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Bright Ideas: Lighting, Well-Being, & Visual Performance Presented by Steven L. Klein IALD, LC, IESNA ASID INTERIORS 08 March 13, 2008

Bright Ideas: Lighting, Well-Being, & Visual Performance
Presented by Steven L. Klein IALD, LC, IESNA
ASID INTERIORS 08
March 13, 2008


Every branch of science has its own technical works which are used to indicate specific details
or physical quantities. In the field of lighting there are such technical words and definitions, the
correct understanding of which will enable one to correlate the full importance of the various
terms used. (
Highlighted
words appear in the glossary).

Light
cannot actually be seen. Light is electromagnetic radiation energy that is capable of
exciting the
retina
and producing a visual sensation. It is visible only when it strikes a surface.
Photometry
is the science of measuring visible light in units that are weighted according to the
sensitivity of the human eye. It is a quantitative science based on a statistical model of the
human visual response to light, that is, our perception of light under carefully controlled
conditions. In photometry, we attempt to measure the subjective impression produced by
stimulating the human eye-brain visual system with
radiant energy
. Photometric theory does
not address how we perceive colors.
Radiometry
is the general study of measuring light,
whereas photometry relates more to how light is seen and perceived by a human viewer. The
light being measured can be monochromatic or a combination or continuum of
wavelengths
:
the eyes response is determined by a
CIE
weighting function. The only difference between
radiometric and photometric theory is in their units of measurement.
Luminance
and
illuminance
are the photometric analogues of the radiometric quantities:
radiance
and
irradiance
respectively. Radiometry and photometry are two related fields which study light.

The human
visual system
is a marvelously complex and highly non-linear detector of electo-
magnetic radiation with wavelengths ranging from 380 to 770
nanometers (nm).
People see
light of different wavelengths as a continuum of colors ranging throught the visible light
spectrum: 650nm is red, 540nm is blue 450 nm is green , and so on. The human visual system
is not equally sensitive to all wavelengths. Depending on whose research you follow, peak
sensitivity is between 555 nanometers and 505 nanometers. Human spectral sensitivity is
measured in terms of the equivalence of visual effect. For example, a person viewing two
equally presented fields of the same wavelength and radiance sees them equal in all respects.
However, whenever wavelengths are the same but radiances differ, then the field with higher
radiance will be seen as brighter.
Given the same output of power at each wavelength, the visual system will sense the yellow-
green region as the brightest and the red or blue regions as the dimmest. This is why, among
equally efficient light sources, a light source that has most of its power in the yellow-green area
will have the highest visual efficacy, i.e., the highest lumens per watt. However, without a
reasonable proportion of red or blue in its output, a light source will not be able to render colors
satisfactorily.


Bright Ideas: Lighting, Well-Being, & Visual Performance
Presented by Steven L. Klein IALD, LC, IESNA
ASID INTERIORS 08
March 13, 2008
Page 2 of 25

How we see color depends on the wavelengths emitted by the light source, the wavelengths
reflected by the object, the surroundings in which we see the object, and the characteristics of
the visual system. Our conception of the color of an object is a constantly changing, highly
dynamic process. It depends on what colors surround the object, how long we have been
exposed to the scene, what we were looking at before, what we expect to see, and perhaps
what we would like to see. Added to that is the fact that about 8% of the male population and
about 0.4% of the female population has a color deficiency or is "color blind" to some degree.
Light and pigments mix differently to form colors. Since the human visual system has three color
receptors, it is possible to pick three suitable colors and generate the other colors by mixing
these. By convention, the primary colors of light are chosen to be red, green and blue (RGB).
Because they produce white light when they are added together, color mixing of light is called
"additive."
Any two primaries of light can be combined to form a secondary color - magenta (red plus blue),
cyan (green plus blue), and yellow (red plus green). When a secondary is mixed in proper
proportions with its opposite primary, the resulting light will be white. Therefore, any primary
color is considered to be complementary with the secondary color produced by mixing the other
two primaries. Yellow and blue are complementary colors of light - as are cyan and red, and
magenta and green.
The color appearance of an object or surface clearly depends on the light used to illuminate it.
Often daylight is considered a "standard" but it is obvious that the color of "daylight" changes
with the position of the sun in the sky, how cloudy or overcast it is and also which direction of
the sky we are sampling, e.g. northern sky or southern sky. In specifying lamp color, the first
thing that must be decided is how" warm" or how "cool" a lamp is to be selected. This is
generally a subjective decision entirely. The
color temperature
of the lamp in Kelvins specifies
the degree of coolness or warmth of the light source. Another color rating of lamps is the
Color
Rendering Index
(CRI). Lamps with high CRI tend to have a "natural" look no matter what the
color temperature.

The amount of light coming to the eye from an object depends on the amount of light striking the
surface and on the proportion of light that is reflected. If a visual system made just a single
measurement of
luminance
, acting as a photometer, there would be no way to distinguish a
white surface in dim light from a black surface in bright light. Yet humans can usually do so, and
this skill is called
lightness

constancy
.
Bright Ideas: Lighting, Well-Being, & Visual Performance
Presented by Steven L. Klein IALD, LC, IESNA
ASID INTERIORS 08
March 13, 2008
Page 3 of 25
The constancies are central to perception. An organism needs to know about meaningful world-
properties color, size, shape, etc. These properties are not explicitly available in the retinal
image, but are extracted by visual processing. The gray shade of a surface is one such
property. To extract it, luminance information must be combined
across space.

In this illustration, you we see the well-known simultaneous
contrast effect which demonstrates a spatial interaction in lightness
perception. The two smaller squares are the same shade of gray,
but the square in the dark surround appears lighter than the square
in the light surround. This is, of course, an illusion, which may be
viewed as a quirky failure of perception. But such illusions are also revealing, allowing us to
examine the inner workings of a system that functions remarkably well. Here we consider how
lightness illusions can inform us about lightness perception.
The following is an example used by the op artist Vasarely, as shown in the illustration: The
visual system processes information at many levels of
sophistication. At the retina, there is low-level vision, including light
adaptation and the center-surround receptive fields of ganglion
cells.
An illusion by Vasarely, left, and a bandpass filtered version, right.
At the other extreme is high-level vision, which includes cognitive processes that incorporate
knowledge about objects, materials, and scenes. In between there is mid-level vision. Mid-level
vision is simply an ill-defined region between low and high. The representations and the
processing in the middle stages are commonly thought to involve surfaces, contours, grouping,
and so on. Lightness perception seems to involve all three levels of processing. When a spatial
ramp in luminance abruptly changes slope, an illusory light or dark band appears.
The image consists of a set of nested squares. Each square is a constant luminance. The
pattern gives the illusion of a glowing
X
along the diagonals, even though the corners of the
squares are no brighter than the straight parts. When a center-surround filter is run over this
pattern (i.e., is convolved with it), it produces the image shown in the illustration. The filter output
makes the bright diagonals explicit.
What we perceive is our visual system's best guess as to what is in the world. The guess is
based on the raw image data plus our prior experience. Lightness constancy is achieved by
inferring, and discounting, the illuminant. A lightness judgment involves the workings of the
whole visual system, and that system is designed to interpret natural scenes. Simultaneous
contrast and other illusions are the by-products of such processing.
Bright Ideas: Lighting, Well-Being, & Visual Performance

Presented by Steven L. Klein IALD, LC, IESNA
ASID INTERIORS 08
March 13, 2008
Page 4 of 25

Consider how that light is going to strike a particular surface and the quality of light you want it
to be, and then select the lamp and fixture that will deliver the ef