Lightness and Brightness and Other Confusions

etween about 380 and 780 nm, which is the span that can activate the receptors
in the human eyes and trigger a neural process resulting in vision.
Physical textbooks and scientists do not, however, fully agree on what wavelength
span to include in the concept of light, and sometimes a distinction is
made between visible light (380–780 nm) and invisible light. ‘Invisible light’
refers in this case to ultraviolet radiation with shorter and infrared with
longer wavelengths than those within the ‘visible spectrum’. Shorter still
(gamma- and X-rays) or longer (radio waves) wavelengths are seldom or
never referred to as light, but are still included in what is called the electromagnetic
spectrum, where the word spectrum originally referred to a visible
range of colours. 58 Even if measurements of energy are limited to what is
called visible light there is, however, no direct correspondence between the
amount of energy and the perceived intensity of light.
Examples of physical concepts related to light
Speed of light, light-year, wavelength, light energy, electromagnetic
spectrum, absorption, radiation, emission, transmission, dispersion,
refraction, diffraction, polarisation, interference, photon
Examples of physical concepts related to colour
Monochromatic, spectral power distribution curve
Physical aspects of ‘colour’
Radiation with wavelengths in the span referred to as visible light can be
isolated into discrete wavelengths by for example a prism. Then they cause
the perception of different hues, as in the rainbow where radiation from the
sun is refracted and reflected by water drops acting as prisms. This has led to
the convention of presenting the different wavelengths in the form of a spectrum,
made up of hues from blue (short wavelength) to red (long wavelength).
Radiation within a very narrow band in this spectrum is called monochromatic.
There is, however, no absolute relationship between wavelengths
and perceived hues 59 – a reason why modern physics often abandons the
traditional colour references for wavelengths in favour of terms such as short,
middle and long wavelength.
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html.
Wavelength information is discarded very early in the visual process. The perceived
hue depends on local contrasts and the total viewing situation and also the intensity of
the radiation.
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