LIGHT CONTROL

Light sources that have built in optical systems are called Directional Sources or Reflector lamps. Ar, MR, PAR and R lamps are a few examples. Light sources which lack such in-built systems require external devices to modify the direction of light in order to make them useful for architectural applications.

These modifications serve two purposes. 1. Direct light to the necessary space and 2. Block light where it is unwanted, thereby avoiding glares. This can be achieved through 3 different methods: Reflection, Transmission and Refraction.

Reflection

Reflection occurs when light bounces back from a surface. Light can be controlled by three different kinds of reflection: Specular, Semi-Specular and Diffuse.

Specular Reflection

When light bounces of a highly polished surface Specular Reflection occurs. In this type of reflection just the direction of the light beam is changed. Example of a Specular surface is the mirror. In Specular Reflection the angle of incidence is always equal to the angle of reflection. This property makes Specular surfaces best for directing light to precise places.

Specular surfaces are invisible in themselves, reflecting back whatever bounces off it. This kind of surfaces may at times appear dark or bright depending on surrounding luminance and angle of the view.

Semi Specular (spread) Reflection

These surfaces partially disperse or spread the light beam that strikes it. Semi-Specular objects have an irregular surface, which have been hammered, sandblasted, brushed etc. Like Specular Reflection, these surfaces reflect back the highest intensity of light at an angle, which is very close to the incidence angle.  

These surfaces generally feature highlights or streak of high intensity light over a background of low intensity light. In interior décor they are used to add sparkles. In luminaries they produce a smooth controlled beam.

Diffuse Reflection

Diffuse Reflection occurs when light is reflected from each point of the surface in all directions. The maximum intensity of reflected light is always perpendicular to the surface in such objects. The best natural example of Diffuse Reflection is sand on the beach – there are no bright spots and light is reflected equally in all directions. In interior décor this type of reflection occurs from walls, ceilings and work surfaces. In luminaries it is used to produce wide distribution of light.

Reflector Contours

Specular and Semi-specular objects use geometric shapes to direct light in luminaries. They use the law of reflection to control light, which states that the angle of incidence is equal to the angle of reflection. Specular reflection is the primary method used in luminaries to re-direct light. It takes light which would otherwise be lost or wasted inside the luminarie and directs it outward to the necessary space. In addition it also produces uniform distribution of light in form of room brightness patterns. The most commonly used geometric shapes in luminaries are ellipses, parabolas and circles. These shapes are either used as themselves or slightly modified to suit the luminarie. 

Elliptical Contours

Ellipses have two focal points. A ray of light originating from one focal point is reflected through the second focal point. This causes a divergent beam to emerge. The spread or distribution of light is dependent on the distance between the two foci points. Elliptical Contours are generally found in Downlighters.

Parabolic Contours

The parabola is a form of ellipse where the two foci points are very far apart. The ray of light here is directed parallel to the axis of the reflector. This produces a series of parallel beams of lights. The area of spread is dependent on the light source and the size (diameter) of the reflector. Such Contours are used in search lights, spot lights and where concentrated beam with limited spread is required.  At times the parallel beams are further passed through a diffusing or a refracting lens as used in Reflector lamps.

Circular Contours

Circles too are a type of ellipse, where the two foci points are together. Therefore it is the opposite of a parabola where the points were very far apart. In Circular Contours a ray of light originating at a foci point is reflected through the same point. Circular Contours are used alone or in combination (with another contour) reflectors called Compound reflectors to re-distribute light which would otherwise be wasted due to misdirection or being trapped.

Other Reflector Contours

Compound Reflectors provide maximum beam spread. They are useful to produce uniform light from a place closer to the surface, For example – lighting up low-height ceilings.

There are innumerable other contour shapes that can be mathematically defined and tailored to suit different types of function.

Reflectors

Specular Reflectors

While finding a suitable size and shape, all Reflectors assume a point to be the source of light, when this point is small and compact like that of Incandescent lamps the reflector design becomes easy. However when the source is large like Discharge lamp, where the entire body emits light, reflector design becomes complicated. In earlier times reflectors were made by trial and error method using mathematical equations on geometric shapes. Today manufactures use computers to design even the most complicated Reflectors easily.

Semi-Specular Reflectors

Although Clear Incandescent lamps use specular reflectors to control light direction, the resultant beam features a few irregularities which needs to be smoothen out. These marks/striations generally are the reflection of the filament coil of the bulb. Semi-Specular Reflectors are used to correct such striations by diffusing the beam of light slightly.  This can be achieved through three types of semi-specular surfaces. 1 An inside-frosted lamp. 2. Etched or faceted surface or 3. By adding a diffusing lens.

Diffuse Reflectors

The reflection of light from diffuse surfaces is multi-directional. Thus, shapes or contours do not affect the distribution or direction of light in Diffuse Reflectors. These are at times used in luminaries to provide uniform brightness, though they cannot direct light towards other specified surfaces.

