MCQs on Illumination

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01․ Candela is the unit of which?
Luminous intensity.
Luminous flux.

A common candle emits light with a luminous intensity of roughly one candela. This is how we are calculating the luminous intensity of the light in terms of candela.

02․ Solid angle is expressed in terms of
radians / meter.

The steradian or squared radian is derived from the Greek stereos for solid and the radius for ray and beam. A steradian is related to the surface area of a sphere in the same way a radian is related to the circumference of a circle. A steradian can be defined as the solid angle subtended at the center of a unit sphere by a unit area on its surface.

03․ Which lamp has the best Colour Rendering Index (CRI)?
High pressure sodium vapour

The visual spectrum of incandescent lamp is continuous and therefore the CRI is 100 which is maximum for any lamp.

04․ Which cells in the eye are responsible for scotopic vision?
Cone cells
Rod cells
Both ‘a’ and ‘b’
None of these

Scotopic vision deals with night vision which is controlled by rod cells.

05․ What is the ratio of illuminance at a point 5m just below a lamp emitting 100 candelas and at a point 5m away from the first point on the same horizontal plane?
2√2 : 1
1: 2√2

From figure shown below Illuminance at point A = 100/52 = 4 Illuminance at point B = 100 × cos 45/OB2 = 50√2/ (5√2)2 = √2.

06․ Which lamps can be directly connected to a solar cell?
Metal Halide
Compact Fluorescent

The output of a solar cell is DC and LEDs operate on DC.

07․ Candela/metre square is the unit of which photometric quantity?
Luminous flux
Luminous Intensity

Luminance is the luminous intensity per unit projected area.

08․ Which lamp is best suited for lighting an International sports arena?
Metal Halide
Compact Fluorescent

Metal halide lamps have luminous efficacy in the range of 80-100 lm/watt and a CRI of above 90. So they are best suited for lighting an International sports arena.

09․ What is the work of a driver in case of an LED?
To convert AC to DC
To convert DC to AC
Both ‘a’ and ‘b’
None of these

LEDs operate on DC. So when they are to be operated on AC supply, a driver is needed which converts AC to required DC value.

10․ What will be the number of lamps, each having 300 lumens, required to obtain an average illuminance of 50 lux on a 4m × 3m rectangular room?

Suppose X number of lamps are required; Total lumens = 300X; Also, Area of the room = 12 square metre; So Illuminance × area = total lumens; 50 × 12 = 300X; X = 2.