Illumination MCQ Quiz - Objective Question with Answer for Illumination - Download Free PDF

The correct answer is option 3.

Specular reflection: The reflection of light from a smooth and shiny surface like a mirror is called specular reflection.

Key Characteristics:

• Occurs on smooth surfaces (e.g., mirrors, calm water).
• Light rays reflect in a single, predictable direction.
• Follows the law of reflection: the angle of incidence equals the angle of reflection.
• Produces clear images, like your reflection in a mirror.

Additional Information 

Other types of reflection are:
Spread reflection: Spread reflection, also known as diffuse or mixed reflection, occurs when light is reflected in a wide range of directions from a surface with imperfections, such as a rough surface or an uneven material. 
Irregular reflection: When a beam of light falls on such a surface which is not perfectly smooth and polished, such as a wall, wood paper etc. is known as irregular reflection.

Diffuse reflection: Diffuse reflection is the reflection of light from a surface such that an incident ray is reflected at many angles, rather than at just one angle as in the case of specular reflection.

Concept:

Electronic ballasts are advanced devices used in Compact Fluorescent Lamps (CFLs) to regulate the current and start the lamp quickly and efficiently. Unlike conventional magnetic ballasts, electronic ballasts operate at high frequencies (typically 20–60 kHz), which helps in eliminating visible flicker and delays during startup.

Benefits of Electronic Ballasts:

- Eliminate slow starting
- Eliminate flickering
- Improve energy efficiency
- Reduce humming noise and heat losses

Evaluation of Options:

Option 1: slow starting and flickering –  Correct
This is the main advantage of using electronic ballasts in CFLs.

Concept:

Outdoor streetlight installations are classified based on practical engineering parameters that influence performance and efficiency. These include the type of light source (e.g., LED, sodium vapor), mounting height, and the specific application area (e.g., residential streets, highways, industrial zones). These factors ensure optimal illumination, energy usage, and safety for each environment.

Classification Factors:

- Light source type (e.g., LED, CFL, HPS)
- Mounting height and spacing
- Application environment (residential, commercial, highway, etc.)

Luminous intensity

• Luminous intensity (Luminance) in a given direction is defined as the luminous flux emitted by the source per unit solid angle.
• It is measured in lumens.
• One lumen is defined as the luminous flux emitted by a source of one candlepower per unit solid angle in all directions.
• The measure of the total light output of a luminous source is known as Luminous Flux. 

Lambert’s Cosine Law

The illumination of a surface is directly proportional to the cosine of the angle between the normal to the surface and the direction of incident light.

E α cos θ 

Here,  the angle θ is measured between the incident light and the normal.

The derived SI unit of illuminance is the lux (lx).

Luminous efficiency: It is also known as radiant efficiency. It is defined as the ratio of energy radiated in the form of light, produces sensation of vision to the total energy radiated out by the luminous body.

Radiant efficiency = energy radiated in the form of light / total energy radiated by the body

Lamp efficiency is measured in lumen/watt.

Important Points:

• Space to height ratio is the ratio of space between luminaires (S) to their height above the working plane (Hm).
• Manufacturers specify the recommended space to height ratio (SHR) for each of their luminaires.
• Typically, a recommended space to height ratio (SHR) is 1 : 2.

Concept:

Illumination is defined as the luminous flux received by the surface per unit area.

It is usually denoted by the symbol ‘E’ and is measured in lux or lumen/m² or meter candle or foot candle.

Illumination, \(E = \frac{\phi }{A} = \frac{{CP \times \omega }}{A}lux\)

Solid angle \(\omega = \frac{A}{{{d^2}}}\)

Illumination \(E = \frac{{CP}}{{{d^2}}}lux\)

Calculation:

Given that, candle power (CP) = 60 CP

Illumination (E) = 15 lux

⇒ 15 = 60/d²

⇒ d = 2 metres

Concept:

Illumination is defined as the luminous flux received by the surface per unit area.

It is usually denoted by the symbol ‘E’ and is measured in lux or lumen/m2 or meter candle or foot candle.

Illumination, \(E = \frac{\phi }{A} = \frac{{CP \times \omega }}{A}lux\)

Solid angle \(\omega = \frac{A}{{{d^2}}}\)

Illumination \(E = \frac{{CP}}{{{d^2}}}lux\)

Calculation:

Given that,

Illuminance = 64 π 

\(E= \frac{P}{4\pi r^2}=\frac{64\pi}{4\pi(4)^2}= 1\;lux\)

The correct answer is option 4.

Sodium vapor lamps are also commonly used for sporting events, as they have a very high lumen-to-watt ratio typically 80–140 lumens/watt.

Applications of different types of lamps

Definition of illumination (E):

illumination is defined as luminous flux received by the surface per unit area. Its unit is Lux or meter-candle or lumens per m²

Therefore,

illumination (E) = Lumen/area

\(=\frac{c.p\ \times\ ω}{A}\)

Where,

c.p = candle power

ω = solid angle = area/ d²

d = distance between the area and the point where the solid angle is formed

A = Area of cross-section

Applications are shown below: 

Concept:

Types of illumination

1.) Direct illumination

• In a direct scheme, about 90 % to 100 % of the total light flux or lumen is made to fall downward directly with the help of deep deflectors.
• This illumination has the highest efficiency but the brightness.
• This scheme is used for industrial purposes (factory lighting), street lamp lighting, and general outdoor lighting.

2.) Indirect illumination

• In an indirect scheme, more than 90 % of the total light flux or lumen is made thrown upward to the ceilings for diffuse reflection.
• This scheme of lighting is achieved by using inverted or bowl-shaped reflectors.
• This type of scheme is mostly used for decoration purposes in places like cinema halls, theaters, and hotels.

3.) Semi-direct illumination

• In a semi-direct scheme, about 60 % to 90 % of the total light flux or lumen is made to fall with the help of semi-direct reflectors.
• The remaining percentage of the light is used to illuminate the ceilings and walls.
• This type of scheme is best suited for rooms with high ceilings.

4.) Semi-indirect illumination

• In a semi-indirect scheme, about 60 % to 90 % of the total light flux or lumen is thrown directly upwards to the ceilings for the defused reflection.
• The remaining amount of light reaches the working plane directly except those absorbed by the fixtures.
• This type of scheme is used for decoration purposes.

Methods for calculating the Illumination of Light

The three main methods are as follows:

1.) Watt per Square Meter Method

• The watt-per-square meter method is fundamentally a rule-of-thumb method. 
• This method is used for rough calculations and checking purposes.
•  It entails allocating watts per square meter of the area to be illuminated in accordance with the desired illumination, based on an average figure of overall system efficiency.

2.) Lumen Method

• The lumen method is also called the light flux method.
• It is the method employed for lighting calculation that is applicable to those cases where the sources of light are such as to produce an approximate uniform illumination over the working plane or where an average value is required.

3.) Inverse Square Law Method

• The inverse square law method is also known as a point-to-point method.
• It is applicable where illumination at a point due to one or more sources of light is required. Though, the candle power of the sources in the particular direction under consideration is known.

The foot-candle is defined as one lumen per square foot. It is the unit of illumination.

Illumination is defined as the luminous flux received by the surface per unit area.

1 lm/m² = 1 lux

1 foot = 0.3048 m

Hence, 1 lm/ft² = 1 lm/(0.3048)²m² = 10.76 lm/m²