Introduction:

Light by definition connotes Electromagnetic radiation that has a wavelength in the range from about 4,000 (violet) to about 7,700 (red) angstroms and may be perceived by the normal unaided human eye. In fact in the prehistoric days, all human activities were coordinated with Sunrise and Sunset. Today, in principle activities are carried out round the clock. All this is made possible because of Artificial Lighting systems. The lighting systems comprise of a source employing any physical phenomenon among.

Incandescence, Electrolumniescence or Flourescence. Some control scheme and a Luminaire. In fact all this has lead to a class of professionals called Lighting Engineers or Illumination Engineers. Unlike other group of professionals they need to be adept at not only at exact sciences of Maths, Physics, Chemistry; but be wary of Physiology and Psychology of users (like a medical professional); have good aesthetic sense and economically utilize resources.Efficacy of these systems is talked in terms of Illuminance per Watt of energy consumed. Efforts are on to reduce energy conmsumption yet have efficient Illumination to enhance productivity. Need less to mention that all these sources employ electrical energy.Trend these days is to employ, modern electronic controls together with energy efficient lamps.These aspects are borne in mind, right from the planning stage of a building. As electrical energy is being used for the purpose, it becomes important for Illuminating Engineer to come up with an integrated system for the complete electrical system of a building.

Importance of lighting:

Humans depend on Light for all activities. Light is a natural phenomenon, very vital for existence, which is taken for granted. In fact, Life involves day night cycles beginning with sunrise and ending with sunset. Pre-historic man had activities limited only to day time. Artificial light enables extended activity period employing in a planned optimized manner, minimizing the resources.

Vision is the most important sense accounting for 80% information acquisition for humans. Information may be acquired through sun/moon light (direct/ reflected) or by using artificial light (closest to natural light). Before we go any further, it is worth looking at Teichmuller’s definition for lighting. “We say the lighting is good, when our eyes can clearly and pleasantly perceive the things around us”. Therefore Artificial light should be Functional and pleasant both physiologically and psychologically. This is often achieved employing multiple sources. It must be borne in mind that the sources should be economic and energy efficient. As all of us are aware, all sources today employ electrical energy.

Electrical energy is supplied as a.c. (alternating current) or d.c. (direct current). Usually electric power supply is a.c. in nature, either single phase or three phases. It must be borne that close circuit is a must for current flow. As it is well known losses exist in all electrical circuits or lines.

Properties of good lighting scheme:

1. Incandescence

Thermo luminescence is by definition radiation at high temperature. The sources employing this process are Incandescent Lamp, Gas Lamp, (flames and in oil Lamps and wax candles). They lead to a continuous spectrum of radiation.

2. Luminescence:

Luminescence Electro luminescence by definition Chemical or Electrical Action on gases or vapour radiation. Here color of radiation depends on the material employed. Usually this process leads to Line or Band Spectrum.

3. Fluorescence:

Fluorescence is a process in which radiation is absorbed at one wavelength and radiated at another wavelength eg: UV impinging on Uranium – Fluorescent oils. This re radiation makes the light radiated visible.

4. Phosphorescence:

Phosphorescence is a process when energy is absorbed at some time and radiated later as glow. Examples of this process are luminous paints that contain calcium sulfide that lead to Phosphorescence. They produce light Radiation after exposure to light.In practice good efficient lighting is obtained by combining Luminescence and Fluorescence. Fluorescent lamp is Luminescent source of low luminous value activating Fluorescent surfaces which lead to visible radiation. Here intensity depends on gas or vapor involved and phosphor material. However, the temperatures o f t h e m a t e r i a l p l a y a role in radiation.

Lumen

The lumen (symbolized lm) is the International Unit of luminous flux. It is defined in terms of candela steradians (cd multiplied by sr). One lumen is the amount of light emitted in a solid angle of 1 sr, from a source that radiates to an equal extent in all directions, and whose intensity is 1 cd.

Luminous intensity

Luminous intensity is an expression of the amount of light power emanating from a point source within a solid angle of one steradian.

Laws of illumination:

The original standard of light was Wax Candle, which is highly unreliable. It was replaced by a Vaporized Pentane Lamp. This is equal to10 original Candles. In the year 1909, Incandescent Lamp was taken as standard by comparison with a Pentane Lamp. Thing to be kept in mind is Primary Standard should be reproducible.It was in1948, Luminous Intensity; based on Luminance (objective brightness) of a small aperture due to Light from a Radiator maintained at1773°c i.e. Solidification temperature of platinum was adopted as Standard. It consists of:

1. Radiator – Fused Thoria – Thorium Oxide. 45mm long internal dia of 2.5mm. Packed with Fused Thoria Powder at the bottom.

2. Supported Vertically Pure Platinum in a Fused thoria crucible with a small aperture of 1.5mm in a large refractory container.

3. Platinum melted by a High Frequency Eddy current. Luminance = 589000 Candles /m2 =600 000 units.