Sight and the Power of Visual Culture

Sight and Visual Perception

The Eye and Sight

• Sight, or visual perception, is a complex process that begins with light reflecting off an object and entering the eye.

• Cornea: The transparent outer layer of the eye that bends the incoming light.

• Pupil: The hole through which light passes.

• Iris: The colored part of the eye that functions like a camera's diaphragm, controlling the amount of light entering the eye by retracting or opening wider.

• Lens: Focuses the light onto the retina.

• Retina: Contains nerve cells shaped like rods and cones.

  • Cones: Translate light into colors, central vision, and details.

  • Rods: Translate light into peripheral vision and motion; enable vision in limited light conditions.

• The translated information is sent as electrical impulses to the brain through the optic nerve.

Compensation for Sight Loss

• Individuals without sight may develop enhanced hearing, taste, touch, and smell.

• Memory and language skills may also be improved in the visually impaired.

• The brain can rewire itself to effectively interact with the environment using available information.

Light, Color Reception, and Perception

• Light and color reception/perception is a three-stage process:

  1. Light Source Color Temperature: The radiation spectrum of the light source used (warm or cold light).

  2. Spectral Filters: How the illuminated object reflects, absorbs, or filters light.

  3. Resolving Detector: The observer's eye or camera sensors.

• Observer metamerism refers to the fact that each observer may perceive colors differently.

Color as a Physiological Response

• Color is not a physical property but a physiological response created in the brain by visible light.

• Color is subjective and depends on individual perception.

Color Vision

• Color vision is provided by cones, which are less sensitive but fewer in number than rods.

• Three types of color-sensitive cones roughly correspond to red, green, and blue.

• Color perception depends on the neural response of these three cone types.

• Visible color can be mapped using the response functions of these three types of cones.

• Color samples can be matched by combinations of monochromatic colors: red (700 nm), green (546.1 nm), and blue (435.8 nm).

• Allowing negative values of red enables matching of all colors, equivalent to adding red to the color sample.

Three-Stage Process of Color and Light Reception:

1. Color Temperature of Light: The radiation spectrum of the light used, influencing the perceived color of objects.

2. Spectral Filters: Reflecting, absorbing, or filtering of radiation by the illuminated object, affecting its perceived color.

3. Resolving Detector: The eye or camera sensors, with individual differences in perception (Observer Metamerism).

Color Temperature

• Understanding color temperature begins with black body radiation and the Kelvin temperature scale.

• A theoretical black body absorbs all incident radiation and emits none at 0° Kelvin.

• $0°K = -273.16° C$ (absolute zero, where all molecular motion ceases).

• As a black body is heated above 0°K, it emits radiation; the spectral distribution depends on its temperature.

• Visible radiation requires the black body to be quite hot.

  • Approximately 1000K: red

  • Approximately 1500K: yellow

  • Approximately 5500K: white

  • Approximately 6500K: bluish-white

  • Approximately 10000K: more bluish

• Black body radiation laws:

  • Planck's law: Gives spectral irradiance at different wavelengths.

  • Wien's law: Provides the wavelength at which peak irradiance occurs.

  • Stephan Boltzman's law: Relates total energy to black body temperature.

Correlated Color Temperature (CCT)

• Lamps are not black body radiators.

• To assign a color temperature to a light source, there must be both color and spectral match to a black body radiator.

• Correlated Color Temperature (CCT) is the temperature of the black body radiator when its color matches that of the light source.

• CCT implies a color match but not a spectral match to a black body at the specified temperature.

Color Specification

• Color is specified by numerical values called color coordinates or chromaticity.

• CCT can be determined by mathematical formulas to find the chromaticity coordinates of black body color temperatures closest to the light source’s chromaticity.

• CCT is a shorthand for reporting the color appearance of light emitted from electric light sources.

• Higher CCT values indicate light that appears “more blue.”

• The International Lighting Commission (CIE) established a colorimetry system for color matching.

Color Vision and Receptors

• Rods are more sensitive but do not detect color.

• Color vision is provided by three types of cones (red, green, and blue).

• Perception of color depends on the neural response of these cone types.

• Color samples can be matched by combinations of monochromatic colors (red, green, blue).

• Negative values of red match all colors.

CIE Chromaticity Diagram

• CIE matching functions derive from Red, Green, and Blue matching functions.

• These functions ensure all positive matching functions, and any color mixes from the three CIE primaries.

• CIE primaries can be shown as mathematical functions of their wavelength.

• CIE color coordinates are derived by weighting the spectral power distribution.

• Tristimulus values (X, Y, Z) are obtained. Y value measures luminosity/brightness.

• Chromaticity coordinates (x, y) are normalized coordinates: $x = X/(X+Y+Z)$, $y = Y/(X+Y+Z)$, and $x+y+z = 1$.