Reflector Materials

Aluminum is the most commonly used material for making Reflectors, as it can be shaped into any geometric contour. It can also be chemically or electronically processed to make it specular, sandblasted or etched to make it semi-specular. Lastly an anodizing process gives it a hard protective surface of high transparency to avoid scratches and abrasions.

Transmission

Transmission of light through a material is affected by two factors: 1. The Rays that the surface reflects and 2. The rays that the object absorbs and re-directs (refraction) within the material. Just like reflection, even the degree of transmission varies, from complete transmission by transparent objects to diffuse transmission by translucent objects.

Direct Transmission

Direct Transmission is seen in transparent objects that leave the light distribution unchanged. These surfaces are used as protective covers to shield out infrared or ultraviolet rays and also to get light of certain colours. Since the light source is visible through these materials (clear glass, plastic etc.) any glare if present in the light source cannot be avoided.

Semi-diffuse (Spread) Transmission

A slight re-direction of the light beam occurs when semi-diffuse materials are used. This happens when light touches the minor irregularities that are present on the surface (by virtue of which these materials get their semi-diffuse properties). These irregularities can be in form of facets and flutes. Other irregularities such as etching, sandblasted, hammering and matte surfaces cause a greater degree of diffusion. Semi-diffuse surfaces conceal the lamp (light source) and also remove glare.

Diffuse Transmission

These surfaces diffuse light in all directions. Opal glass and plastics with microscopic particles that remove directionality of the light beams are two surfaces that allow complete diffusion.

Refraction

When light rays pass through different mediums, the speed of light changes, which causes refraction. Thus a straw in a clear glass of water will appear to bend, as light moves from air to water. A similar refraction occurs when light passes from air to glass or plastic. These materials can be manipulated to change the direction and distribution of light.

Prisms

If light is passed through two refracting surfaces, which are parallel to each other, the two refractions (along the parallel surfaces) cancel each other out.  However if light is made to pass through two similar surfaces, which are not parallel to each other, it will change the direction of the beam.

When light enters a medium which has a higher refraction index (capable of causing a higher degree of refraction), than its initial medium, the direction of change is more perpendicular. For light entering a medium with lower refraction index, the resultant direction is way from the perpendicular.

Prisms are transparent bodies which have non-parallel surfaces. A light beam that enters through one surface, exits in a completely different direction through the other surface. The direction of the exit beam can be controlled by manipulating the angle between the two surfaces.

Lenses

A lens is formed by two opposite refracting surfaces that share a common axis. There are two types of basic lenses Concave and Convex.

The Convex lens takes its name from the word converging, and can be referred to as the positive type of lens. These types of lenses are thicker in the middle than their edges. Convex lens converge diverging light beams into parallel beams.

Concave lens, also known as negative or divergent lens is thinner in the middle and thicker at the edges. It causes parallel light beams to diverge. 

Fresnel Lens, named after its French inventor Agustin Jean Fresnal, consists of a series of concentric lenses. When used in luminaries, they produce a concentrated beam with reduced brightness, thereby controlling glare. In these kind of lenses if the light source is placed on the focal point it will emit parallel beams of light. If the light source is placed in any other place than the focus point, it emits asymmetrical or diffused rays.

Glasses and plastics that feature small prism patterns or other such refractive surfaces are called Prismatic lenses. These lenses increase the distribution/spread of light and reduce the luminance of the light source, thereby controlling glare.

Total Internal Reflection

Total internal reflection occurs when light passes through a transparent surface such as plastic or glass and travels forward by getting continuously reflected within it from side to side. Edge reflection and light transmission through rods are two examples.

Fiber Optics

In fiber optics, light entering one side of the wire is transmitted to the other end through Total Internal Reflection. Light rays are reflected back and forth inside the core and travel from one end to the other. 

Instead of using a single core with a large diameter, fiber optics uses multiple small cores. This gives it more flexibility. In order to avoid light leaking from one core to another, a sheath separator is used that has lower refraction index than the fiber. 

There are two types of optical fibers: coherent and incoherent. In Coherent bundles the entrance and exit cores at both ends are placed exactly in the same way. Since each core transmits light to the same position from one end to another, images can be transmitted through such fiber optics. Incoherent bundles feature a random arrangement of fibers. Hence, these are incapable of transmitting images. 

A typical fiber optics lighting system contains the following: 1. A light projector. 2. A tungsten Halogen or Metal Halide light source. 3. An optical fiber harness. 4. A fitting for each bundle. And 5.Bundles of optical fibers

Glare Control

Sometimes the lens or reflector of the light source can be used to control the glare and conceal the light source. Some ways to achieve this is as follows:

Baffles and Louvers

Baffles

Baffles and Louvers shield light at normal viewing angles, making the experience more pleasant. Baffle is a type of panel that shields light from one direction. If the light source is small Baffles are capable of shielding it all around. Louvers are a series of baffles placed geometrically such that direct light is shielded across all directions.

