ILLUMINATION ENGINEERING - PART – 14 – DIFFERENCE BETWEEN ILLLUMINANCE AND LUMINANCE – THREE PROBLEMS IN FLOODLIGHTING DESIGN

DIFFERENCE BETWEEN ILLUMINANCE AND LUMINANCE

ILLUMINANCE OR ILLUMINATION (E) is the measure of light falling on a surface (lumens per square metre or lux)

E is measured by the units fc (lx) ; fc being the symbol for foot candles and (lx) the symbol for lux in the International System (SI).

LUMINANCE OR BRIGHTNESS (B) is measured by the units fL(cd/sq.metre); fL being the symbol for foot Lamberts and (cd/m2) the symbol for candela per square meter.

Luminance refers to light reflected from it or emitted by it (candela per square metre or alternatively a possible illuminance x reflection factor).

LUMINAIRE is the name given to a complete light source and the light fixture.

REFLECTION FACTOR

In case of diffused reflecting surface the brightness or luminance gets reduced by an amount known as reflection factor.

Reflection Factor (R.F) is the ratio of reflected luminous flux to total incidental luminous flux.

Brightness (B) = [Reflection factor x Illumination (E)]/ ∏ in cd/sq.metre.

Reflection factor = [∏ x B] / E

PROBLEM – 01

A building measuring 40 m x 25 m is to be floodlit on the front side with brightness of 25 lumens per sq. metre. Coefficient of reflection of building surface is 0.25. Lamps of 500 W having lumens output of 8000 each are used. Assuming beam factor as 0.6, waste light factor 1.2 and maintenance factor as 0.75, determine the number of lamps required.

PROBLEM – 02

It is desired to flood-light the front of the building 40 m wide and 15 m high. Projector of 1000-W lamps giving 20 lumen/watt are available. If the desired level of illumination is 75 lm/sq. metre and if the projectors are to be located at ground level 12 m away, design and show a suitable scheme.

Assume the following – Coefficient of utilization is 0.4, depreciation factor is 1.3 and waste light factor is 1.2.

PROBLEM – 03

Estimate the number of 1000-W floodlight projectors required to illuminate the upper 75 m of one face of the 95 m tower of width 15 m if approximate initial average luminance is to be 6.85 cd/sq.metre. The projectors are mounted at ground level 50 m away from base of the tower. Utilization factor is 0.2, reflection factor of wall is 25% and efficacy of 1000-W lamp is 18 lm/watt.