• 1931 CIE chromaticity diagram shows the color temperature of a true black body.

• The black body color temperature path is called the black body locus.

Limitations and Uniformity

• Two points on either side of the black body locus can have the same CCT but different color coordinates.

• A tolerance zone is specified near the black body locus to limit color differences.

• The 1976 uniform chromaticity CIE chart provides uniform color spacing for colors at the same luminance.

• Coordinates used are denoted (u',v') and computed from 1931 x,y coordinates by the following transformation:

  • $u'= 4x / (-2x + 12y + 3)$

  • $v'= 9y / (-2x + 12y + 3)$

Psychophysics and Standard Observer

• Psychophysics transforms physical stimuli into psychological responses (detection, identification, discrimination, and scaling).

• A standard observer is a theoretical human visual system described using equations relating quantity to measurable statistics of light stimuli.

Luminance and Brightness

• Luminance expresses that lights with equal power and different wavelengths do not appear equally bright.

• Brightness varies with wavelength, peaking at approximately 555 nm.

• For graphics resembling surface colors (e.g., colored/printed paper), lightness is the key variable.

Psychophysics Concepts

• Absolute threshold: The point at which a stimulus is first detected as it increases in strength.

• Signal detection theory: The interaction of sensory capabilities and decision-making in stimulus detection.

• Difference thresholds: The point at which two stimuli can be differentiated (just-noticeable difference).

• Scaling: Assigning relative values to sensory experiences.

Luminance and Spectral Dependence

• Luminance is a measurement expressing variable brightness of equal-power lights at different wavelengths.

• Even if lights of different wavelengths are equal in power, the visual system is not equally sensitive to them.

• For photopic vision, peak brightness is around 555 nm.

• Photopic luminance defined as: where P is spectral power and V is the photopic spectral sensitivity of the standard observer.

CIE and Lightness

• L* is the lightness variable of the CIE Luv* system, scaling from black to white for achromatic patches.

• Value is the lightness variable of the Munsell color order system, also scaling from black to white.

• Formula for L*

  • $L^*= 116(Y/Y_n)^{1/3}-16$ where Y is the luminance of the patch and Yn is perfect diffuser luminance

• Formula for Value

  • $Value = <br>Y/Y<em>n$ where Y is the luminance of the patch and $Y</em>n$ is the luminance of a perfect diffuse reflector (ideal white patch).

Color Matching

• Color matching involves adjusting the intensity of component sources until the matching light equals the test light.

• Standard human observers can match any test light with a mixture of three sources.

Chromaticity Diagram and Color Gamuts

• Chromaticity diagrams show 2 out of 3 colour dimensions the third being luminance

• Chromaticity diagrams show two dimensions of color, with luminance as the third dimension.

• Mixtures of two lights lie on the straight line connecting their chromaticities.

• The (xy) gamut of a monitor is the triangle connecting the primaries.

Luminance and Chromaticity Limitations

• Their mathematical properties are derived from assumptions.

• Limited by

  • Size and Shape

  • Edges and uniformity

  • Surrounding areas, previously viewed areas

  • Observer characteristics

Size, Shape, Edges and Observer Variations.

Strictly speaking, chromaticities indicate which lights will match for the standard observer under the exact laboratory conditions that generated the data on which the system is based, given further assumptions about the mathematical properties of human color vision.

• The 1931 CIE System is based on matches for patches subtending two degrees of visual angle, or about 1.75 cm viewed from 50 cm (or 0.7 inches viewed from 20 inches).

• For larger areas there is a second CIE system, the CIE 1964 System, based on matching with 10 degree patches

Colorimetry

• Colorimetry measures the tristimulus values of a color using a tristimulus colorimeter.

• Spectroradiometers measure the absolute spectral radiance or irradiance of a light source.

• Spectrophotometers measure spectral reflectance, transmittance, or relative irradiance.

• Spectrocolorimeters are spectrophotometers that can calculate tristimulus values.

• Densitometers measure the degree of light passing through or reflected by a subject.

• Color temperature meters measure the color temperature of an incident illuminant.

Munsell Color System

• The Munsell color system is a color space specifying colors based on hue, chroma, and value.

  • Hue: Basic color.

  • Chroma: Color intensity.

  • Value: Lightness.

• Perceptually uniform and independent dimensions.

Munsell Dimensions and Characteristics

• Hue is measured by degrees around horizontal circles.

• Chroma is measured radially outward from the neutral gray vertical axis.

• Value is measured vertically on the core cylinder from 0 (black) to 10 (white).

• Munsell's system is based on measurements of human visual responses.

Munsell Hue and Chroma.

