Contents

 i. Definitions And Laws
ii.Properties Of Good Illumination
iii.Mean Horizontal Candle Power(MHCP)
iv.Electric Lamps
v. Fluorescent Lamps
vi.Illumination Level

Definitions and laws

Luminous Flux: It is the rate of energy radiation in the form of light waves, the unit is LUMEN

Lumen: It is the unit of luminious flux, it represents the  flux emiited in the unit solid angle of one steradian by a point source having a uniform intensity of one candela. Thus a uniform point source of one candle power emmits 4Ï€ Lumen

Lumen= candle power× solid angle

Candle Power:  It is the capacity of a source to rediate light and is equal to the number of lumens  emmited in a unit solid angle by a source of light in a direction.

Luminious Intensity: In a given direction is the quotient of the luminous flux emitted by a source(or by an element of a source), in an infinitesimal cone containing the given direction by the solid  angle of that cone. Hence it is the luminious flux/unit solid angle

Candela: Is the unit of luminious intensity. The luminious of a full radiator(back body) at the temperature of solidfication of platinum is 60 candels per square meter.

Luminious:  The luminious flux reaching a surface,per unit area of that surface.One lumen per square meter is termed as one lux .

Illumination= Flux/area or lumen/area or candle power× solid angle/Area

Luminance:  Is the luminious intensity in a given direction of an element of a surface, per unit projected area that surface.

PROPERTIES OF GOOD ILLUMINATION:

i. illuminating source should have sufficient light.
ii. it should not strike the eyes.
iii. is should have suitable shades and reflectors.
iv. it should be installed at such a place so as to give uniform light.
v. is should not produce glareneres in the eyes.

MEAN HORIZONTAL CANDLE POWER(MHCP): 

It is the mean of the candle power in all directions in the horizontal plane obtaining the source of light.

Mean spherical candle power(MSCP):
It is the mean or average of the candle power in all direction below the horizontal.

INVERSE SQUARE LAW: The illumination upon a surface varies inversely as the square of the distance from the source. Thus if the illumination at a surface one meter from the source is X units ,then the illumination at 2meters will be X/4 at 3meters will be X/9 and so on.
strictly the inverse square law operates only when the light rays are form a point source and are incident normal upon the surface.

Thus illumination in lamberts/meter^2 on a normal place = candle power/(distance in meters)^2

However,illumination from a uniform diffusing area such as indirectly lighted ceiling is independent of distance, provide the distance concerned is small in relation to the size of the source.

COSINE LAW: The illumination recived  on a surface is propotional to the cosine of the angle between the direction of the incident light rays and the normal to the surface at the point of incidence. as the angle of incidence Thus.
Eh= EnccospIcosP/D^2
Where, 
Eh= illumination on horizontal plane
En= illumination due to light normally incident
P(thita)= the angle of incidence
D= distance from the source
what is illumination


Electric Lamp 

There are three types of electric lamp
i.Filament lamp
ii. Electric discharge lamp
iii. Carbon arc lamp

Filament Lamp
Filament lamp may be 
(a) vacuum lamp (b) gas filled

Both vacuum and gas filled lamp use tungsten filament the life of such lamps largely depends on the rate of filament evaporation. To reduce the rate of filament evaporation, innert gas like argon or nitrogen is filled inside the shell for the same power output and the same filament size the temparature attained by the filament of the gas filled lamp would be less than that of a vacuum lamp.
The filament lamps are  normally available in the wattage range of 25,40,60,100,200.500,750,1000 and 1500. Single coiled filament is used on low wattage lamps and coil filament is used on high wattage lamps.

Electric Discharge Lamp

The operation of these lamps is based on the phenomenon of exciation and ionisation in a gas or vapour. The colours given by various vapours are given below:
They are two types
(a).Cold cathode lamp: no filament is used to heat the electrode for starting e.g neem lamp, sodium vapour lamp etc.
(b).Hot cathode lamp:  a filament is used to heat the main electrode at the time of starting e.g mercury vapour lamp, fluorescent tube etc.


Carbon Arc Lamp

Here 2 carbon electrodes are placed in contact end to end in which DC is flowing . By separating apart by about 0.6cm, gives out a luminous are shown the operating voltage of these lamps vries batween 40 to 60V.


FLUORSCENT LAMPS

It is used to correct the deficiency of red  colour in the spectra of mercury lamp,Further fluorscent powders converts ultra-violet radiations of the mercury discharge at 3654A and2537A into visible light perticularly at the red end of the spectrum. these powders are usually in white colours and are coated on the inside surface of the glass tube.


ILLUMINATION LEVEL


The factors on which the degree of illumination necessary for a specific task depends are
(i) Size of the object
(ii) speed of movement
(iii)duration of task
(iv) brightness contrasts
(v) colour
The deleterious effect on the observer of the brighter portion of the visual fields is called glare.
glare in its mildest form results in the distraction of the worker away from his task. however,when glare is intense physical discomfort is experienced. Excessive glare are causes fatigue and reduces the ability to perceive. Adequate care mus6 be taken while deciding about illumination level at a place to avoid glare