01 - Light and radiation

POLI.DESIGN Overview

Founded by: Politecnico di MilanoFocus: Specializing Master in Lighting Design and Technology for the academic year 2024/2025

Introduction to Light and Radiation

Lecturers: Arch. Chiara Bertolaja, Prof. Andrea SiniscalcoPurpose: The course aims to delve into the intricate nature of light, exploring its physics, emotional resonance, and its crucial role in design and technology. It raises pivotal questions on measurement techniques and the multifaceted utilization of light across various fields such as architecture, interior design, and visual arts.

What is Light?

Definition: Light refers to the visible portion of the broader electromagnetic spectrum, ranging approximately from 380nm (violet) to 780nm (red). Throughout history, numerous scientists, including Christiaan Huygens, Isaac Newton, James Clerk Maxwell, Max Planck, and Albert Einstein, have contributed foundational theories regarding the nature of light.

Historical Theories of Light

Wave Theory

  • Proposed by: Christiaan Huygens (1690)

  • Description: Huygens described light as a wave propagating through a hypothetical medium referred to as the Aether. This theory was initially marginalized in favor of Newton’s corpuscular theory but gained acceptance due to Fresnel's experiments validating wave behavior through phenomena like diffraction and rectilinear propagation.

  • Huygens' Life: 1629 - 1695

Corpuscular Theory

  • Proposed by: Sir Isaac Newton

  • Description: Newton suggested that the behavior of light, including reflection and refraction, could only be adequately explained if light were composed of particles known as corpuscles. This perspective, while powerful, failed to adequately address phenomena like diffraction, interference, and polarization, which later led to its decline until it was reconciled with the wave theory through the formulation of duality.

  • Newton's Life: 1642 - 1727

Electromagnetic Radiation Theory

  • Formulated by: James Clerk Maxwell

  • Description: Maxwell's groundbreaking theory interlinked electricity, magnetism, and light, treating all three as manifestations of electromagnetic phenomena. Light propagation is described as dual waves (electric and magnetic) that exist perpendicularly to each other and to the direction of propagation, encapsulated through four equations known as Maxwell's equations:

Maxwell's Equations:

  1. $ abla ullet E = \frac{\rho}{\epsilon_0}$

  2. $ abla ullet B = 0$

  3. $ abla \times E = -\frac{\partial B}{\partial t}$

  4. $ abla \times B = \mu_0 J + \mu_0 \epsilon_0 \frac{\partial E}{\partial t}$

  • Maxwell's Life: 1831 - 1879

Quantum Theory

  • Developed by: Max Planck and Albert Einstein

  • Description: Planck's investigation of black body radiation led to the revolutionary idea that electromagnetic energy is quantized. Einstein expanded on this by introducing the notion of photons, revealing that the energy of light is proportional to its frequency, articulated by the formula:

Energy of a Photon:$E = h u$where $E$ = energy, $h$ = Planck's constant, and $ u$ = frequency.

  • Key experimental validations like the photoelectric effect (discovered by Heinrich Hertz in 1887) highlighted inconsistencies in prior theories, redefining our understanding of light.

  • Planck's Life: 1858 - 1947, Einstein's Life: 1879 - 1955

Wave-Particle Duality

  • Hypothesized by: Louis de Broglie (1924)

  • Description: De Broglie proposed that all matter, akin to light, displays both wave-like and particle-like characteristics. This hypothesis generated extensive experiments, such as electron diffraction, conducted by Davisson and Germer in 1927, which supported his theory.

  • De Broglie's Life: 1892 - 1987

Quantum Electrodynamics

  • Developed by: Key physicists including Paul Dirac and Richard Feynman

  • Description: Quantum Electrodynamics (QED) describes the unique interaction between matter and light, concluding that light exists as photons. At microscale levels, light behaves as a wave, whereas at macroscale it exhibits particle-like properties.

  • Feynman's Life: 1918 - 1988

Light and Visual Perception

The qualitative experience of light is influenced by intensity and wavelength. The human eye's response varies across the spectrum, which significantly alters color perception. This is explained through the sensitivity curve from 380 nm (violet) to 780 nm (red).

Luminous Efficacy and Eye Sensitivity

  • Purkinje Shift: Exhibits that the spectral sensitivity of the human eye fluctuates based on varying light levels (photopic vs. scotopic). The eye demonstrates peak sensitivity at 550 nm (yellow) during photopic vision, which is substantially higher than for other wavelengths.

Light Sources

Classification

  • Natural Sources: Include sunlight and diffuse light from the sky, essential for ecological balance and human health.

  • Artificial Sources: Encompass a broad range of lighting fixtures with distinct spectral characteristics impacting brightness and color perception.

Thermal Radiation

Explained via emission curves that vary with temperature; as temperature increases (beginning from 525°C), emitted color transitions from dull red to bluish-white, reflecting elevated energy output and varying spectral distribution.

Correlated Colour Temperature (CCT)

CCT signifies the color impression derived from an incandescent object's temperature, expressed in Kelvin (K). Importantly, CCT does not adequately apply to colored light sources, necessitating an appreciation of perceptual color differences.

Considerations for Light Sources

  • Gaseous Radiators: These emit light when electrons collide with gas atoms, generating specific spectra crucial to various applications, including neon lights and gas discharge lamps.

Quality of Light

  • Colour Rendering Index (CRI): Ranges from 0 to 100; this metric indicates a light source's effectiveness in accurately rendering colors in comparison to a reference source. The CIE standard rates sources based on particular color samples, which is vital for accurate color portrayal in design fields.

  • Consistency and Variability: Perceptual discrepancies can occur even among identical LED sources, necessitating evaluation through precise metrics like Duv to maintain quality and expectation in design applications.

Conclusion

The study of light integrates crucial aspects of physics and perception, which are fundamental for applications in design, particularly in lighting technology. Key considerations such as CCT, CRI, and spectral distribution are pivotal, directly influencing visual quality and energy efficiency in modern lighting applications.