Hue,Measured by degrees around horizontal circles

• Each horizontal circle Munsell divided into five principal hues: Red, Yellow, Green, Blue, and Purple, along with 5 intermediate hues (e.g., YR) halfway between adjacent principal hues. Each of these 10 steps, with the named hue given number 5, is then broken into 10 sub- steps, so that 100 hues are given integer values.
  Two colors of equal value and chroma, on opposite sides of a hue circle, are complementary colors, and mix additively to the neutral gray of the same value.
  CHroma

• Chroma, measured radially from the center of each slice, represents the “purity” of a color (related to SATURATION), with lower chroma being less pure (more washed out, as in pastels).

• Note that there is no intrinsic upper limit to chroma. Different areas of the color space have different maximal chroma coordinates.

Light Reception/Perception and the Visible Spectrum

1. Color Temperature of Light: The radiation spectrum of the light used (warm or cold light), influencing the colors we see.

2. Spectral Filters: How objects reflect, absorb, or filter light, affecting their perceived color.

3. Resolving Detector: The eye or camera sensors, each perceiving colors differently (Observer Metamerism).

Visible Light Spectrum

• The visible light spectrum spans approximately 400 nanometers (violet) to 700 nm (red).

• Spectral colors are produced by visible light of a narrow band of wavelengths (monochromatic light).

• Unsaturated colors, such as pink, magenta, can only be made from a mix of multiple wavelengths.

• Photon Frequency and Wavelengths

  • Violet: Wavelength 380-450nm

  • Blue: Wavelength 450-485nm

  • Cyan: Wavelength 485-500nm

  • Green: Wavelength 500-565nm

  • Yellow: Wavelength 565-590nm

  • Orange: Wavelength 590-625nm

  • Red: Wavelength 625-750nm

Display Spectrum

• Computer monitors and TVs cannot reproduce all discernible colors of by a human eye.

• Colors outside display gamut can only be approximated.

White Light and Splitting

• Shining white light through a prism causes wavelengths to bend at different angles.

• The order of wavelengths represented by “Roy G Biv” for red, orange, yellow, green, blue, indigo and violet

Human Vision and Extended Spectra

• Purple and magenta are the brain’s interpretation as a mixture of red and violet.

• Bees can see UV which flowers use to attrack pollinators.

• Birds see UV marks on plumage

Color Vision Defects - Dichromacy - Types

• Dichromacy is having two types of functioning cone cells in the eyes.

• Types of Color Blindness

  • Protanopia: Impairment in perceiving long wavelengths. Reds are seen as beige or grey. Caused by lacking red cone cells.

  • Deuteranopia: Impairment in perceiving medium wavelengths. Caused by lacking green cone cells.

  • Tritanopia: Inability to perceive short wavelengths. Trouble distinguishing yellow and blue. Caused by lacking blue cone cells.

Color Perception Across Species

• Bees and other insects see ultraviolet light.

• Birds see ultraviolet light marking on Plumage but sometimes cant see the longer wavelengths such as red.

Infrared and Ultraviolet Radiation

• Infrared (IR) Radiation: Wavelengths from 700 nm to 1 mm. Approx. frequency range of 430 THz to 300 GHz.

• Ultraviolet (UV) radiation Wavelength 10 nm to 400 nm.

Photography and Sensors

• Digital sensors and films can record non-visible UV and IR radiation.

Ultraviolet Photography Principles

• Ultraviolet filters are divided into:

  • near UV (380–200 nm wavelength; abbrev. NUV)

  • far UV (or vacuum UV) (200–10 nm; abbrev. FUV or VUV)

  • extreme UV (1–31 nm; abbrev. EUV or XUV)

• Photographers subdivide the near UV into:

  • Long wave UV that extends from 320 to 400 nm, also called UV-A,

  • Medium wave UV that extends from 280 to 320 nm, also called UV-B,

  • Short wave UV that extends from 200 to 280 nm, also called UV-C.

SWIR, MWIR and LWIR explained

• Short-wavelength infrared SWIR Water absorption increases significantly at 1450 nm. The 1530 to 1560 nm range is the dominant spectral region for long-distance telecommunications

• Mid-wavelength infrared MWIR In guided missile technology the 3–5 µm portion of this band is the atmospheric window in which the homing heads of passive IR 'heat seeking' missiles are designed to work, homing on to the Infrared signature of the target aircraft, typically the jet engine exhaust plume. This region is also known as thermal infrared

• Long-wavelength infrared LWIR The "thermal imaging" region, in which sensors can obtain a completely passive image of objects only slightly higher in temperature than room temperature - for example, the human body - based on thermal emissions only and requiring no illumination such as the sun, moon, or infrared illuminator

Infrared Technology

• Active-infrared night vision involves illuminating the scene at infrared wavelengths invisible to humans.

• Infrared radiation is longer wavelengths than visible and cannot be seen by the human eye.

Infrared Thermography

• Thermography allows remote sensing of temperature based radiation emitted according to the black body radiation law.

• Cameras are available with smartphones.