Robert Collins Robert Collins CSE486, Penn State CSE486, Penn State Physics of Light and Color • Light is electromagnetic radiation Lecture 26: – Different colors correspond to different wavelengths λ – Intensity of each wavelength specified by amplitude Color and Light • Visible light: 400-700nm. range not in textbook (sad but true) V I B G Y O R (ROYGBIV) Robert Collins Robert Collins CSE486, Penn State What is Color? CSE486, Penn State Sketch: Light Transport Source emits photons • Objects don’t have a “color” And then some reach an eye/camera and • Color is a perception; what we “see” are measured. Photons travel in a • It is a function of straight line – light source power at different wavelengths – proportion of light at each wavelength reflected off object surface – sensor response to different wavelengths They hit an object. Some are absorbed, some bounce off in a new direction. Robert Collins Robert Collins CSE486, Penn State Light Transport CSE486, Penn State Color of Light Source Source emits photons Spectral Power Distribution: Relative amount of light And then some reach energy at each wavelength an eye/camera and Amplitude Illumination are measured. Photons travel in a Wavelength λ straight line UV Visible IR They hit an object. Some are absorbed, some bounce off in a new direction. 1 Robert Collins Robert Collins CSE486, Penn State Some Light Source SPDs CSE486, Penn State Robert Collins Robert Collins CSE486, Penn State Light Transport CSE486, Penn State (Ir)radiance Source emits photons And then some reach an eye/camera and are measured. IRRADIANCE RADIANCE Photons travel in a straight line Incoming Outgoing light light surface Surface Reflection They hit an object. Some are absorbed, some bounce off in a new direction. Robert Collins Robert Collins CSE486, Penn State Specular Surfaces CSE486, Penn State Lambertian Surfaces Light rays purely reflect via Snell’s Purely “matte” surface. law (angle of reflection = angle of incidence) Properties: Properties: Apparent brightness is proportional to cosine of angle between Outgoing light has same SPD observer’s line of sight and the (“color”) as incoming light. surface normal (Lambert’s Law) If you stand in the right place you Outgoing light has SPD that see a little picture of the light depends on spectral albedo of source reflected off the surface. surface (what wavelengths get absorbed vs transmitted). 2 Robert Collins Robert Collins CSE486, Penn State More General Surfaces CSE486, Penn State Spectral Albedo Ratio of outgoing to incoming radiation at different wavelengths. Have both a specular and diffuse reflections. (proportion of light reflected) Spectral albedo for several different leaves embedded colorant e.g. paint Robert Collins Robert Collins CSE486, Penn State Spectral Radiance CSE486, Penn State Light Transport Source emits photons And then some reach an eye/camera and are measured. to a Sensor Response Spectral Spectral Spectral Irradiance Albedo Radiance They hit an object. Some are absorbed, some bounce off in a new direction. Robert Collins Robert Collins CSE486, Penn State Human Vision CSE486, Penn State Human Eye: Rods and Cones • Human eyes have 2 types of sensors: near fovea – CONES • Sensitive to colored light, but not very sensitive to dim light – RODS • (very) Sensitive to achromatic light rods (overall intensity) near periphery S cones (blue) M cones (green) L cones (red) 3 Robert Collins Robert Collins CSE486, Penn State Putting it all Together = Color CSE486, Penn State Simple Example Relative Spectral Power Spectral albedo Distribution of White Light of apple (red) 1 1 .* albedo 0 0 400 700 600 700 Relative power Wavelength (nm) Wavelength (nm) 3 cones Spectral Radiance 1 COLOR! 0 continued 600 700 Relative power Wavelength (nm) Robert Collins Robert Collins CSE486, Penn State Simple Example (continued) CSE486, Penn State Simple Example relative numeric Red Cones Red Cones response (area) Relative Spectral Power 1 1 Spectral albedo Distribution of Blue Light of apple (red) .* =100 1 1 0 0 response Relative response 400 500 600 700 400 500 600 700 Wavelength (nm) Wavelength (nm) Spectral Radiance .* albedo Green Cones Green Cones 0 0 1 400 500 700 .* 1 1 600 700 = 0 Relative power Wavelength (nm) Wavelength (nm) 0 (no response) 600 700 0 0 response Relative response Relative power Wavelength (nm) 400 500 600 700 400 500 600 700 Wavelength (nm) Wavelength (nm) Spectral Radiance Blue Cones Blue Cones 1 1 1 .* = 0 (no response) (no radiance) 0 continued 0 0 looks response 400 700 Relative response 400 500 600 700 400 500 600 700 Wavelength (nm) Wavelength (nm) “red” Relative power Wavelength (nm) Robert Collins Robert Collins CSE486, Penn State Simple Example (continued) CSE486, Penn State The Abyss Clip relative numeric Red Cones Red Cones response (area) 1 1 .* = 0 (no response) 0 0 “One-way ticket” clip from DVD response Relative response 400 500 600 700 400 500 600 700 Wavelength (nm) Wavelength (nm) Spectral Radiance Green Cones Green Cones 1 .* 1 1 = 0 (no radiance) 0 (no response) 600 700 0 0 response Relative response Relative power Wavelength (nm) 400 500 600 700 400 500 600 700 Wavelength (nm) Wavelength (nm) Blue Cones Blue Cones 1 1 .* = 0 (no response) 0 0 looks response Relative response 400 500 600 700 400 500 600 700 Wavelength (nm) Wavelength (nm) “black” 4 Robert Collins Robert Collins CSE486, Penn State What is Going On in This Clip? CSE486, Penn State Black/Yellow under Yellow Light yellow .* black 0 Under yellowish green light, Irradiance Spectral Albedo both the blue/white wire and the black/yellow wire look identical. black Radiance Now for the spectral explanation yellow .* yellow of why this happens… Irradiance Spectral Albedo yellow Radiance Robert Collins Robert Collins CSE486, Penn State Blue/White under Yellow Light CSE486, Penn State Lesson Learned yellow .* blue 0 Irradiance Spectral Albedo Surfaces materials that look different under white light can appear identical under colored light. black Radiance yellow .* white Irradiance Spectral Albedo yellow Radiance Robert Collins Robert Collins CSE486, Penn State Metamers CSE486, Penn State Sample Metamers Metameric curves for human eye under daylight Test pattern viewed under daylight Definition: two different spectral reflectances that appear indistinguishable to a given observer under given illumination conditions. Illumination metamerism: two color distributions look the same under a given illumination Overcoming metamerism by Overcoming metamerism by viewing having a different observer Observer metamerism: two color distributions look the under different illumination same to a given observer. Viewed by camera Viewed under with more sensitive incandescent red response than lighting human eye From http://www.iitp.ru/projects/posters/meta/ 5 Robert Collins Robert Collins CSE486, Penn State Metamers CSE486, Penn State Perception: Color Constancy To further explore observer metamerism, see the Humans are very good at recognizing the same material interactive metamer applet at: colors under different illumination. Not clear how this http://www.cs.brown.edu/exploratories/freeSoftware/catalogs/color_theory.html is achieved in the general case. Robert Collins Robert Collins CSE486, Penn State Why is Color Constancy Hard? CSE486, Penn State Color Blindness Normal color perception Red/Green color blindness Want to factor this signal into these two components. Need prior knowledge! 6.