COLOUR PHYSICS AND MEASUREMENT (TE 509) LECTURE 2
COLOURis a sensory perception produced in brainIt requires: • A Light Source • An Object • An Observer
Electromagnetic SpectrumLight 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
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
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)
• 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%
• 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
• The Sun produces light with a Colour Temperature at around 5800 K• Light from Sun gets reflected and refracted by the earths 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
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.
RefractionWhen light passesthrough materials ofdifferent densities,the velocity of thelight changes slightlyand this causes abend in the ray at theinterface between thetwo 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 ReflectionSpecular reflection istypical of that obtainedfrom a mirror and is highlydirectional.
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 = εclA = absorbance or optical density.c = concentration.l = path length or thickness.ε = extinction coefficient.