Attributes of colour: Colour is a matter of perception and subjective interpretation. Even if they are looking at the same object, people will describe it in vastly different words. Verbal expression of colour is too complicated. However, to make it simple, based on its three elements: hue, saturation (chroma) and its lightness, the colour description has been standardized by CIE Lab model sphere.

Hue: This is how we perceive an object’s colour – red, orange, green, blue, etc. The Hue Colour chart (wheel) shows the continuum of colour from one hue to the next.

Saturation (Chroma): It describes the vividness or dullness of a colour — in other words, how close the colour is to either gray or the pure hue.

Lightness: Colours can be classified as light or dark when comparing their value.

CIE Colourimetry: This is the metric of psychophysical colour stimulus (CIE 2006). Colour stimuli can be produced by two fundamental methods: additive and subtractive colour mixing. Basic Colourimetry is built on additive colour mixture (CIE 2006). There are four basic empirical laws of additive colour mixing formulated by HG Grassman in 1853 and used to obtain the colour match (CIE 2006). They are:

  • The three mathematically determinable elements, the hue, the brightness of colour, and the brightness of the intermixed white can be used to analyze the every impression of colour.
  • If one of two mingling lights is continuously altered (while the other remains unchanged), the impression of the mixed light is also continuously changed.
  • The two colours that have same hue and the same proportion of intermixed white, give identical mixed colours regardless of the composition of colours.
  • The total intensity of any mixture is the sum of the intensities of the lights mixed.

Luminous Flux (F, Phi, Lumens): It describes the quantity of light emitted by a light source.

Luminous Intensity (cd, Candela): The luminous intensity describes the quantity of light that is radiated in a particular direction. This is a useful measurement for directive lighting elements such as reflectors. It is represented by the luminous intensity distribution curve (LDC).

Luminance (Cd/m2): Luminance is the only basic lighting parameter that is perceived by the eye. It specifies the brightness of a surface and is essentially dependent on its reflectance (finish and colour).

Illuminances (Lux): Illuminance describes the perceived quantity of luminous flux falling on a surface. It decreases by the square of the distance (inverse square law). Illuminance: E (Lux) =Luminous flux (lm)/ area (m2).

Luminous Efficiency (Lumens/Watt: The luminous efficiency is the ratio of the luminous flux to the electrical power consumed (lm/W).

Life Expectancy (Hours): Although a lamp may continue to function electrically, the light output and the efficiency degrade over time. The life of a lamp is defined as the operating time for light output to fall to 50% of its original lumen figure.


Nominal Power (Watts): This is the power consumption at the lamp input and is typically 10 to 20% below the nominal rating.

The Behaviour of Light

Reflection: Light reflecting off of a polished or mirrored surface obeys the law of reflection: the angle between the incident ray and the normal to the surface is equal to the angle between the reflected ray and the normal. When light obeys the law of reflection, it is termed a specular reflection. However generally, no surface is perfect and hence there are diffused reflections, and in cases where the surface is highly irregular, the reflection could be completely scattered (spread).

Absorption: Beer-Lambert or Bouger’s Law: Absorption by a filter glass varies with wavelength and filter thickness.

Refraction/ Total Internal Reflection: Snell’s Law: When light passes between dissimilar materials, the rays bend and change velocity slightly, an effect called refraction. Refraction is dependent on two factors: the incident angle, q, and the refractive index, n of the material, as given by Snell’s law of refraction.

Diffraction: Diffraction is another wave phenomenon that is dependent on wavelength. Light waves bend as they pass by the edge of a narrow aperture or slit.


Interference: When wavefronts overlap in phase with each other, the magnitude of the wave increases. When the wavefronts are out of phase, however, they cancel each other out.

Manipulation of Light


Diffusion: It is often necessary to diffuse light, either through transmission or reflection. Diffused transmission can be accomplished by transmitting light through roughened quartz, flashed opal, or poly tetrafluoroethylene (PTFE, Teflon). Diffusion can vary with wavelength.

Collimation: Some lamps use collimating lenses or reflectors to redirect light into a beam of parallel rays. If the lamp filament is placed at the focal point of the lens, all rays entering the lens will become parallel. Similarly, a lamp placed in the focal point of a spherical or parabolic mirror will project a parallel beam.

Focusing Lenses/ Mirrors: Lenses & mirrors are often employed to redirect light or concentrate optical power. Simply put, all rays parallel to the optical axis pass through the focal point. Since the index of refraction is dependent on wavelength, chromatic aberrations can occur in simple lenses. The most common lenses are Concave & Convex Lenses.

Prisms: Prisms use glass with a high index of refraction to exploit the variation of refraction with wavelength.

Diffraction Gratings: Gratings rely on interference between wavefronts caused by microscopically ruled diffraction lines on a mirrored surface.

