Light Theories, CRI & Gaseous Radiation

Light

• Light is the visible portion of a wider electromagnetic spectrum.
• Over time, scientists like Huygens, Newton, Planck, and Einstein have proposed various hypotheses to describe light.
• The main questions scientists have attempted to answer, include defining light, understanding how we perceive it, and how to measure it for our purposes.

Wave Theory

• In 1690, Christiaan Huygens theorized light as a wave propagating through a fluid called the Aether, permeating the universe.
• The Wave theory marked the first mathematical approach to understanding light.
• Newton's corpuscular theory gained favor, but Augustin-Jean Fresnel's observations later revived Huygens' theory.
• Fresnel showed Huygens's theory explained phenomena like rectilinear propagation and diffraction of light.

Corpuscular Theory

• Sir Isaac Newton posited that light consists of particles, called corpuscles, to explain reflection and refraction.
• The simplicity of the theory was offset by its failure to explain diffraction, interference, and polarization
• It was later partially revived with the concept of duality of light.

Electromagnetic Radiation Theory

• James Clerk Maxwell unified electricity, magnetism, and light as manifestations of the same phenomenon.
• Light propagates as two waves (electric and magnetic) carrying electromagnetic radiant energy, perpendicular to each other and the direction of propagation.
• Maxwell developed four equations describing light's electromagnetic nature.

Quantum Theory

• In 1887, Heinrich Rudolf Hertz discovered the photoelectric effect, where electromagnetic radiation hitting a metal causes electron emission, which the wave theory could not explain.
• Max Planck, studying the black body, stated in 1900 that electromagnetic energy emits only in quantized form, establishing the basis for quantum mechanics.
• Albert Einstein expanded on Planck's concept, stating the energy carried by a "quantum of light", called a photon, equals the Planck constant times the radiation frequency.
• In the theory, high-frequency radiation (like x-rays or gamma rays) transmits large amounts of energy, whereas low-frequency radiation carries smaller amounts.

Wave-Particle Duality

• In his PhD thesis in 1924, Louis de Broglie suggested electrons possess an undulatory nature associated with wavelength.
• De Broglie reworked Planck's theories, theorizing that light and matter behave as both a wave and particle.
• Clinton Joseph Davisson and Lester Halbert Germer experimentally confirmed De Broglie's theory in 1927 by observing diffraction from a beam of electrons (particles), birthing what is known as wave-particle duality.

Quantum Electrodynamics

• Physicists like Paul Dirac, Wolfgang Pauli, and Werner Heisenberg quantized the electromagnetic field, leading to quantum electrodynamics (QED).
• This theory's exponents include Richard Feynman.
• QED studies the interaction between matter and light.
• Summarizing light, we can describe it as a set of particles (photons) with corpuscular behavior on a macro-scale, while also having wave-like properties on the micro-scale, related to wavelengths.

Light And The Visual System

• Visual sensation depends on the intensity and wavelength of radiation, and on how the eye responds to it.
• Within the eye's visible range, sensitivity varies with different wavelengths of the same energy content.
• Under photopic vision, the eye has 20x greater sensitivity to light at 550nm (yellow) compared to 700nm (deep red) or 450nm (violet-blue) wavelengths.
• The difference between the peaks of the spectral eye sensitivity curves for photopic and scotopic vision, according to CIE, is called the Purkinje shift.
• The eye perceives wavelengths as colors, ranging from violet to yellow, orange, and red as wavelength increases.
• At the wavelength of 555nm, the human eye's maximum sensitivity (100%) is registered.
• This maximum luminous flux corresponds to 683 lumens

Light Sources

• It is possible to transform energy into light energy using a light source capable of emitting a luminous flux
• Light sources are divided into:
  • Natural, such as sunlight or diffuse sky light
  • Artificial, such as lighting fixtures

Thermal Radiation

• When heated to 525°C, a solid body emits dull red light, changing to bright red, orange, yellow, white, and finally blueish white with increasing temperature.
• The efficacy of theoretical "metal", grows as the curve approaches the middle of the range of "visible radiations".
• The eye's sensitivity reaches its maximum value is represented by the V(λ) curve, at a wavelength of 555 nm.

Correlated Color Temperature (CCT)

• Color temperature is a color impression obtained from an incandescent body at that temperature.
• The term only applies to white light sources.
• At low temperatures, high wavelengths are prevalent, giving a reddish light.
• At high temperatures, low wavelengths are prevalent, giving a white-light-blue hue.
• CCT is expressed in Kelvin (K), and is unrelated to the light source's actual temperature.
• An ideal black body (Plackian radiator) emits light as it overheats.
• The color shade of emitted white light relates to that radiator's temperature, measured in Kelvin (K).
• These colors can range from 1000 K (very warm) up to 20000 K (very cold).
• When the temperature varies from 1,000 K to 20,000 K, the blackbody moves on a curve known as the Planckian locus.
• If a light source's color deviates from the Planckian locus, its color temperature is undefined.
• Color temperature is important in lighting because of its impact on the color appearance of illuminated objects.
• Spectral radiant output is spectral radiant output provides information on: the amount of energy emitted and the wavelengths in which light is expressed.
• Spectral characteristics are expressed by the distribution of relative power

Colour Rendering Index

• CRI measures the degree of color shift objects undergo when illuminated by a light source, as compared to illumination by a reference source of comparable color temperature.
• The higher the CRI value, the more accurately a light source renders the color of an object.
• Fullness and continuity of the spectrum of the light source.
• CRI is calculated using 8 + 6 colour samples taken from the Munsell colour system
• The CIE method evaluates a light source's color rendering by comparing it to a sample source (standard illuminants) close in color temperature to the test source and using CIE colour plates.

ANSI/IES TM-30-20

• TM-30 is an alternative to the EU Colour Rendering Index.
• TM-30 considers 99 samples.
• To measure color with the TM-30, consider the accurate colour rendering.
• This accurate rendition of the colour, measures the Fidelity Index (Rf), that can span from 0 to 100.
• The Color Gamut: the measure of saturation relative to standard illuminants; measured with the Gamut Index (Rg) that can range from 40 to 140.

Other

• Observers tend to accept sources that increase saturation more than those that decrease it.
• Apart from CRI and TM-30, the Color Quality Scale considers colour preference.
• Perceptual disuniformity of the CIE diagram: regions exist in the chromaticity diagram where the visual system does not detect color differences.
• Within each ellipse, color differences remain unseen.
• The lines are called isotherms and are used to evaluate Duv along the curve of the planckian locus

Light source properties

• Low Pressure Sodium Lamps
• Efficacy: Very high luminous (up to 200 lm/W)
• Lifetime : Long (10000 hours)
• Colour: Warm, with warm light appearance
• CRI: Almost inexistent

Explore light theories from Huygens and Newton to Maxwell. Learn why only the first eight Munsell color samples are used for CRI. Understand gaseous radiator emission and spectral differences.