Such shielding helps to conceal the light source and 

Louvers

reduces glare in a certain zone, this is called the shielding angle. This is the highest angle that the eye can view without seeing the primary light source. However Baffles and louvers do not prevent horizontal surfaces below the light source from reflecting back glare.

Baffles and Louvers can be coated black or made of reflective or transmitting materials, depending on the desired result. To control glare, conceal lamp and get maximum diffusion of light use open louvers, or plastic or glass with a slight degree of diffusion.

Reflectors

An opaque (light-blocking), black Reflector will also act like a Baffle. A reflector’s shape, will affect how the visible surface of the lamp’s interior looks. If the reflector’s surface redirects light towards the eye, it will give high luminance and produce glare. If light is directed downwards away from the eye, it will result in low luminance without any glare.

Most efficient reflector designs incorporate source shielding by extending the surface of the reflector. This increase in depth of the reflector reduces a little efficiency due to light absorption along the extended surface. But since the light that is emitted is controlled, it is much more useful.

Parabolic Reflectors are also used to control glare. In this type of reflectors most of the light is directed downwards away from the eye. This gives it the illusion of lower brightness when seen from normal viewing angles.

Diffusion along the reflector surfaces caused due to etched or brushed surface will increase the luminance of the lamp but will also direct light towards the eye.

Auxiliary Equipment

All discharge lamps and low-voltage incandescent lamps require auxiliary equipment to provide them with electric current to light up.

Auxiliary equipments are of two types, Ballasts and Transformers. Both consume a little amount of power, adding to the total wattage of the Light Source.

Transformers

Transformers are used in low-voltage light sources. These low-voltage lamps work on current between 6 to 24 V (12V being the most common). However a building generally receives anything from 120 to 125 V. Transformers convert the high voltage to low voltage before directing the current to the light source/lamp.

Transformers can be installed within a lighting system or it can be external. With smaller transformers available today, it is possible to embed them into small and compact lighting fixtures. Transformers in Recessed luminaries are hidden out of view above the ceiling. For surface luminaries and pendants the transformer is generally hidden within the lighting system. Where ceiling conditions (i.e. low ceilings) allow pendants too can have their transformers hidden above the ceiling.

Track mounted lighting systems can either have their transformer hidden along the track or at an external place like above the ceiling. If external transformer is used, it is necessary to connect it with the lamps with thick wires, to prevent voltage drop.

High amperage level (high strength of electric current) of low-voltage lamps, limit the number of track lights per transformer. This can be calculated via the formula: Watts divided by Voltage equals amperes (W/V=A). Thus a 50 wattage, 12 volts lamp (60/12 = 4.16amps) draws the same amperage as a 500 watts 120 V lamp (600/120 =4.16 amps).

Two types of transformers are used for low-voltage lamps: Magnetic (core-and-coil) and electronic (solid-state). In Magnetic Transformers copper wire is wound around a steel core. Magnetic transformers are large and heavy, but have a long life span. They also emit noises at times.

Torodial transformers are a type of doughnut-shaped Magnetic transformers, which are relatively quieter than normal magnetic transformer. But they also emit sounds when the dimmer option in lamp is used or when they are used on too many lamps simultaneously. The sound becomes louder if there is a fault in the design of the lamp, as the lamp too tends to resonate with the transformer’s sound.  

Electric Transformers are based on electric circuits. They are quieter, light-weight and more compact. But their life spans are shorter than magnetic transformers and are more likely to fail. They are also incompatible with certain kind of dimmers. However due to their compact size they are used more often than magnetic transformers.

Ballasts

All discharge lamps require Ballasts (incandescent lamps being the only exception). If the electric arc is allowed to form without a regulated supply from the Ballasts, it will draw a large amount of current and thereby burnout in no time. The Ballast regulates the amount of light received by discharge lamps and also the amount of light required for the lamp to start.

Traditionally different types of Ballasts were created for different types of lamps. Where Ballast manufactured for a certain lamp didn’t work with other types of lamps.  Newer Ballasts allow various lamps to work with the same Ballast, and are also capable of operating 2 to 3 discharge lamps at the same time.

In discharge lamps, electricity wattage is dependent on the Ballast. If a 50 wattage lamp is given 500 watts by the Ballast, the lamp will work at 500 watts, but will burnout or experience ballast failure. Lamp wattage for discharge lamps thus represents the wattage at which it is designed to operate and not the wattage it can operate on. Simply changing the bulb wattage of a discharge lamp to higher or lower watts is of no use, as the Ballast also needs to be changed accordingly.

Two-Lamp Ballasts are used to reduce ballast cost and installation cost. These are available in two forms: Two-Lamp Series design and Two-Lamp Parallel design. Two-Lamp Series are used more often as they are more cost effective. In such lamps a single Ballast provides current to two lamps. When one lamp goes off, the other one stops working too.  

Two-Lamp Parallel design lamps feature two ballasts providing current to the two different lamps. These are more expensive, but when one lamp goes off the other keeps working, in this kind of set-up.

