Light
Light is the natural agent
that stimulates sight and
makes things visible.
Light is electromagnetic
radiation that the unaided
human eye can perceive
and having a wavelength in
the range of 370- 800 nm
and travelling at a speed of
2,99,972 km./sec
Light and Perception
Light reveals to our eyes the
shape, color and texture of
objects in space
Light radiates equally in all
directions and spreads over
a large area as it emanates
from its source
As it spreads it also
diminishes in intensity
Light
Natural light consists of seven colors
having different wavelengths. The
average human eye is most sensitive
to a wave length of 5500 0A.
The relative sensitivity of eye for a
particular wave length is the visual
effect produced by the light on the
average human eye as compared
with the effect of light having wave
length 5500 0A on human eye.
This is also known as Relative
luminosity Factor.
Light Sources
Light entering a space can
either be NATURAL or
ARTIFICIAL
Natural Light can be either
direct sunlight or
diffused daylight
Artificial Light is produced
by various sources like
incandescent lamps,
fluorescent lamps, LED’s
, Optic fibers, CFL’s etc
Lighting Terms
Candela: luminous intensity of a source that emits
monochromatic radiation at a frequency of 540x
1012 Hz and has a radiant intensity of 1/683 watts/
stradian
Luminous Intensity: is the rate of flow of visible
light per unit of time. Expressed in Lumens.
Illumination: is the intensity of light falling at any
given point on a lighted surface. Illumination =
luminous flux incident per unit area.
Lux is the SI unit of illumination
Lighting Terms
Luminance: is the quantitative measure of the
brightness of a light source or an illuminated surface
luminance = luminous intensity per unit projected area
of the source or surface from a given direction. Unit of
luminance is Lambert
Brightness: is the sensation by which the observer
can distinguish between difference in luminance
To perceive shape and form, Contrast of brightness
ratio is required
Lighting Terms
Lighting Terms
Lighting Terms
Lighting Terms
Lighting Terms
Whole of the light incident on a reflecting surface is
not reflected. Some portion of it is absorbed by the
surface.
The ratio of the reflected light to the incident light is
called reflection factor.
Lighting Terms
When the contrast or brightness
ratio is too high, Glare can result
Glare: is the sensation
produced by any brightness
within the visual field that is
sufficiently greater than the
luminance to which the eye can
adjust , resulting in discomfort
Glare can be either Direct or
Reflected/ Indirect
Electrical Lighting
Daylight varies with season, time of the
day, latitude and weather conditions.
To compensate for these variations we
rely on Electrical light sources also
termed as LUMINAIRE
Luminaire: is a light fixture
consisting of one or more electric lamps
with all the necessary parts and wiring
for positioning and protecting the
lamp, connecting the lamp to the power
source and distributing the light
Electrical Lighting
Luminaire = Lighting fixture which
includes the following:
Lamps
Lamp sockets
Ballasts
Reflective material
Lenses, refractors, louvers
Housing
Lamps
Choice of light source (LAMP) influenced by:
Color
Heat generation
Energy usage
Cost
Technical Terms associated with
Luminaire
Lamp Color Temperature: is an indication of the lamps
color appearance i.e. Yellow, white , blue- white etc.
Guide to whether the light source is regarded as
warm, mid range or cool
Measured in Kelvin’s (K)
Higher the K value/ color temperature, cooler the light
source
Color Temperature
Light Source Color Color Temperature
Quality
Candle Flame Warm 1750 K
Incandescent Medium 2600 – 3000 K
Lamp
Fluorescent Cool 4250K
Lamp
Clear Blue Sky Very Cool 10,000 K
Technical Terms associated with
Luminaire
Color Rendering Index (CRI) is a measurement of the
color rendering capability of a lamp and is useful for
comparing lamps with the same or similar color
temperature
Indicated as a percentage
CRI of 80% and higher indicates that there is little shift in
test colors when illuminated by a lamp as compared with
a reference light source of the same color temperature
Eg. Tungsten lamp at a color temp of 3200 deg. K, noon
sunlight at 4800 deg. K and avg. daylight with a color
temp. of 700 deg. K have a CRI of 100% and render
colors perfectly
CRI
A relative scale indicating how
perceived colors illuminated by
the light source match actual
colors. The higher the number
the less color distortion from the
reference source.
85 -100 CRI = Excellent color rendition
75 - 85 CRI = Very Good color rendition
65 - 75 CRI = Good color rendition
55 - 65 CRI = Fair color rendition
0 – 55 CRI = Poor color rendition
Technical Terms associated with
Luminaire
Efficacy is a measure of the relationship between the
amount of light and the amount of heat produced by
both daylight and electric light source
Efficacy expressed as ratio of lumens provided to watts
of heat produced by a light (lumens/ watts)
Source Efficacy (lm/w) Source Efficacy (lm/w)
Candle 0.1 Metal halide 80-125
Incandescent 8-22 lamp
Lamp High pressure 55 - 115
Tungsten 18-22 Sodium lamp
Halogen Lamp Induction Lamp 48- 70
Fluorescent 35-80 Sulfur Lamp 90 - 100
Lamp
Sunlight 90 - 115
CFL 55-75
White LED 20 - 60
Efficacy
Coefficient of Utilization
A surface to be illuminated receive light either directly
from the lamps or reflected from the ceiling and walls
or both.
In this case, the total flux reaching the surface will
never be equal to the flux emitted by the lamp, due to
absorption by reflectors, ceiling and walls.
Utilization factor = lumens reaching at the working place
total lumens emitted by the source
Usually it varies from 0.5 to 0.8.
Lamp Rated Life
No. of hours the lamp is rated as being efficient
Beyond the rated life, the efficiency of the lamp drops
Lamps with higher rated life are usually costly, but last for a
longer duration
As lamps age, they lose a certain amount of output.
Eg. Old T12 fluorescents can lose up to 30% of output over
their life.
This depreciation must be accounted for when installing new
lighting system.
Lamp Rated Life
No. of hours the lamp is rated as being efficient
Beyond the rated life, the efficiency of the lamp drops
Lamps with higher rated life are usually costly, but last for a
longer duration
As lamps age, they lose a certain amount of output.
Eg. Old T12 fluorescents can lose up to 30% of output over
their life.
This depreciation must be accounted for when installing new
lighting system.
Electrical Lighting
Electrical lighting has following
advantages :
1. Easy to control
2. Economical
3. Easy to handle
4. Steady output
5. Better reliability
6. Suitable for almost all purposes etc.
