4. LIGHT
• Perceived COLOUR is due to the Energy of
Photons
• Energy level of Photon is based on Frequency
and Wavelength
5. Ultraviolet Light
• The ultraviolet part of the solar spectrum has
several beneficial effects in the overall
environment but it may also be harmful if UV
exceeds ”safe” limits
• If the amount of UV radiation is sufficiently high
the self-protection ability of some biological
species is exhausted and causes severe
damage
• This also concerns the human in particular the
skin and eyes
7. Infrared Light
• Infrared light contains the least amount of
energy per photon of any other band
• An infrared photon often lacks the energy
required to pass the detection threshold of a
quantum detector
• Infrared is usually measured using a thermal
detector
8. LIGHT INTENSITY
• Number of Photons hitting an area over time is
Intensity
• There is a difference between Light Output of a
light source and the intensity of light reaching
the surface
• LUMEN (lm) – measure of power of visible light
• Lumen is the photometric equivalent of watt
9. • Yellowish-green light receives the greatest
weight because it stimulates the eye more than
blue or red light of equal photometric power.
1 watt at 555 nm = 683.0 lumens
• The human eye can detect a flux of about 10
photons per second at a wavelength of 555 nm.
• However, a lumen does not measure intensity. It
is generally used to measure light output
10. • watt (W), is the fundamental unit of optical power
and is defined as “rate of energy of one joule (J)
per second”.
• Optical power is a function of both the number of
photons and the wavelength. Each photon
carries an energy that is described by Planck‟s
equation:
Q = hc / l, where:
Q is the photon energy (joules),
h is Planck‟s constant (6.623 x 10-34 J s)
c is the speed of light (2.998 x 108 m s-1)
l is the wavelength of radiation (meters)
12. • The loss of intensity due to distance is
predictable and is known as “Inverse Square
Rule”.
• The Inverse Square Rule states that:
“the light intensity will be in inverse
proportion to the square of the distance
from the light source”.
• That is, if the distance from light source is
doubled, the intensity will be reduced to 25%
13. Colours of Light
Colour Wavelength (nm)
Violet 390-450
Blue 450-490
Green 490-570
Yellow 570-590
Orange 590-620
Red 620-770
14. • Colour Temperature is based on
radiation from a theoretical black body
• The Colour Temperature of the light
produced by the black body is actually the
temperature of the body in Kelvin
• The colour temperature describes the
spectrum of the light and the relative
quantities of different wavelengths
17. • The Sun produces light with a Colour
Temperature at around 5800 K
• Light from Sun gets reflected and refracted by
the earth's atmosphere, the actual colour
temperature of the Sun will vary with different
conditions
• At noon, on a clear day, the direct light from the
Sun is around 5500 K, but with the light from the
sky included, it is around 6500 K. For this
reason 'Daylight' is usually defined as 6500 K
18. • In 1931, in order to define the artificial light
sources used in colour evaluation, the
Commission Internationale de l‟Éclairage (CIE)
established three „standard‟ illuminants
• These three standards have spectral
characteristics similar to natural light sources
and are reproducible
• A = Indoor artificial illumination, 2856 K
• B = Daylight plus sunlight, 4870 K
• C = Average daylight, 6770 K
24. Incandescence
• Electricity runs through the filament
• Electrical energy changes into heat
• Filament emits photons “light”
• Filament incandesces and glows
25. Fluorescence
• Electrodes at both end of the phosphor coated
tube.
• Gas containing Argon and Mercury vapors
inside the tube.
• A stream of electrons flows through the gas from
one electrode to other.
26. • Electrons interact with mercury atoms and these
atoms get excited.
• When back to ground state, mercury atoms
release photons “ultraviolet region”.
• These photons hit phosphor atoms.
• Phosphor fluoresces light.
27. Refraction
When light passes
through materials of
different densities,
the velocity of the
light changes slightly
and this causes a
bend in the ray at the
interface between the
two materials.
28. Reflection
• Light reflects off a
mirror or similar
surface, the rays
reflecting off the
surface will exit at
the same angle
on the other side
of line
perpendicular to
the surface as
the incident rays.
30. Diffuse Reflection
• If most light rays do
not follow the law of
reflection and instead
are reflected in
multiple directions.
31. Transmission: Beer Lambert Law
• Light absorption in transparent material is
related to concentration of colourants in the
material and the thickness of material.
A = εcl
A = absorbance or optical density.
c = concentration.
l = path length or thickness.
ε = extinction coefficient.