Electromagnetic Ballasts

Till 1980’s all lamps featured Electromagnetic Ballasts. These Ballasts convert electric current to the necessary amperes to start the lamp and thereafter to regulate it. The parts of the Ballasts can be divided into three parts: Transformer, Inductance coil and a capacitor.

The transformer converts the voltage received from the electric source to a voltage that is suitable to start the lamp. The inductance coils limits the amount of voltage that can be used by the lamp. Whenever current is changed in a Ballast (inductor) it gives rises to an opposing electromotive force. The capacitor is built to counter this force and bring about the original balance. 

Electromagnetic Ballasts equipped with Capacitors are called High Power-Factor Ballasts.

Power Factor

Power Factor calculates how efficiently a Ballasts converts electricity received by it, to electric current required by the lamp. The Perfect relationship is 100%. The formula used for calculating Power Factor is: Power Factor = Input Watts (w)/ Line Volts (V) x Line Amperes (A).

The Power Factor of Inductive Ballasts lags behind; while Power Factor of Capacitive Ballasts is much better. Magnetic Transformers are Inductive, while Electronic Transformers are Capacitive by nature. Adding a capacitor to Inductive systems can convert them into capacitive systems.

On the basis of Power Factor, Ballasts can be divided into three groups: High Power Factor – Ballasts that have a Power Factor of 90% and above; Power Factor Corrected Ballasts, with 80 to 89% Power Factor; and Low (normal) Power Factor Ballasts, for Ballasts with Power Factor below 79%.

High Power Factor Ballasts use the minimum amount of current needed and can be used to power a number of lamps. Low Power Factor use much more power (twice the level) limiting the number of lamps that it can operate.

Power Factor doesn’t determine the amount of light emitted by the luminarie but the amount of current that can be regulated by the Ballast. As such it is not a determining factor when calculating light output/intensity.

Lamp Ballasts System Efficacy

The Initial-Lumen and Mean-Lumen ratings that you see displayed on lamp boxes are based on lamp operations by a Reference Ballast. Reference Ballast is an instrument used to measure lumen output under laboratory conditions; this measurement forms the Baseline, which helps us to compare various lamps in the market against it. Some commercially manufactured Ballasts operate more efficiently than the Reference Ballasts and a few with lower efficiency.

In practice lamp efficiency of market Ballasts is always lower than that of Reference Ballasts. This is due to electrical resistance, which happens when electric current passes through the core-and-coil. As the core-and –coil transforms a little amount of power into heat energy, while converting electricity into light. This loss in power is called Ballast Loss.

The difference between light provided by Reference Ballast and light provided by market Ballast is called Ballast Factor. Some Ballast have more than one Ballast factor, this occurs when the Ballast can be used for different types of lamps say a standard lamps and an energy-saving lamp.

Ballast Efficacy Factor is the ratio between the Ballast Factor and the power input (watts) of the Ballast.

Electromagnetic Ballasts produce a low level noise called the Hum. The degree of the sound level/hum varies from Ballast to Ballast. For easy identification and selection of Ballasts, manufactures rate Ballasts by their sound level from A to F; where Hum A is the quietest with 20 decibels sound and F the loudest with 49 decibels.

Electronic Ballast

Electronic Ballasts are more efficient, lighter and more compact than Electromagnetic Ballasts. Made of electronic components they convert power to light more efficiently than the traditional core-and-coil set-up. It is to be noted that they are not used to produce more light but instead use less power for the same amount of light, thereby reducing energy costs. Since they consume less power they tend to produce less heat.

HID Electronic Ballasts also regulate and help maintain a good level of colour rendering –CCT and CRI. They provide better voltage regulation to lamps, even when operating with low power input; and also ensure more efficient starts.

The humming sound associated with Electronic Ballasts is 75% lower than Electromagnetic Ballasts. They also operate Fluorescent lamps at a higher frequency thereby increasing efficiency and eliminating flicker.

Fluorescent Dimming Ballasts

Electronic Fluorescent Dimming Ballasts can dim T5 (including T5 high output lamps), T8 and T12 lamps to 1% of measured light (which equals 10% of perceived light). T4 triple and quad lamps, and Twin T5 lamps can be dimmed up to 5% of measured light (or 22% of perceived light) using Electronic Dimming Ballasts.

Fluorescent Heater-Cutout Ballasts

Heater Cut-out electromagnetic Ballasts in Rapid Start Lamps feature an electric circuit which cuts out the electricity supply to the electrode heaters (present in the cathodes) once the lamp switches on. This enables a lamp to consume less power (20%) and save energy.

Other Ballasts

There are various kinds of energy saving Ballasts in additions to the ones discussed. These Other Ballasts fall into two categories: 1. Ballasts that save energy by reducing power (wattage) consumed and reducing light output and 2. Ballasts that save energy by reducing power (wattage) loss in its operation. The latter is achieved through more efficient design.

Class P Ballasts come with automatic thermal protectors, which turn off power if the heat level increases beyond a certain point to avoid overheating. The limit of desired voltage is set by UL (Underwriters Laboratories).