Reflection, Transmission & Mirror Losses: Reflection is the process by which light is returned either at the boundary between two media (surface reflection) or at the interior of a medium (volume reflection), whereas transmission is the passage of electromagnetic radiation through a medium. Both processes can be accompanied by diffusion (also called scattering), which is the process of deflecting a unidirectional beam into many directions. In this case, we speak about diffuse reflection and diffuse transmission. Where Mirror Losses is the lost which happens since all reflective surfaces have conduction electrons that are responsible for the reflection process. As they move and react to the light’s electric field, they lose a fraction of energy due to the non-zero resistivity hence reflected light has a bit of lesser energy that the falling light.

Principles of Light Manipulation

The Inverse Square Law: The inverse square law defines the relationship between the irradiance from a point source and distance. It states that the intensity per unit area varies in inverse proportion to the square of the distance. E = I/ d2 In other words, if one measures 16 W/cm2 at 1 meter, he will measure 4 W/cm2 at 2 meters. Distance is measured to the first illuminating surface – the filament of a clear bulb, or the glass envelope of a frosted bulb.

The Inverse Square Law
Lambert’s Cosine Law

Lambert’s Cosine Law: The irradiance or illuminance falling on any surface varies as the cosine of the incident angle. The perceived measurement area orthogonal to the incident flux is reduced at oblique angles, causing light to spread out over a wider area than it would if perpendicular to the measurement plane.

Lambertian Emission and Reflection: A lambertian surface reflects or emits equal (isotropic) flux in every direction, however as the reflections follow the cosine law – the amount of reflected energy in a particular direction (the intensity) is proportional to the cosine of the reflected angle. Since both intensity and apparent area follow the cosine law, they remain in proportion to each other as the viewing angle changes. Therefore, luminance remains constant while luminous intensity does not.

Evolution of Light Sources: Fire (B~.C. 1.4 million years) was probably the first artificial light and heat source by humans and also used for cooking as well as to make tools. Then came the wicked candles (3000 B.C.) used by Egyptians and ancient Romans. Fire Torch (B.C.1300) was the next stage when early humans were found to burn wooden log and use it as a simple carrier of light and fire. A major improvement in candles came in the middle ages when beeswax candles were introduced in Europe having no bad smell & smoke. Slowly oil lamps started appearing (& can be found even today) and by early 1790, William Murdoch was the first man who used flammable gas as lighting when he began experimenting with various types of gas and finally settling on coal gas as the most effective.

After discovery of electricity in lightning by Benjamin Franklin in the year 1752, it was a matter of time when electricity was to become a proven source of lighting. As in 1705 Francis Hauksbee, had already demonstrated a gas-discharge lamp using static electricity. By 1801, Humphry Davy did develop an arc lamp for lighting. With more development came the famous incandescent bulb and the first person to make a working light bulb was Humphry Davy himself but it was neither bright enough nor did it last long enough to be practical – but it laid to the foundation for modern light bulbs. By 1880, it was Englishman Joseph Wilson and American Thomas Alva Edison invented light bulbs within the same year. Later, Thomas Edison went on to improve the light bulb with a much longer lifespan making it an industry standard. After which there has been no looking back and the development of light sources has been at an increasingly fast rate and with improved efficacies and also the light output.

Performance Changes/ Improvement of Light Sources

The lamps convert energy into visible light, which also depends upon the type of lamps and in most of the lamps if the energy is wasted in to heat (convection & conduction) as well as in generation of other radiations like UV & infrared and the actual energy used for light output is very less (Energy Conversion Efficacy). The chart below provides the Sankey Diagram of growth of the lamps.

Energy Balance Diagram of Few Lamps (Sankey Diagram)

Understanding Light Distribution Curves

They are also known as photometric polar diagrams and the name points out, it is a visual representation of the light diffused by a luminaire. This graph tries to transpose a three-dimensional concept (the light diffused by a lamp or fixture in a 3D space) onto a two-dimensional medium (a sheet of paper or a computer screen).

Lamp Wise Electricity To Light Conversation Ratio

At first glance, a light distribution curve may look quite complicated but they are (relatively) simple to read. To understand it, let’s start from the middle of the diagram (see below). This marks the lamp‘s position. Usually, you will see two lines radiate from the center, a solid line and a dotted line. The solid line indicates the frontal view (C00/1800), the dotted line the side view (C900/2700). The shape of both lines may differ based on the profile of the lamp or its fixture. When the two curves are the same (Symmetrical Light Distribution) for any lamp or fixture, both of these profiles overlap and hence the dotted line is invisible.

As stated above in this article, luminous efficacy of any light source is measured in ratio of emitted light (lumens) divided by the electrical power it draws (watts). A light source that is 100% efficient at converting energy into light would have an efficacy of 683 lm/W. To put this in context, the lumen/ watt ranges of various lamps are given hereafter.

…To be continued

Prabhat Khare possesses a BE (Electrical) degree from IIT Roorkee (Gold Medalist). Now, he is the Director of KK Consultants.

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