LIGHT AND LIGHTING FUNDAMENTALSPrepared by Engr. John Paul Timola

LIGHT

• a form of radiant energy from natural sources and artificial sources.

• travels in the form of an electromagnetic wave, so it has wavelength and a known speed.

• can be reflected and refracted like other electromagnetic radiation.

LIGHTING

• the application of light to illuminate objects, surfaces, scenes, pictures and people.

• Since it is an application, it is both a science and an art.

• Science, because it makes use of the science of light and employs methods and techniques developed through time.

• It is an art because the personal taste (preference) and artistic sense of the designer and owner greatly influence the manner by which lighting is applied.

Radiant Energy, Light and Color

• Light is that portion of the electromagnetic spectrum to which the eye responds.

Radiant Energy, Light and Color

• Visible energy is a small part of the total spectrum, which ranges from cosmic rays with extremely short wavelength to electric power frequencies with wavelength in hundred kilometers

• The visible portion lies between 380 and 770 nanometers.

• Nanometers --- a unit of wavelength equal to 1 x 10-9

meter or one- billionth of a meter.

• The color of light is determined by its wavelength.

• Visible energy with the shortest wavelengths (380 to 450 nm) produces the sensation of violet

• Those with longest wavelengths (630 to 770 nm) produce sensation of red.

• In between light blue (450 to 490 nm), green (490 to 560 nm), yellow (560 to 590 nm), and orange (590 to 630 nm).

The Electromagnetic Spectrum

• Region with slightly longer wavelengths immediately adjacent to the red end of the visible spectrum is known as the infrared

• Region with slightly shorter wavelengths immediately adjacent to the violet end of the visible spectrum is the ultraviolet.

Human Visual System

• responds to the very small part of the electromagnetic spectrum that lies between 380 and 760 nanometers.

• However, it does not respond uniformly. Why?

• Given the same output of power at each wavelength, the visual system will sense the yellow-green region as the brightest and the red and blue region as the darkest.

Light Source

• most of its power in the yellow-green area

• have the highest visual efficiency(highest lumens per watt)

• will not be able to render colors satisfactorily without a reasonable proportion of red or blue in its output

• With most sources, the wider the range of wavelengths present, the lower the efficiency.

How to see colors

• depends on the wavelengths emitted by the light source, the wavelengths reflected by the object, the surroundings in which we see the object and the characteristics of the visual system.

How Colors Mix in Pigment

• In pigments, a primary color is defined as one that subtracts or absorbs a primary color of light and reflects or transmits the other two.

• So the primary colors in pigments (sometimes called subtractive primaries) are magenta, cyan, and yellow –the secondary colors of light.

How Colors Mix in Light

• The primary colors of light (red, green, and blue) can be added to produce the secondary colors of light–magenta (red plus blue), cyan (green plus blue), and yellow (red plus green).

• Thus, colors of light are called “additive”. A secondary color of light mixed in the right proportions with its opposite primary will produce white light.

• Thus, yellow and blue are complimentary colors of light as cyan and red, and magenta and green.

How Colors Mix in Light versus Pigment

• Additive (Light) Subtractive (Pigment)

The Eye and Vision

The Eye and Vision

The Eye

• The rays of light enter the eye through the Cornea, which is the transparent membrane that bulges out at the front of the eye. They then pass through the Pupil, which is a round opening in the colored Iris.

• The eye reduces the size of this opening to limit the rays of light to the central and optically best part of the Lens, except when the full aperture is needed for maximum sensitivity.

• The pupil also closes for near vision to increase the clarity of near objects.

• It can change the area of the opening over a ratio of about 16:1 although the eye works efficiently over a range of brightness of about 1,000,000:1.

• The ability of the eye to adjust to higher or lower levels of luminance is termed Adaption.

• The whole eye is filled with a jelly like substance and the rays pass through this onto the lens, which focuses the image.

• Muscles around the lens make it fatter or thinner so the eye can focus sharply on distant or close objects. This ability is called Accommodation and ensures that sharp, clear image is focused onto the light sensitive cells of the Retina.