The UL symbol on lamps indicates that Underwriters Laboratories has tested the sample unit of the concerned lamp and that it complies with the standard safety standards. The UL symbol can also appear on Ballasts and Transformers where it indicates that the product measures up to the set safety standards. When applied to a luminarie the UL symbol indicates that the lamp as a whole including its various parts (Ballasts etc) meet the required safety standards.

DISCHARGE LAMPS

Electric Discharge lamps work by passing electric current through vapour or gas.

The light produced by passing electric current through vapour or gas is of a higher intensity than light produced by passing electricity through filaments. Electric Discharge Lamps are more efficient and have a longer life than filament based lamps (Incandescent lamps).

Fluorescent Lamps

Fluorescent Lamps work by passing an arc/wave of electric current through mercury vapour. A pair of Cathodes is located at each end of the tube. When there is sufficient voltage difference between the two (brought about by a Ballast, which is also responsible for regulating the voltage) it creates an arc of electric energy through the mercury vapour, connecting both the Cathodes. This arc in turn increases the energy levels in the mercury molecules present in the vapour causing them to change energy levels, during which they discharge light and heat/UV waves. The generated waves hit the phosphor coating on the inner walls of the tube causing them to fluoresce (become luminous) and emit light.

As light is emitted from the tube walls, the entire length of the tube turns into a light source. The selection of kind of phosphor and other additions determine the type of light emitted: Ultraviolet, Coloured or the commonly seen white light.

There are two types of Fluorescent Lamps, differentiated on the basis of their Cathodes. There are Cold Cathodes and Hot Cathodes lamps. It is to be noted that the two names are very misleading, as Cold Cathodes dissipates more heat than Hot Cathodes.

Cold Cathodes

Cold Cathode Lamp

The Cold cathode lamp is a thimble-shaped cylinder of soft iron, which is sometimes coated with emissive materials. The voltage drop/difference between the cathodes needs to be higher in cold cathodes than their counterpart. Therefore it needs more wattage, which in turn generates more heat. The efficiency of these lamps is low.

Cold Cathode Fluorescent Lamps (CCFL) are easily dimmable and provide instant start. They also have a longer life span than Hot Cathode Lamps. Cold Cathode lamps are generally used for decorative purposes.

Neon Lamps are an example of Cold Cathode lamps. These lights are easily bendable, which makes them apt for signs and artwork. Neon lamps do not have any form of coating on their inner walls, with light being produced simply by exciting the gas molecules.

Hot Cathodes

In Hot Cathodes Lamps the Cathodes are made of tungsten and the tube coated with emissive materials. These lamps are more efficient and are virtually used for all lighting purposes. They give out better output at lower costs than CCFL’s, as wattage required by them is low.

Hot Cathode Lamp

During the first 100 hours of burning Fluorescent Lamps the lumen output drops by 5%. Thereafter, the fall in lumen rate decreases. The published ‘Initial Lumens’ in Fluorescent Lamps refer to the lumen intensity after these first 100 hours.  The lifetime depreciation in light output of the lamp is approximately 15% of the initial lumen. This fall occurs due to the gradual deterioration of the Phosphor coating and evaporation of the electron emissive materials from the Cathodes.

Different colours of light are achieved by using different mixtures of Phosphor. In a few cases an extra filter is required to block out mercury rays which tend to de-saturate the emitted colour. This is generally done for Red and Deep Blue lamps by applying an outer coating to the lamp. Whereas Gold Fluorescent lamps get their colours through subtraction, as no phosphor emits mainly yellow light. A yellow filter coating inside the tube absorbs the unwanted wavelengths from the phosphor. Whenever subtractive filtering is used there is a decrease in luminance. Black Light Fluorescent lamps use a special kind of phosphor that emits ultra-violet rays along with a few visible blue wavelengths.

These Fluorescent lamps are identified by a ‘F’ followed by their wattage, shape, bulb diameter in eighth of an inch and colour. For example in F32T8/RE830: F means Fluorescent Lamp; 32 stands for 32 watts; T8 denotes diameter in eighth of an inch, which is 1 (8/8); RE refers to the use of Rare Earth Phosphor coating, and the first digit 8 in 830 refers to 80 CRI and 30 signifies 3000K colour temperature.  Thus F32T8/RE830 is a small sized Fluorescent lamp with a diameter of 1 inch, which works at 32 watts. It gives out warm white light with emphasis on reds (3000k) with a colour accuracy of 80%. (If the second half of the name read RE765, it would have signified 70 CRI and 6500 K colour temperature which is the colour of cool white light with emphasis on blues)

Lamp Ballast Circuits

Fluorescent Lights require a Ballast to regulate their flow of electricity through their tubes. There are three kinds of Ballast Circuits: Preheat, Instant-Start and Rapid-Start.

