University of Asia Pacific

Arch 231:
ED 2-Visual & Sonic Environment

Topic 02:
Basics of Light

Udday Datta
Lecturer,
Department of Architecture
University of Asia Pacific
 Presentation Summery
 Light
 The Attributes of Light
 The transmission of Light
 Reflection of light
 Colour of Light
 Surface Colours
 Coloured Light
 Photometry
 The Illumination of Light

Basics of Light
 Light

• The form of matter is known to us primarily by the way it reflects light.

• Sensitive designers have always understood that what we see is a consequence of both the
quality of the physical design and the quality of light falling on it.

• Sometimes the architect must accept the light as it is and design the form in response to it.

• Other times both the form and the light source are under the architect's control.

• This is true not only for interiors, but also the exterior at night. Thus, the architect creates the
visual environment by both moulding the material and by controlling the lighting.
 Light

• Light is a form of radiant energy that travels in waves made up of vibrating electric and
magnetic fields.

• These waves have both a frequency and a length, the values of which distinguish light from
other forms of energy on the electromagnetic spectrum.

• Visible light, as can be seen on the electromagnetic spectrum, represents a narrow band
between ultraviolet light (UV) and infrared energy (heat).

• These light waves are capable of exciting the eye's retina, which results in a visual sensation
called sight.

• Therefore, seeing requires a functioning eye and visible light.

 Light

• So, Light is defined as

that portion of the
electromagnetic spectrum
to which our eyes are
visually sensitive. What we
perceive as light is a
narrow wavelength band of
electromagnetic radiation
from about 380 to 780 nm.
Light
 The Attributes of Light

• Two main attributes of light are its quantity and its quality.
• Its quantitative aspects are discussed later in Section photometry

• As for any other electromagnetic radiation, the velocity of light (c) is approximately 3×108
ms-1 (or 300 000 kms-1).

• More precisely (in ms-1):

In Vacuum 299 792 000 ms -1
In air 299 724 000 ms -1
In water 224 915 000 ms -1
In glass 198 223 000 ms -1
 The Attributes of Light

• Its quality is characterised by wavelength (λ, in m) and its reciprocal, the frequency (f, in Hz).
The product of these two always gives the velocity:

c = f × λ,

• So, if one is known, the other can be found by dividing the known one into the velocity,
 The Transmissions of Light

• Material bodies exposed to light behave in various

ways.
• Some materials when exposed to light transmit a large
part of it- these are referred to as transparent.
• Others, the opaque materials, block the passage of
light. Behind an opaque object there will be no light (no
direct light), i.e. it will cast a shadow.
• The term translucent is applied to materials which
transmit a part of the incident light, but break its
straight passage, scatter it in all directions, creating
diffuse light.
• A sheet of glass is said to be transparent while a sheet
of plywood is opaque and a sheet of ‘opal’ Perspex
(frosted sheet) is translucent.
 The Transmissions of Light

• Light incident on the surface can be distributed three ways: reflected, absorbed or transmitted.
The corresponding properties are:
Reflectance (r),
Absorbance (a) and
Transmittance (t).

• In all cases, r + a + t = 1.
• In case of opaque objects: t = 0, thus r + a = 1.

• Materials, which in a small thickness appear to be transparent, may become opaque in a large
thickness.

• The term ABSORPTIVITY is a property of the material, indicating the absorption per unit
thickness, whilst ABSORBANCE is the property of a body of given thickness or a surface of an
opaque body.
 Reflection of Light

• Surfaces may be classified, in terms of their reflective properties, as :

Specular (a mirror),
Diffuse (ordinary building surfaces),
Transitional or Spread: giving a spread reflection (basically diffuse, but
with some specular component),
Semi-diffuse (all diffuse, but with some directional bias)

• If parallel rays of incident light remain parallel after reflection from a

surface, the surface is a plane mirror and we speak of specular
reflection.

• Light reflected from a matte surface will be diffused. Most often a

mixture of two kinds of reflections will occur, termed as semi-diffuse
or spread (transitional) reflection, depending on the relative magnitude
of the two components.
 Reflection of Light

Some materials have practically the same reflectance for all wavelengths of light. These do not
change the wavelength composition of light after reflection surfaces with such neutral reflection
properties will be seen as white light:

White If r is above 0.75

Grey If r is between 0.05 to 0.75
Black If r is below 0.05
 Colour of Light

• The colour of light is determined by its spectrum or

spectral composition.

• Light of a particular wavelength or a narrow band of

wavelengths is referred to as monochromatic. In
physics, monochromatic describes light that has the
same wavelength so it is one color. Broken into Greek
roots, the word shows its meaning: monos means one,
and khroma means color.

• The colour of broad-band light depends on the relative magnitude of its components, on its
spectral composition.

• A continuous spectrum white light can be split by a prism into its components, which are
perceived as colours.
 Colour of Light
• White light, such as sunlight, is a combination of all colours.

