Basics of illumination and Laws of illumination 

Introduction:

Light is a Electromagnetic radiation Which has a wavelength in the range from about 4,000 (violet) to about 7,700 (red) angstroms and may be seen by the normal human eye.

what is light?

It is the part of radiant energy from a hot body which produce the visual sensation on human eye.

Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is perceived by the human eye.

Luminous flux: it is the light energy radiated out per second from the lamp. Its unit is lumen.

Lumen: it is defined as the luminous flux emitted in a unit solid angle by a source of one candle power. i. e.
Lumen=candle power x solid angle= cp x ω.

Luminous intensity (I) or Candle- power:

Luminous intensity (I) or Candlepower of a point source in a given direction is the luminous flux (number of lumens) radiated out per unit solid angle. Its unit is Candela (cd) or lumens per steradian.

Mean spherical candle-power (MSCP):

Generally, the luminous intensity or candle power of a source is different in different directions. The average candle-power of a source is the average value of its candle  power in all the directions. Obviously, it is given by flux (in lumen) emitted in all directions in all planes divided by 4π. This average candle-power is also known as mean spherical candle-power (MSCP).

MSCP= total flux in lumen/ 4 π

Mean Hemispherical candle-power (MHSCP):

 It is given by the total flux emitted in a hemisphere (usually the lower one) divided by the solid angle subtended at the point source by the hemisphere.

MHSCP= total flux emitted in hemisphere/ 4 π

Lux :  It is unit of illumination and it is defined as luminous flux falling per unit.

Lux = 1 Lumens/ m²

 

Illumination (E) or Illuminance:

 When the luminous flux falls on a surface, it is said to be illuminated. Illumination is the luminous flux received by a surface per unit area. Its unit is Lux or meter-candle or lumens per m2.

Refer following table for standard lux level

Residential Building	Commercial Building
Kitchen 200	Classroom 300-400
Bathroom 300	Jewlery Work 700-800
Bedroom 300	Entrance Foyers 150-200
Dining 150	Office 200-300
Stairs 100	hospital Treatment Room 400-500
Study 300	Stairs 80-100
Drawing Hall 300	Laboratories 300-400
Living 300

 

Specific output or Lamp efficiency:  Lamp efficiency of a lamp is the ratio of luminous flux to the power intake. Its unit is lumens per watt (lm/w).

Types of light source and Typical Luminous Efficiency (lm/W)

Incandescent bulb                8-18
Fluorescent Lamp                46-60
Mercury Vapour Lamp         44-57
CFL                                             40-70
Sodium Vapour Lamp (Low Pressure or LPSV)  101-175
Sodium Vapour Lamp (High Pressure or HPSV)  67-121
Metal Hallide                                60-80
LED                                              30-50
Best LED                          105

 

Plane angle- A plane angle is subtended at a point and is enclosed by two straight lines lying in the same plane. In radians, plane angle is the ratio arc/radius. It is expressed in degree or radians.

Solid angle: angle subtended at the center of the sphere by a part of its surface having an area equal to (radius) 2.

 

Space to Height Ratio : This is the ratio of the distance between the center of two adjacent luminaires to the mounting height of the luminaires above the working  plane.

UTILIZATION FACTOR:

Utilization Factor or Co-efficient of utilization. It may be defined as “the ratio of total lumens received on the working plane to the total lumens emitted by the light source”.i.e.

 Utilization factor =Lumens received on the working plane/Lumens emitted by the lamp

FACTOR EFFECTING UTILIZATION FACTOR:

Type of light, light fitting, Colour surface of walls and ceiling, mounting height of lamps, Area to be illuminated.

Its value lies between 0.4 and 0.6 for direct fittings it varies from 0.1 to 0.35 for indirect fittings

Maintenance factor

It may be defined as “the ratio of illumination under normal working condition to the illumination when everything is clean or new” i.e.

MF = Illumination under normal working conditions / Illumination when everything is clean.

 

Depreciation Factor

DF is inverse of  Maintenance factor

DF = Illumination when everything is clean. / Illumination under normal working conditions

REFLECTION FACTOR:

It may be defined as “the ratio of luminous flux leaving the surface to the luminous flux incident on it”. It’s value will be always less than 1

ABSORPTION FACTOR:

When the atmosphere is full of snow or smoke fumes, it absorbs some light. Hence absorption factor may be defined as “the ratio of net lumens available on the working plane after absorption to the total lumens emitted by the lamp”. It’s value varies from 0.5 to 1

Laws of Illumination:

 (laws of energy conservation)

1) Inverse square law- According to this law, the illumination of a surface is inversely proportional to the square of the distance  between the source and the surface.

This law states that the Illuminance (E) at any point on a plane perpendicular to the line joining the point and source is inversely proportional to the square of the distance between the source and plane.
                                                                        E= I/d²
Where, I is the luminous intensity in a given direction.

2) Lambert cosine law-

The law states that Illuminance at a point on a plane is proportional to the cosine of the angle of light incident  (the angle between the direction of the incident light and the normal to the plane).
E= I/d²_{* COS Thita}

It is the point source Illuminance equation.

Where, I_θ is the luminous intensity of the source in the direction of the illuminated point, Ɵ is the angle between the normal to the plane containing the illuminated point and the line joining the source to the illuminated point, and d is the distance to the illuminated point.

FEATURES OF GOOD ILLUMINATION SCHEME

(i)             Provide adequate illumination,

(ii)           Provide light distribution all over the working plane as uniform as possible,

(iii)         Provide light of suitable colour.

(iv)          Uniformity of Illumination

(v)           composition of  the light should be such that the colour appears natural

(vi)          formation of long and hard shadows is considered to be a shortcoming. shadow less light is essential.

(vii)        No ageing

(viii)      Easy maintenance etc.

 

ADVANTAGES OF GOOD ILLUMINATION SCHEME

 It save work hours of the job

It save money

It increase profit

It reduce product reject

It avoid dark shadow, glare

It provide good colour combination

It improves moral and motivation

It avoid obstacle and accidents, etc.

CONCEPT OF PHOTOMETRY :-

we can say that It is the technique for measurement of a candle power of a light source.

This is the method of comparison of candle power of a light source with a standard (known) candle power source.

The experiment is carried in a dark-room of which the walls and ceiling are dark-black coloured to avoids reflection of light and hence error is eliminated and results are more accurate.

Principle of Measurement:

It is based on the principle of "Inverse Square Law".

 

Procedure:

At this position, distance of standard lamp from photometer head (l1) = distance of test lamp from photometer head (l2).

These distances are accurately noted to avoid errors on the graduated scale.

By inverse square law, the candle power of the source under test is found from the

 

The illumination is measured in the unit Lumen/m² i.e. in Lux.

In the recent development a meter is available which directly measure the illumination at any spot surface in 'Lux'. The meter is called as "Lux Meter".

Lux meter is compact and portable meter.

It is an illumination photometer calibrated to measure illumination in the standard Unit.

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