Physics of Light and Color. Lecture 26: Color and Light. What is Color? Sketch: Light Transport. Light Transport. Color of Light Source.

Transcription

1 Lecture 26: Color and Light Physics of Light and Color Light is electromagnetic radiation Different colors correspond to different wavelengths λ Intensity of each wavelength specified by amplitude Visible light: 4-7nm. range not in textbook (sad but true) V I B G Y O (ROYGBIV) R What is Color? Sketch: Light Transport Objects don t have a color Color is a perception; what we see It is a function of light source power at different wavelengths proportion of light at each wavelength reflected off object surface sensor to different wavelengths Photons travel in a straight line Light Transport Color of Light Source Illumination Spectral Power Distribution: Amplitude Relative amount of light energy at each wavelength Photons travel in a straight line Wavelength λ UV Visible IR

2 Some Light Source SPDs Light Transport (Ir)radiance IRRADIANCE RADIANCE Photons travel in a straight line Incoming light Outgoing light Surface Reflection surface Specular Surfaces Lambertian Surfaces Light rays purely reflect via Snell s law (angle of reflection = angle of incidence) Properties: Outgoing light has same SPD ( color ) as incoming light. If you stand in the right place you see a little picture of the light source reflected off the surface. Purely matte surface. Properties: Apparent brightness is proportional to cosine of angle between observer s line of sight and the surface normal (Lambert s Law) Outgoing light has SPD that depends on spectral albedo of surface (what wavelengths get absorbed vs transmitted). 2

3 More General Surfaces Have both a specular and diffuse reflections. Ratio of outgoing to incoming radiation at different wavelengths. (proportion of light reflected) Spectral albedo for several different leaves embedded colorant e.g. paint Spectral Light Transport to a Sensor Response Spectral Spectral Albedo Spectral Human Vision Human Eye: Rods and Cones Human eyes have 2 types of sensors: CONES Sensitive to colored light, but not very sensitive to dim light RODS (very) Sensitive to achromatic light near fovea near periphery rods (overall intensity) S cones (blue) M cones (green) L cones (red) 3

4 Putting it all Together = Color Simple Example Relative Spectral Power Distribution of White Light 4 7 albedo Spectral albedo of apple (red) cones Spectral COLOR! 6 7 continued Spectral 6 7 Simple Example (continued) (no ) (no ) relative numeric (area) = = = looks red Relative Spectral Power Distribution of Blue Light Simple Example albedo Spectral (no radiance) 4 7 Spectral albedo of apple (red) 6 7 continued Spectral (no radiance) 6 7 Simple Example (continued) relative numeric (no ) (no ) (no ) (area) = = = looks black The Abyss Clip One-way ticket clip from DVD 4

5 What is Going On in This Clip? Black/Yellow under Yellow Light Under ish green light, both the blue/white wire and the black/ wire look identical. black black Now for the spectral explanation of why this happens Blue/White under Yellow Light Lesson Learned black blue Surfaces materials that look different under white light can appear identical under colored light. white Metamers 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 Observer metamerism: two color distributions look the same to a given observer. Metameric curves for human eye under daylight Overcoming metamerism by viewing under different illumination Viewed under incandescent lighting Sample Metamers Viewed by camera with more sensitive red than human eye Test pattern viewed under daylight Overcoming metamerism by having a different observer From 5

6 Metamers Perception: Color Constancy To further explore observer metamerism, see the interactive metamer applet at: Humans are very good at recognizing the same material colors under different illumination. Not clear how this is achieved in the general case. Why is Color Constancy Hard? Color Blindness Normal color perception Want to factor this signal into these two components. Need prior knowledge! Red/Green color blindness 6