1. The Essence of Light
The word light can evoke many different meanings and associations. Some common
associations include energy, electromagnetic radiation, waves, particles, color, the sun,
light bulbs and light fixtures. It can be a powerful element in the telling of stories and a
vital component of visual arts and theater. It is seemingly ubiquitous and familiar and at
the same time complex and often misunderstood.
The following document is intended to be an introduction to what we understand about
light by examining aspects of the physical description of light and the psychological
components of how we perceive light. The physical description of light includes the
fundamental physics related to the practice of lighting science and how we harness that to
engineer light into the devices that we encounter every day. The psychological
components point to the perception of light that can only be described in connection with
the sensory cells of the eyes and visual processing that results from it. The blending of
both the physical and psychological are what make up the practice of lighting science.
1.1. Light as a Spectrum
One of the most recognizable aspects of light is that we perceive it as coming in a range of
different colors. The color of the light is a result of the different wavelengths that make up
visible light radiation. This is typically referred to as the visible spectrum and is the band
of the electromagnetic spectrum that is visible to the human eye. Visible light occupies a
narrow band of wavelengths in the electromagnetic spectrum from approximately violet
to red (380nm-780nm).
[insert image of spectrum with visible light and identified]
For a given light source the distribution of energy at each wavelength in the spectrum can
be characterized and results in the spectral power distribution of the source. These are
usually represented in graphical form with the power (absolute or relative) on the Y axis
and the various wavelengths on the x axis.
[Insert Image of example spectrum]
Full characterization of the source will include wavelengths outside the visible range to
include important radiation characteristics in other wavebands including ultraviolet
radiation (<380 nm) and infrared radiation (>780 nm). .
Formally the spectral power distribution can be quantified using Φλ to represent the
power in watts at a given wavelength as:
Φλ= dΦ/dλ
which represents a spectral radiation quantity over the smallest possible interval in the
width of the wavelength. The width of that interval will be an important consideration for
colorimetric aspects among others to be discussed in later sections.
Integrating the spectral quantity over a given wavelength range gives the total power
measured through the range.
Φ = ∫λ1
λ2 Φλ dλ
The human eye does not respond equally to each wavelength in the visible spectrum, but
according to the sensitivities of the different receptors and this will be discussed in more
detail in section xx.xx
1.2 Light as Waves and Particles
There are applications in lighting science that require the knowledge that light has both
the properties of waves and as a stream of particles.
A. Light as a wave.
This description is most applicable for describing the propagation of light through a
uniform medium (such as air) as a periodic oscillation in a straight line. Similar to the
behavior of waves that radiate outward from dropping a pebble into water.
As a wave, light is characterized by its wavelength (λ) and frequency (ν), which are related
by the speed of light (c):
c = λν
where c ≈ 3.0 × 108 m/s in vacuum.
When light encounters a different medium (for example going from air to water) it will
refract and change direction when entering the second medium.