light to Enlightenment – the physic of light absorption and scattering Leonardo da Vinci’s ‘The virgin on the rocks' is an early example of the deliberate use of atmospheric perspective in pictorial art Kees van Overveld -17-

From light to Enlightenment – the physic of light perspective Geometric properties of light rays: • conservation of direction • eventually any non-parallel beam will diverge • mapping a point in 3-space onto a point in a (2D) image is a central projection (i.e., a projection whereby projecting rays all pass through a so-called projecting centre) Kees van Overveld -18-

From light to Enlightenment – the physic of light perspectief By why is there a pojection centre? Kees van Overveld -19-

From light to Enlightenment – the physic of light perspective Properties of central projection Classical perspectivef: (Italian renaissance, Brunelleschi (1377-1446)): • horizon, • lines  lines, • points  points, hence: • intersections  intersections Kees van Overveld -20-

From light to Enlightenment – the physic of light perspective The development in perspective in pictorial art Egyptian art has used more or less the same style for 30 centuries; no need for rendering of geometric perspective Kees van Overveld -21-

From light to Enlightenment – the physic of light perspective The development in perspective in pictorial art Classical Greek art had partial understanding of projection and the geometry of 3D (Euclid) – in particular of ‘things’ that were small enough so that no visible size reduction occurs (=so called isometric perspective: parallel lines stay parallel) http://www.ottobw.dds.nl/filosofie/perspect.htm Kees van Overveld -22-

From light to Enlightenment – the physic of light perspective The development in perspective in pictorial art Byzantine pictorial art: inverted’ perspective for religious reasons Kees van Overveld -23-

From light to Enlightenment – the physic of light perspectief The development in perspective in pictorial art master of Flemalle (Merode altaarstuk – ca. 1427) Late Gothic art: Limited success in depicting geometric perspective. The principle “one painting = one viewpoint”had not yet been discovered Kees van Overveld -24-

From light to Enlightenment – the physic of light perspectief The development in perspective in pictorial art Break through: Massacio applied single (vanishing-) point perspective (early Italian Renaissance). The concept worked in virtue of the known location and gazing direction of the viewer http://en.wikipedia.org/wiki/Holy_Trinity_(Masaccio) Kees van Overveld -25-

From light to Enlightenment – the physic of light perspectief The development in perspective in pictorial art Full control of multi-vanishing point perspective of Dutch masters in 17th century (pioneered by Simon stevin, although theoretical underpinning had to wait until the 19th century: Gaspard Monge, projective geometry). http://en.wikipedia.org/wiki/Johannes_Vermeer Kees van Overveld -26-

From light to Enlightenment – the physic of light perspectief http://mrl.nyu.edu/projects/npr/mpr/ The development in perspective in pictorial art .. But is photo realistic perspective convincing? Sometimes the eye wants to be deceived Kees van Overveld -27-

From light to Enlightenment – the physic of light perspectief The development in perspective in pictorial art Early 20th century: cubism - dropping the assumption of a single view point per painting Shifting the responsibility from the painter to the viewer Looking = sampling, i.e. a dynamic, attention-driven process http://burgessart.files.wordpress.com/2012/09/picasso3.jpg Kees van Overveld -28-

From light to Enlightenment – the physic of light perspectief The development in perspective in pictorial art The Chirico (and others) deliberately used ‘wrong’ perspective for pictorial purposes, manipulating the view and creating an eery, dream-like atmosphere http://uima.uiowa.edu/giorgio-de-chirico/ Kees van Overveld -29-

From light to Enlightenment – the physic of light perspectief The development in perspective in pictorial art Inverting perspective: creating a pictorial illusion on a 3-D background (Julian Beaver, England) http://www.s-anand.net/blog/pavement-drawings-in-perspective/ Kees van Overveld -30-

From light to Enlightenment – the physic of light Summary; essential concepts: • Point source: centre where speherical waves originate; 1/r2 behavior of intensity relative to the point source • Power: energy per time interval • Intensity: light power per surface area • Radiance: transported light power per surface area per solid angle • Irradiance: received or emitted light power per surface area • Spectrum: distribution of light energy over the wavelengths (continuous or discrete) Kees van Overveld -31-

From light to Enlightenment – the physic of light • Reflection: interaction of light with a surface • Diffuse reflection (Lambert): BDR is more or less constant • Mirroring: reflection where BDR only differs from 0 when incoming and outgoing rays have about the same angel with normal • Scattering: interaction of light with a spatial medium where light rays no longer are straight lines • Dispersion : velocity of light, and therefore refraction index varies in dependence of wavelength • Diffraction and interference: deviation from the straight line-behavior of light rays due to their wave character • Absortpion: decrease of light power due to reflection or passage through a medium Kees van Overveld -32-

From light to Enlightenment – the physic of light • Pupil: centre of perspective projection, where all light rays have to pass • Collimator: enforces light rays from different directions to fall onto different sensory cells • Perspective: transformation from 3D to 2D where distances are represented by angles • Straight lines and points stay straight lines and points due to perspective • Parallellism in 3D: coïncidence in perspective image • Distances and angles are not preserved • Vanishing point: limiting case for the projection of a point that, in 3D, moves along a straight line towards infinity • Horizon: collection of vanishing points of all 3D directions parallel to the ground plane Kees van Overveld -33-

From light to Enlightenment