• Materials that are black , absorb all the visible light which is shone on them.
• Materials that are white or colourless , absorb no visible light.

• The colours of other objects are selective in their reflectance. They may absorb the particular
wavelengths of incident light thus the remainder reflected will show a colour.

• In mixing coloured pigments the absorptions are additive and the reflections are subtractive, for
example:

Yellow paint Absorbs blue Reflects red, yellow, green

Blue paint Absorbs red and yellow Reflects blue and green
A mixture of two Absorbs blue, red and yellow Reflects only green
 Surface Colours

• Whilst coloured light from various sources would be additive, surface colours are subtractive,
or rather their absorbance are additive.
• A surface painted red appears to be this colour, as it absorbs everything else, reflects only the
red component of the incident light. If a red surface is illuminated by white light, which is the
addition of the above yellow-orange and blue-green, it will appear to be a dirty grey, as the light
has no red component, no red will be reflected.
 Surface Colours

• However, if this same red paint is illuminated with monochromatic blue light, it will appear
black because the colour red absorbs all colours except red. Unless the red paint is illuminated
with light that contains red, it will not appear red.

• In the real world, where the colours are not completely saturated (pure), the situation is more
complicated.

• Ordinary colours, such as red, reflect not only most of the red light, but also small amounts of
the other colours. This can create problems when the illumination does not have a good
mixture of the various colours.

• A bright red car reflects plenty of red lights when it is illuminated by daylight. However, when
this same bright red car is parked at night under a clear mercury street light, it will appear to be
brown. Since clear mercury lamps emit mostly blue and green light, there is little red light that
the car can reflect. Although much of the light of the other colours was absorbed, enough blue
and green was reflected to overwhelm the red light.
 Basic Colour Theory

COLOR WHEEL

• A color wheel, based on red, yellow and

blue, is traditional in the field of art.

• A color wheel (also referred to as a color

circle) is a visual representation of colors
arranged according to their chromatic
relationship. Begin a color wheel by
positioning primary hues equidistant from
one another, then create a bridge between
primaries using secondary and tertiary
colors.
 Basic Colour Theory

• Sir Isaac Newton developed the first circular diagram of colors in 1666. Since then, scientists
and artists have studied and designed numerous variations of this concept.

• Differences of opinion about the validity of one format over another continue to provoke debate.
In reality, any color circle or color wheel which presents a logically arranged sequence of pure
hues has merit.
Basic Colour Theory
 Basic Colour Theory

 There are also definitions (or categories) of colors based on the color wheel. We begin with a
3-part color wheel.

 Primary Colors: Red, yellow and blue are the primary colors. In traditional color theory (used in
paint and pigments), primary colors are the 3 pigment colors that can not be mixed or formed
by any combination of other colors. All other colors are derived from these 3 hues.

 Secondary Colors: Green, orange and purple are the Secondary colors. These are the colors
formed by mixing the primary colors.

 Tertiary Colors: Yellow-orange, red-orange, red-purple, blue-purple, blue-green & yellow-green

are the tertiary colors. These are the colors formed by mixing a primary and a secondary color.
That's why the hue is a two word name, such as blue-green, red-violet, and yellow-orange.
 Basic Colour Theory

COLOR HARMONY

 Harmony can be defined as a pleasing arrangement of parts, whether it be music, poetry, color,
or even an ice cream sundae.

In visual experiences, harmony is something that is pleasing to the eye. It engages the viewer and
it creates an inner sense of order, a balance in the visual experience.

 When something is not harmonious, it's either boring or chaotic. At one extreme is a visual
experience that is so bland that the viewer is not engaged. The human brain will reject under-
stimulating information. At the other extreme is a visual experience that is so overdone, so chaotic
that the viewer can't stand to look at it. The human brain rejects what it can not organize, what it can
not understand. The visual task requires that we present a logical structure. Color harmony delivers
visual interest and a sense of order.
 Basic Colour Theory

A color scheme based on complementary colors

 COMPLEMENTARY COLORS are any two colors

which are directly opposite each other, such as
red and green and red-purple and yellow-green.

 These opposing colors create maximum

contrast and maximum stability. The high
contrast of complementary colors creates a
vibrant look especially when used at full
saturation.

 Complementary colors are really bad for text.

 Basic Colour Theory

A color scheme based on analogous colors

ANALOGOUS COLORS are any three colors which are

side by side on a 12 part color wheel, such as
yellow-green, yellow, and yellow-orange.

Usually one of the three colors predominates, a

second to support. The third color is used (along
with black, white or gray) as an accent.

They usually match well and create serene and

comfortable designs. Analogous color schemes are
often found in nature and are harmonious and
pleasing to the eye.
 Coloured Light

• The most comprehensive classification of surface colours is the Munsell system. This
distinguishes three attributes:

• Hue:
• It is the concept of colour, using the common terms, red, yellow, blue, etc., with transitional
colours (e.g. green/yellow) and further numbered subdivisions.

