As per the diffusion principle of street lighting installations, which of the following methods is used to cal

Question

As per the diffusion principle of street lighting installations, which of the following methods is used to calculate the illumination at any point on the road surface?

Examoogle · Accepted Answer

The correct answer is Option D: Inverse-square law method. The illumination at any specific point on a road surface is calculated using the inverse-square law, which relates the intensity of a light source to the distance and the angle of incidence. This method is fundamental to determining the photometric performance of street lighting luminaires as it accounts for point-source emission behavior.

Step-by-Step Solution & Concepts:
The illumination at any specific point on a road surface is calculated using the inverse-square law, which relates the intensity of a light source to the distance and the angle of incidence. This method is fundamental to determining the photometric performance of street lighting luminaires as it accounts for point-source emission behavior.

Working Principle: The illumination $E$ at a point is directly proportional to the luminous intensity $I$ of the source in that direction and inversely proportional to the square of the distance $d$ from the source to the point. It is also modified by the cosine of the angle $	heta$ between the light ray and the normal to the surface, expressed as $E = \frac{I \cos(	heta)}{d^2}$.

Key Formulas: $E = \frac{I \cos(	heta)}{d^2} = \frac{I \cos^3(	heta)}{h^2}$ — where $h$ is the mounting height and $	heta$ is the angle with the vertical; $I = E 	imes d^2$ — calculation of luminous intensity required for a desired illumination

Key Points: The law assumes the light source acts as a point source.; The illumination at any point is significantly affected by the mounting height of the luminaire.; Calculations often involve the 'cosine-cubed' rule for points on a horizontal plane relative to the source.; Street lighting design requires uniform horizontal illumination distribution.

Advantages: Provides precise point-to-point illumination estimation.; Essential for calculating uniformity ratios in road lighting design.

Disadvantages: Not suitable for light sources that act as large area surfaces.; Becomes complex when accounting for multiple light sources simultaneously.

Applications: Street and highway lighting design.; Architectural floodlighting.; Interior lighting layout planning.

Additional Info: Light flux method is generally used for average illumination in interior spaces, not point-specific road surface calculations.. Specular reflection relates to polished surfaces (like wet roads), not the general calculation of incident illumination.. Watt per square metre is a crude estimation method for rough power density requirements and does not provide photometric accuracy.

Answer: D is correct — The inverse-square law method is the standard approach for determining the illumination at a specific point on the ground relative to a point-source street luminaire.

Exam Tip: Always remember the difference: use the Lumen method for average illumination in rooms and the Inverse-Square Law method for point-specific illumination (like street lights).

Examoogle · Answer

Option A: Watt per square metre method

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Option B: Light flux method

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Option C: Specular reflection method

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