The Eye

• We don't "see" with the retina—it is only when the information it collects has been transferred along the Optical Nerve to the brain that a conscious visual image is formed and this is the time we "see".

Rods and Cones in the Retina

• The retina has two basic types of Receptors—Rods and Cones for collecting this information. By a chemical process in the retina, the eyes are able to work over the enormous range of brightness we see.

Rods and Cones in the Retina

• Cones can differentiate between the different wavelengths of light and therefore enable us to see in color. The rays of light are not actually colored. The more sensitive rods only give us black and white vision.

Vision

Vision

• Photopic or daytime Adaptions

— vision when the cones operate during the day and nominal daylight conditions, enabling us to see in detailed color.

• Mesopic vision

• — If light conditions are not bright, as the rods can only "see" a black and white image, the overall impression is much less brightly colored.

• Scotopic, or nighttime Adaptions

— At even lower levels, much lower than the average street lighting or moonlight, the cones cease to function.

— The eye losses all its facility to see in color and the rods take over giving completely black and white vision.

Relative Spectral Sensitivity of the Eye

• These different adaptions are important because not only does the eye discriminate between different wavelengths of light with the sensation of color, but it is also more sensitive to some wavelengths than others.

• For Photopic vision, the eye has peak sensitivity at 555nanometers, which is yellow-green color.

• Scotopic vision, peak sensitivity moves to 505 nanometers, which is blue-green light, although the vision is in terms of black and white.

• The Mesopic vision peak will be somewhere between the two.

Test your self

1. The rays of light enter the eye through the ______.

2. The ability of the eye to adjust to higher or lower levels of luminance is termed ______.

3. The vision when the cones operate during the day and nominal daylight conditions, enabling us to see in detailed color is ______.

4. A unit of wavelength equal to 1 x 10-9 meter or one- billionth of a meter is the ______.

5. Receptors that can differentiate between the different wavelengths of light and therefore enable us to see in color is the ______.

Definitions

Transmission

• When light passes through an object, it is called transmission

Reflection

• process by which electromagnetic radiation is returned either at the boundary between two media (surface reflection) or at the interior of a medium (volume reflection)

Diffusion

• Both transmission and reflection 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

Refraction

• bending of a wave when it enters a medium where its speed is different

Absorption

• Instead of completely transmitting the light, some material can absorb part or all of the incident light, converting it usually into heat.

• Many materials absorb certain wavelengths while transmitting others.

• This is referred to as selective absorption.

Filters

• Transmissive filter

a material that absorbs some wavelengths and transmit others

• Reflective filter

a material that absorbs some wavelengths and reflects others

Generation of Light

Phenomena for the generation of light 1. Incandescence

2. Electric discharge

3. Arc formation

4. Electro-luminescence

5. Photo-luminescence

Incandescence

• Lat, “to glow white”.

• Method of producing artificial light by applying heat or raising the temperature of a solid body or vapor.

Incandescence

• A body which is gradually heated will begin to radiate energy in the surrounding medium in the form of electromagnetic waves of varying wavelengths.

• Initially, at lower temperatures, the radiated energy will only be in the form of heat waves.

• Further increase in temperature will cause light waves to be radiated out and the body becomes luminous.

Incandescence

• Increasing the temperature will cause the color of the light to change from bright red, to orange, to yellow and then to white if the temperature is high enough.

• The higher the temperature, the shorter the wavelengths of the visible radiation becomes.

• Note - heat energy is generally considered as wasted energy.

Electric Discharge

• When the electric current is passed through a gas, the atoms and molecules emit radiation whose spectrum is characteristic of the elements present

Electric Discharge

• Electrons fall from high to low energy levels resulting in infrared, visible light or ultraviolet radiation

Electroluminescence

• Light is generated when electric current passes through certain solids such as a semiconductor or phosphor material.

• Applications include automotive dashboard lighting, LCD display back lights, advertising billboards etc.

Photoluminescence

• Radiation at a specific wavelength is absorbed, usually by a solid, then re-emitted at different wavelength.

• When the re-emitted radiation is visible, it is referred to as fluorescence or phosphorescence.