## 07/07/2020

### Illumination Interview Question Answer - 1

Explain the term: Light

Light : It is small part of electromagnetic radiation which is sandwich between ultraviolet and infrared radiation.

Explain the laws of illumination.

Laws of illumination

Inverse square law : This law states that the illumination of a surface is inversely proportional to the square of the distance between surface and a point of light source. This law is applicable if the surface is normal to the line of flux.

Lambert’s cosine law : This law states that the illumination at any point on the surface is directly to the cosine of angle between the normal at that point and line of flux. This law is applicable if the surface is inclined at angle θ to the line of flux.

Explain the following terms with regard to illumination: Luminous flux, Lumen and Luminous intensity

Luminous flux : It is defined as the light wave emitted per second from a luminous body.

Lumen : It is an unit of luminous flux.

Lumen = candle power of light source × solid angle

Luminous intensity : It is defined as the luminous flux emitted by the electrical source per unit solid angle.

Explain the following terms with regard to illumination: Candle power, Illumination and Lux

Candle power :  It is defined as the total luminous flux lines emitted by light source in a unit solid angle. Its unit is candela or lumen/steradian

Illumination : It is defined as the luminous flux received by the unit surface area. Its unit is Lux.

Illumination = Luminous flux / Area

Lux : It is defined as the illumination flux of one lumen uniformly distributed over a radius of 1 meter from the center of sphere.

Explain the following terms with regard to illumination: MHCP, MSCP and MHSCP

Mean horizontal candle power (MHCP) : It is defined as the mean of candle power of source in the direction of the horizontal plane.

Mean spherical candle power (MSCP) : It is defined as the mean of candle power of source in all the direction.

Mean hemispherical candle power (MHSCP) : It is defined as the mean of the candle power of source in all the direction above or below of the horizontal plane.

Explain the following terms with regard to illumination: Specific consumption, Utilization factor and Depreciation factor

Specific consumption : It is defined as the ratio of the electrical power input to the source to the average candle power of it.

Specific consumption = Electrical power input / Average candle power

Utilization factor : It is defined as the ratio of the total lumen reaching to the working plane to the total lumen emitted by the light source.

Utilization factor = Total lumen at working plane / Total lumen emitted by light source

Depreciation factor : It is defined as the ratio of the initial illumination to the ultimate illumination.

Depreciation factor = 1 / Maintenance factor

Explain the following terms with regard to illumination : Maintenance factor, Absorption factor and Lamp efficiency

Maintenance factor : It is defined as the ratio of the illumination under normal condition to the illumination when the system is new.

Maintenance factor = Illumination at normal condition / Illumination when system is new

Absorption factor : When atmosphere is full of smoke and fumes, there is a possibility that absorption of light. Therefore, total lumen available after absorption is reduced. The absorption factor is defined as the ratio total lumen available after absorption to the total lumen emitted by the light source.

Absorption factor = Lumen available after absorption / Lumen emitted by light source

Lamp efficiency (Luminous efficiency) : It is defined as the ratio of the luminous flux emitted by the light source to the electrical input supplied to the light source.

Lamp efficiency = Luminous flux emitted by light source / Electrical input to the light source

Describe the function of lamp.

Lamp : The lamp is equipment which produces light.

Describe the main disadvantages of filament lamps.

• Very sensitive to voltage fluctuation
• Short lamp life
• High power consumption (15Watt / lamp)

Describe the advantages of LED lamps over filament lamps.

Advantages of LED lamps over filament lamps

• Withstand against voltage fluctuation
• Long life
• Low power consumption (< 1 Watt / LED)

Describe the function of luminaries in a lighting system. What does the luminaries consist of?

Function of luminaries :The luminaries are a device which transforms the light emitted from one or more lamps. The luminaries consist of lamp, lamp socket, lamp housings, electronics Ballast and reflective material.

Write the full form of CRI.

The full form of the CRI is colour rendering index.

Write the full form of CFL lamp.

The full form of the CFL lamp is compact fluorescent lamp.

Describe the advantages of electronic ballasts over conventional ballasts.

Advantages of electronic ballast over conventional ballasts

• Instant start or lighting
• Less heat generated
• Higher power factor
• Consume very less energy as compared to conventional ballast
• Lamp life increases
• Less weight
• Noiseless operation

Describe the advantages of CFL lamps over Incandescent lamps?

Advantages of CFL lamps over incandescent lamps

• Longer lamp life (Approx. 10,000 hours)
• Higher illumination ( 50 – 55 lumen / watt )
• Less heat generation
• Energy saving