• Value (V) or lightness:

• It is the subjective measure of reflectance, light or dark appearance, measured on a scale of 0
(absolute black) to 10 (the perfect white). In practice, values from 1 to 9 are encountered. It can
be converted into reflectance:
 Coloured Light

• Chroma or saturation:

• It is the fullness or intensity of colour.

All colours have at least 10 classes
(e.g. blue-green), but some colours
can be very ‘strong’, having a chroma
up to 18.

• Any colour can be designated by the

three facets, hue-value/chroma, e.g.
5R − 4/10 = a hue of red 5 – value
of 4/chroma of 10.
 Coloured Light

• The Munsell ‘colour wheel’ shows the framework of two (irregular) cones (joined at their
bases), where the radial direction is the hue (as shown by the ‘plan’ view of the base circle),
and the vertical scale gives the value and the radial distance from the axis indicates the chroma
or intensity.

• The vertical axis itself would contain the neutral colours, from black, through shades of grey to
brilliant white.

• Better catalogues of paints would give the Munsell designation as well as the more ‘poetic’ (or
gimmicky) colour names.
 Photometry

• The simplest luminous system consists of a light source (a lamp), a surface illuminated and an
eye perceiving the light, both from the source and reflected by the surface. The four measurable
photometric quantities are:

• The photometric term for the time rate of light flow is luminous flux. The luminous flux or Φ
(phi), is measured by the unit lumen (lm), which is defined as the flux emitted within 1
steradian (sr) by a point source of I = 1 cd, emitting light uniformly in all directions. Therefore,
1 cd emits a total of 4π lumens

A simple luminous system: a light source, a lit surface and an eye.

 Photometry

• The rate at which a light source emits light energy is analogous to the rate at which water sprays
out of a garden hose. The power with which light is emitted from a lamp is measured in lumens. We
can say that the quantity of light a lamp emits in all directions is indicated by the lumen value.

I, the luminous intensity of a source, measured in units of candela (cd). It is the basic assumed and
agreed unit in the international standard (defined as the intensity of a black body of 1/60 cm2, when
heated to the melting point temperature of platinum) - all other units are derived from this.
 Photometry

• E, or illuminance (the symbol E comes from the French Éclairage) is a measure of the
illumination of a surface (note that illumination is the process, illuminance is the product). The
lumens from a light source will illuminate a surface. A meaningful comparison of various
illumination schemes is possible only when we compare the light falling on equal areas.
Illuminance is, therefore, equal to the number of lumens falling on each square meter of a surface
(i.e. the incident flux density of 1 lm/m2). The unit is lux (lx).

• L, or luminance, is a measure of brightness of a surface, when looked at from a given direction.

Its unit is cd/m2 (sometimes referred to as nit, rarely used in English), which is unit intensity of a
source of unit area (source intensity divided by its apparent area viewed from the nominated
direction).
 The Illumination of Light
• The inverse-square law for light intensity states:
The intensity of illumination is proportional to the inverse square of the distance from the light source.

• It means illumination from a point source reduces with the square of the distance. A source of I
candela emits a total flux of 4πl lumens. At a distance (r) this flux will be distribute over a
sphere of radius r, i.e. a surface 4πr2.Thus the illumination at a distance r is

• This is known as the inverse square law and is applicable when the illuminated plane is
perpendicular to the direction of light, that is when the angle of incidence, β = 0° in the
formula,
Eβ = En x Cos β
 The Illumination of Light

• Where
• En= illumination on a normal plane,
• Eβ = illumination on a plane tilted by β° and
• β = angle of incidence

• Illumination of a surface from several sources will be the simple sum of the component
illuminations: E= E1+E2+E3
The Illumination of Light
 The Illumination of Light

• Illumination: Quantity

Comparison of American Standard and System International (SI) Lighting units

 The Illumination of Light

• Illumination: Quantity

• The eye responds to a range of illumination levels extending over a million orders of magnitude
from 0.1 lux ( full moonlit night) to 100 000 lux (bright sunshine). Common light levels
outdoor at day and night can be found in the table below:
 The Illumination of Light

• The outdoor light level is approximately 10,000 lux on a clear day.

• In the building, in the area closest to windows, the light level may be reduced to
approximately 1,000 lux.

• In the middle area it may be as low as 25 - 50 lux. Additional lighting equipment is often
necessary to compensate the low levels.

• Earlier it was common with light levels in the range 100 - 300 lux for normal activities.

• Today the light level is more common in the range 500 - 1000 lux - depending on activity.

• For precision and detailed works, the light level may even approach 1500 - 2000 lux. The table
below is guidance for recommended light level in different work spaces:
The Illumination of Light