Preheat lamps are the oldest type of Discharge Lamps. In this type of lamps the cathodes need to be heated in order for them to emit electrons and form the arc. As this requires heating the cathodes before the formation of the electric arc, these lamps are called Preheat Lamps. This preheating process takes a few seconds and is controlled by an automatic Starter, which switches off once the deed is done. Thereafter electricity is directed to the cathodes to form the electric arc. Once the arc is formed it maintains the cathode temperature.

Instant-Start lamps operate without the Starter. This requires the ballast to direct a sufficient jolt of energy to the cathodes to form the arc instantly. This is a violent action and the cathodes need to have the ability to withstand it. Since preheating is not needed only one external contact is located at each end of most of these lamps. Such Slimpin (lamps with one pin) Lamps can be operated on more than one wattage and voltage as against Bipin (lamps with two pins). Hence, they are identified by length and not wattage. For example F96T12/RE830 is a 96 inch (8 feet) long slimpin lamp with a diameter of 1.5 inches (12/8). It is coated with Rare earth phosphor and has a CRI of 80 and colour temperature of 3000K.

Rapid Start Lamps combine the technology used in Preheat and Instant-Start Lamps. The Ballast has separate windings/sections that heat the cathodes continuously while in operation, switching on the lamp instantly using less voltage than the jolt used in Instant Start lamps. Since the cathodes are continuously heated, these are the only Hot Cathode lamps that can be dimmed or flashed.

Rapid Start Ballast uses less electricity compared to Instant Start and Preheat lamps. They are also smaller and cheaper. At times Rapid Start Lamps are lit using Instant Start Ballast, as the latter does not provide continuous electricity to the cathodes, such arrangement saves 2 to 3 watts per lamp. But at the same time they lose their dimmable and flash features.

T8 and T5 Lamps

For good Colour Rendering (CRI) Fluorescent Lamps require Rare Earth Phosphor, which is expensive than standard Phosphor. This is the reason why smaller diameter (T8 = 1 inch and T5 =5/8 inches) lamps are produced. Smaller diameter means lesser Rare Earth coating and it also places the phosphor closer to the electric arc, increasing efficiency. In addition Rare Earth also increases the lumen efficiency of the lamps. Due to these two factors T8 lamps today have replaced T12 lamps in most applications. As tube diameter decreases and luminance increases a need for better shielding applications arises.

There are three kinds of Rare Earth Phosphor for T8 lamps – RE-70, RE-80 and RE-90. RE-70 has a coating of conventional Phosphor, and a thin coating of Rare Earth triphosphors.RE-80 has a thick coat of triphosphors, along with the conventional coat. RE-90 has four coatings of wide-emission phosphors, along with a blue light filter to reduce the blue wavelengths. Though the filter lowers the lamp’s luminance it lends higher colour accuracy, making the fall in luminance unnoticeable.

The smaller diameter of the T5 adds to the lamp’s optical control and focus. Its small diameter also allows it to produce light at lower wattage, making them more compact. T5 lamps are available only with RE80 Phosphors.

Variations

Standard Fluorescent lamps can be replaced with energy saving Fluorescent lamps, which consume less power and emit lesser light. The fall in wattage is about 10 to 12 % and the fall in light output is between 10 to 20%.

Lamp Variations

T8 and T12 U-bent lamps are regular 4 feet lamps bent in a U-shape. This allows two to three 4 feet lamps to be configured into a 2 feet-square luminaire. Circular or Circline lamps are T9 lamps shaped into a circle. T5 lamps also can be made into Circular lamps.

High output Rapid-Start lamps produce about 45% more light than standard Rapid-start lamps. These lamps are indentifies by Bulb diameter, colour and the letters ‘HO’. For example F96T12/RE830/HO, is a High output (HO) Fluorescent lamp (F), with 96 wattage and 12/8 diameter. It CRI is 80 and has a colour temperature of 3000K. Very High Output (VHO) lamps also work on rapid-start Ballast giving out twice the amount of lamp than the standard Fluorescent lamp, using thrice as much power.

Reflector Lamp

Reflector and aperture lamps contain an inner reflector to provide built-in direction light control. These lamps have an un-reflectorized part known as the window, which is devoid of reflector and coating. The intensity of light that is emitted through the window is high but the overall output of light is low (due to the presence of the reflector). These lamps are used only for special purposes.

Compact Fluorescent Lamps (CFL)

CFL’s have higher efficacy and a CRI of 82. They also have a longer life span with 10,000 to 20,000 hours of life. Their Ballasts features either Preheat or Rapid-start circuits. These lamps have high lumen output due to their thicker Phosphor coating. Their cornered shape and smaller diameter make the thicker phosphor coating a must. 

These lamps use the same Rare Earth Phosphors like T5 and T8 lamps. Their colour temperature varies with the relative balance of the phosphor coating. It can be anything between 2700K (similar to Incandescent lamps) to 4100K Fluorescent Lamps.

