Reflections, Diffusion and Refraction

Reflection
Whenever light falls onto surface, some of it is absorbed and the remainder is either reflected or transmitted. If the surface is opaque and smoothly polished, the specularly reflected light leaves the surface at the same angle as is arrived (as a billiard ball striking a cushion), and by suitably shaping the surface it is possible to redirect the light in any desired direction (e.g. a motor car headlight, with lamp placed at the focal point of a polished parabolic mirror directing most of the light forward).

Diffuse reflection occurs from matt surfaces. The light is reflected most strongly at right angles to the surface (whatever the direction from which the light arrives) and progressively more weakly at other angles. Matt surface show no highlights. Most painted and many other surfaces are partly specular and partly diffuse reflectors of a light and are classified according to which type or reflection predominates.

Diffusion
Light passes straight through a transparent material, but it scattered or diffused to a greater or lesser extent in a translucent material. Flashed opal glass or its plastics equivalent scatters it completely so that it emerges in all directions, and complete concealment of lamps behind a panel of this material is easy achieved. Frosted glass diffuses the light less perfectly, so that it emerges mainly in the same general direction as when it entered the glass; in effect, it is usually possible to see vaguely the positions of lighted lamps behind frosted panels. Hammered and rolled glasses and clear plastics with a similar finish generally have less diffusing and concealing power than frosted glass but have a sparkle that may be preferred in many cases.

Refraction
If light passes through a transparent material which does not have parallel side, it will be bent away from its original direction by a process known as refraction. Ribbed glass or plastic fittings in which each rib is a carefully designed prism can therefore be made to control light very accurately in a required direction, and this principle is very widely used in electric street lighting fittings.

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Lighting and common terms used in lighting design

Virtually all buildings have electric lighting which serves two purpose. It help us to recognise objects quickly and in sufficient detail to learn all we need to know about them, and it contributes to making buildings safe and pleasant place in which to work or take part in other activities.

There must always be enough light to make object visible but other factors are no less important. The directions from which light come, the brightness and color contrasts created between details of interest and their background, the presence or absence of bright reflections in the part of the object being looked at, and changes in color resulting from the type of lamp used can all effect ease of recognition.

Some of the more common terms used in lighting design and the associated units are given below
Luminous flux. The light emitted by a source, or received by a surface. It is expressed in lumens. Symbol: Ø
Lumen. This is the SI (Standard International) unit of luminous flux. An ordinary 100W lamp for example emits about 1200 lumen. One lumen is the luminous flux emitted within solid angle (one steradian) by a point source having intensity of one candela. Symbol: lm
Luminous intensity. The quantity which describes the power of a source or illuminated surface to emit light in a given direction. It is the luminous flux emitted in very narrow cone containing the given direction divided by the solid angle of the cone. The result is expressed in candelas. Symbol: I
Candela. The SI unit of intensity. It is lumen per steradian. Symbol: cd
Illuminance. The luminous flux density at a surface, i.e. the luminous flux incident per unit area. The quantity was formerly know as the illumination value or illumination level. It is expressed in lux (lumens/m2 or lm/m2). Symbol: E
Lux. SI unit of illuminance. It is equal to one lumen per square meter.
Room index. An index related to the dimensions of a room, and used when calculating the utilization factor and the characteristics of a lighting installation. It is given bellow
lw / hm (l + w)
where l is the length and w the width of the room and hm the height of luminaires above the working plane.

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Application for Thyristor: Excitation of synchronous motor

In the past the overload torque requirement has been met by using a large frame size than necessary to meet full load torque requirements, the torque being proportional to the product of the AC supply voltage and the DC field produced by the excitation current.

With thyristor control of excitation current it is possible to use a smaller frame size for a given horse power rating and arrange to boost the excitation by means of a controller to avoid loss synchronism under torque overload conditions.

The excitation current of a synchronous motor may be controlled by supplying the motor field winding from a static thyristor bridge, using the motor supply current to control the firing angle, figure below. A pulse generator varies the firing angle of the thyristors in proportion to a DC control signal from a diode function generator. Variable elements in the function generator enable a reasonable approximation to be made to any of a wide range of compensating characteristics.

Simple compensated excitation circuit
When the motor operates a synchronously, i.e. during starting, a high emf is induced in the field winding, and the resulting voltage appearing across the bridge must be limited to prevent the destruction of the bridge elements. This may be done by using a shunt resistor connected as shown.

Where more exacting requirements have to be met, current feed back can be applied to eliminate effects of non-linearity in the pulse generator and rectifier bridge and it will also improve the response of the system to sudden changes of load. Automatic synchronizing is possible without relays by incorporating a slip frequency sensing circuit to control the gate which supplies the control signal to the pulse generator.

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Passive components and active components

Electronic components are divided into two types, namely passive components and active components. Passive component is a component that can not lead to increase in the electric power circuit, examples of passive components are resistors, capacitors, and inductors.

While the active component is a component that can lead to increase in the electric power circuit, examples of active component is tansistor.

Resistor has the ability to convert electrical energy into heat. Inductor has the ability to convert electrical energy into magnetic force. However, neither of the two components that can lead to the addition of power in the circuit, it is called passive components.

Instead of a transistor receives the low power input (small current) and converts it into a high power output (large current), the transistor is called the active component.

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Gain transistor

Gain transistor or the acquisition of the current generated by the transistor, commonly referred to as the small signal current gain, while the magnitude of the gain is written with symbols HFE. This gain value is determined when the transistor is made, and with a very diverse range. For example, type BC548 NPN transistor has a range between 110 and 800.

Under conditions of saturation, the gain is the ratio of collector current to base current, or by the equation

HFE = IC / IB

Example question:
Calculate the gain transistor in the circuit below


completion:
Known value of IC = 60 mA, IB currents obtained by Ohm's law
IB = VBB - VBE / RB
IB = (6 - 0.7) V / 10 kΩ
IB = 5.3 V / 10 kΩ
IB = 0.00053 A
IB = 0.53 mA
The amount of gain or HFE = IC / IB = 60 mA / 0.53 mA = 113

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Cut off condition and saturated condition of transistors

In the switch circuit, transistor sometimes be in the inactive state, where there is no any current flows through it. These conditions is called transistor's cut off condition.

Figure A shows the state of the transistor in a cut off condition


At other times when the transistor is in the fully active state, where there are only a relatively small potential difference between the emitter and collector terminals. These conditions is called transistor's saturated condition.

Figure B shows the state of the transistor in saturated condition.

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