COLOUR PHYSICS AND MEASUREMENT

(TE 509)

LECTURE 2

COLOUR

is a sensory perception produced in brain

It requires:

• A Light Source

• An Object

• An Observer

Electromagnetic Spectrum

Light in vacuum has wavelengths between 380 to 760 nm

LIGHT

• Perceived COLOUR is due to the Energy of

Photons

• Energy level of Photon is based on Frequency

and Wavelength

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

ULTRAVIOLET

• 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

Infrared Light

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

• 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

• 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)

Photon Energy Vs Wavelength

• 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%

Colours of Light

Colour Wavelength (nm) Violet 390-450

Blue 450-490

Green 490-570

Yellow 570-590

Orange 590-620

Red 620-770

• 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

Blackbody radiation in visible region

• 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

• 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

Spectral distribution of Sources

In 1966, a fourth series of illuminants was adopted,

the D series.

Spectral distribution curve of D65

Spectral distribution curve of TL84

Incandescence

• Electricity runs through the filament

• Electrical energy changes into heat

• Filament emits photons “light”

• Filament incandesces and glows

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.

• 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.

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.

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.

Specular Reflection

Specular reflection is typical of that obtained from a mirror and is highly directional.

Diffuse Reflection

• If most light rays do

not follow the law of

reflection and instead

are reflected in

multiple directions.

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.