There are six different families of CFL’s:

  1.      T4 twin-tube lamps use pre-heat ballasts and a starter. Their Ballasts are cheap and operate on 5 to 7 watts of power

Twin Tube; Quad Tube and Triple Tube

  2.      T4 or T5 quad-tube preheat lamps also feature an integrated starter. They are available from 13 to 26 watts. Featuring four tubes they produce more light than the twin-tube T4 lights.

  3.      T4 quad-tube electronic lamps do not have an in-built starter. Like T4 preheat lamps they are also available from 13 to 26 watts. These lamps can be dimmed with electronic dimming Ballasts.

  4.       T4 triple-tube rapid start/preheat lamps do not include any starter. They operate at 18 to 70 watts. These lamps are frequently used in recessed luminaries replacing incandescent downlighters and wall washers.  These lights too can be dimmed.

  5.      T5 twin-tube rapid start/preheat lamps do not need any starters and operate from 18 to 80 watts. These are high output lamps with increased lumen output. These too can be equipped with electronic dimmers.

Long and Modular Lamps

 6.       Self-ballasted CFL’s are designed to directly replace incandescent lamps. They save more energy and are easier to maintain, consuming ¼ to 1/3 energy used by Incandescent lamps and lasts up to 10 times longer.  They include double folded fluorescent lamps, instant-start electronic ballast and an outer diffuser. There are two kinds of Self-Ballasted CFL’s - Modular and non-modular.

Light Output

The end is reached when the Cathode materials completely dilapidates. When this happens Preheat lamps flash on and off or extinguish themselves; Instant-start and rapid-start lamps extinguish, flicker or operate with very low lumen output.

Lamp Life

Lamp life of Fluorescent lamps varies with different kinds of lamps. Average life of a Fluorescent lamp is based on the average life of a large representative group of lamps tested in the laboratory under controlled conditions. This average span is called ‘Burning Hours’

Preheat lamps have an average of 7500 to 9000 burning hours; Slimline (single pin lamps) 7500 to 12,000 hrs; Rapid start 14,000 to 24,000; High Output lamps 9000 to 12,000 hrs; and very High Output lamps 10,000 to 12,000 hrs.  Every time a hot-cathode lamp is switched on, it dilapidates a certain amount of cathodes, thus the life span of the lamp is also affected by the number of lamp starts. The life of cold cathode lamp is not affected by such lamp starts. The lamp-start (switching on) of Fluorescent lamps is also affected by ambient temperature, with low temperature requiring higher voltage.

Coloured Lamps

Coloured Fluorescent lights vary widely in lumen output. For example 25 red-lamps are required to equal the lumen output of one green lamp. Thus different lamps have different effectiveness when it comes to coloured Fluorescent lights.

Flicker and Stroboscopic Effect

The mercury arc inside Fluorescent lamps operates on 60Hz operating current, going on and off 120 times per second! We see a continuous beam of light because of the phosphorescent (carry on) quality of phosphor which carries on emitting a reduced amount of light when the arc goes off. This cyclic variation of light is called Flicker.

60Hz produces 120 flickers per second along the lamp’s body, but this effect is weaker at the ends of the tube where the flicker rate is 60 per second. The human eye cannot detect the 120 flickers but can detect 60 flickers per second but only by peripheral vision of the retina. That’s why when we see the ends of Fluorescent light from the corner of our eye we can detect it flickering. 

When rapidly moving objects are seen under discharge light, they appear blurred with a ghost like quality. This is known as the stroboscopic effect. Due to this an object moving at high speed under discharge light will appear to move in jerks. Under extreme conditions circling objects may appear to standstill or even to move in the reverse direction.

Stroboscopic effect rarely causes difficulty as modern phosphors have long carry-over periods. If a problem does occur, than the Ballasts rectifies it.

High Intensity Discharge (HID) Lamps

High Intensity Discharge Lamps

HID refers to those lamps where the power density inside the lamp is high. Normally the arc is passed through vapour or gas in standard Fluorescent lamps, as against HID lamps where the arc is passed through a high pressure environment of gases or vapours. There are three types of HID lamps: Mercury Vapour, Metal Halide and High-pressure Sodium Lamps. Working along the same principal as Fluorescent lamps they consist of an arc tube and cathodes with emissive electrodes. The tube is filled with one or more vaporized or ionized metals. In most HID lamps this arc tube is enclosed inside another glass bulb. These lamps have a warm up process which takes between 3 to 7 minutes depending on the ambient temperature.

Each kind of HID lamp is unique. In Mercury Lamps, light is produced by passing the arc through mercury vapour. The electrodes are made of tungsten embedded with emissive materials. The electrodes are heated through an electric discharge which results in emission of electrons and formation of the arc. Once formed the arc’s heat vaporizes the mercury which gives off a poor quality light of greenish hue (15 to 20 CRI). Phosphor coating on the inside of the tube improves its CRI levels to 45 to 50.

In metal Halide lamps, the electric arc is passed through mercury vapour and metal halides. The metal halides give off their respective colour spectrums when heated (colour dependent on the type of metal halides used). The light emitted by metal halides is of better quality with 65 CRI. The addition of Phosphor coating further increases it to 70 CRI.  Pulse-start metal halide lamps have higher inside pressure than normal metal halide lamps, this reduces tungsten evaporation, which in turn stops blackening of the tube giving off better luminance.

