4/15/2013

Prepared by : Saima 2010-Ba-18

Submitted to : Sir Arslan

(L&I) WHAT ARE POLAR CURVES & THEIR USES

What are Polar curves?

Polar distribution diagram, also called a polar curve, is a graph showing how luminous intensity

values vary with increasing angles from the imaginary axis of the lamp.

It is sometimes customary to show the curve for only one half of the distribution because in nearly all

cases the other half would be an exact mirror image (i.e. the polar curve is symmetrical about the lamp

axis).

A polar curve is schematic figure of the luminous intensity

(CANDELA) distribution of the luminaire. The shape of the

polar curve indicates the way in which the luminaire controls

the light distrbution from lamp. Figure 2.2 illustrates the polar

curve from an industrial type of luminaire.

Axially symmetrical luminaires produce a uniform intensity

distribution as shown in figure 2.2. the linear luminaire for

fluorescent lamp does not have an axially symmetrical

distribution. At least two polar curves are required- one on

shorter axis and one on longer axis. Polar curves can be

obtained from photometric data issued by manufacturer.

Types of polar curves:

There are two types of polar curves

1. Horizontal polar curve:

A curve is plotted between the candle power and the regular position, if the luminous intensity i.e.

candle power is measured in the horizontal plane about the vertical axis called horizontal polar curve.

2. Vertical polar curve:

A curve is plotted between candle power, if it is measured in vertical plane and the angular position is

known as vertical polar curve.

Uses of polar curves:

How to find MHCP and MSCP?

Polar curves are used to determine actual illumination of the surface by employing the candle power in

that particular direction as read from the vertical polar curve. These are also used to determine mean

horizontal candle power (MHCP) and mean spherical candle power (MSCP). The mean horizontal

candle power (MHCP) of lamp can be determined from the horizontal polar curve by considering the

mean value of all the candle power in horizontal direction. The mean spherical candle power (MSCP)

of a symmetrical source of a light can be found out from the polar curve by means of a Rousseaus construction.

Rousseaus construction:

Let us consider a vertical polar curve is in the form of two lobes symmetrical about YOY axis. A

simple Rousseaus curve is shown in figure 1.27.

i. Draw a circle with any convenient radius and O as center.

ii. Draw a line PQ parallel to axis YOY and is equal to diameter of circle.

iii. Draw any line OEA in such a way that line meeting the circle at point E. let the projection be S.

iv. At S erect an ordinate ST = OE. v. By similar construction draw other ordinates. The

curve PSQUTVP obtained by joining these

ordinates is known as Rousseaus curve. The mean

ordinate of this curve gives the mean spherical candle power (M.S.C.P)

The mean ordinate of curve =

The area under the Rousseaus curve can be determined by Simpsons rule or by graph paper.