In High Pressure Sodium Lamps (HPS) the arc is passed through combined mercury and sodium vapour (with the latter being more dominating). This produces the orange-tint colour that we see on street lights, having 21 to 22 CRI. In these lamps the inner tube is made of Polycrystalline Alumina (PCA) a type of ceramic, which is sodium resistant at high temperature and has a high melting point. By increasing the lamp pressure inside these lamps one gets a CRI of 85 with a colour temperature of 2700 k emitting light similar to that of incandescent lamps.

Ceramic Metal Halide Lamps combine the ceramic arc tube (PCA) technology with metal halide chemistry. PCA allows lamps to work at higher temperature with better colour rendering (CRI) and output. Many high colour rendering HID lamps have shorter life span and lower light intensity than standard HID lamps, but their superior CRI makes them desirable. High Pressure sodium lamps have a life span of 10,000 hrs, and a CRI of 85. Ceramic metal halide high pressure lamps have a life of 6000 to 15,000 hrs, with a CRI between 81 and 96.

Bulb Shapes.

Bulb Shapes

HID bulbs are shaped similar to Incandescent Bulb shapes along with 4 other different shapes especially produced for HID bulbs. They are B (Bulged), BT (Bulged-Tubular), E (Elliptical) and ED (Elliptical-Dimpled). The shapes common to both HID and Incandescent lamps are: A (Arbitrary), PAR (Parabolic aluminized reflector), Reflector (R) and Tubular (T).

HID lamps are identified with a multi-letter code that includes the kind of lamp, wattage, followed by suffixes that may include lamp shape, the diameter in eighth of an inch, outer bulb finish, operating position, base and colour. Example: CMH100/C/U/MED/830 – CMH stands for Ceramic Metal Halide Lamp; 100 equals 100 wattage; C= Phosphor coated; U=burning position universal; MED =Medium Screw Base; and 830 denotes 80 CRI and 3000K colour temperature.

Lamp Operation

Like standard Fluorescnet Lamps HID lamps also need Ballasts to start the arc and maintain it. They use electric Ballast which gives them better colour and longer life. It takes some time for HID lamps to switch on immediately after being switched off.  The inside of the lamp needs to cool sufficiently before the arc can re-strike. The time required for this operation is between 3 to 10 minutes for Mercury lamps; Metal Halides take 10 to 20 minutes; HID Sodium lamps take less than a minute; while instant strike HID lamps are equipped with a second arc which can strike immediately.  

Light Output

Depreciation of light occurs overtime due to loss/erosion  of emissive particles and tungsten from the cathodes. This depends on the number of time a lamp is switched on and off; thus long burning cycles help to elongate the life of these lamps. Other factors that determine light output are operating current and the current wave produced by the ballast design.

Among HID lamps, light output of Metal Halides decrease the fastest. While frequent lamp-starts is also very harmful to metal halides, followed by HPS and it is least harmful to Mercury Vapour lamps.

Lamp Life

The rated average of HID lamps is the time when 50% of a representative group of lamps have burned out and 50% remain burning, under controlled conditions. For example Vapour Lamps have an average of 24,000+ hrs – the plus indicates that 50% of Vapour lamps work beyond 24,000 hrs (with 50% burning out at 24,000 hrs).  Metal Halides have a rated average of 7,500 to 20,000 hrs. And HPS lamps stand at 24,000 hrs. Metal Halides burnout the fastest due to loss of emissive materials during start-up and because of the presence of iodides in the tube. Just like Fluorescent Lamps the first 100 hrs of all HID lamps is also the time when light output depreciate the fastest.

Dimming

It is possible to dim a few HID lamps with special Ballasts, but operating HID lamps below their standard lumen output will produce colour shifts and lower efficacy of the lamp. As wattage decreases the CRI of Metal Halides comes down and matches that of Mercury Vapour lamps; while HPS lamps act like low-pressure Sodium lamps giving out a yellow-amber colour. Mercury Lamps will retain the same CRI, which in any case is very low, but low wattage will affect life span and lumen capacity.

Low Pressure Sodium (LPS) Lamps

LPS Lamps

Technically speaking LPS lamps are not HID Discharge sources but are used for a few similar applications.  They have high lumen output per unit but they emit a very narrow colour spectrum. As such they are never used for interior applications.

This kind of lamp consists of two tubes, one inside the other, and contains a mixture of neon and argon gases. Plus sodium metal in the inner tube. The arc is passed through the gases which cause the sodium metal to heat up giving out a yellow-amber glow. The light thereby produced is monochromatic in nature, emitting only the yellow waves of the spectrum.

As against HID lamps LPS lamps continue to emit the same amount of light throughout their life; nor are their light output affected by ambient temperatures. They continue to work till they fail to start or warm up to full light output.