Candela/metre square is the unit of which quantity? | Electrical MCQ | Examoogle

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ElectricalUtilization Of Electrical Energy

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Candela/metre square is the unit of which quantity?

Illuminance

Luminous flux

Luminance

Luminous Intensity

Correct Answer

⚙️ TE • Technical Concept & PrincipleElectricalUtilization Of Electrical Energy

Option C

Luminance

Quick Summary:

Luminance represents the intensity of light emitted from or reflected off a surface per unit area in a given direction. Its SI unit is the candela per square metre ( $cd/m^2$ ), also historically known as the nit.

⚙️TETechnical SolutionConcept & Principle

💡 Explanation

🔢 Key Formulas

$L = \frac{I}{A \cos \theta}$ — definition of luminance where $I$ is luminous intensity in candela and $A$ is surface area in $m^2$

$L = \frac{d^2\Phi}{dA \cdot d\Omega \cdot \cos \theta}$ — differential form related to luminous flux $\Phi$

⚙️ Working Principle

Luminance ( $L$ ) is defined as the luminous intensity ( $I$ ) of a source in a given direction divided by the projected area ( $A_p$ ) of the source as seen by an observer. Mathematically, it is expressed as $L = \frac{I}{A \cos\theta}$ , where $\theta$ is the angle between the surface normal and the viewing direction. This quantity accounts for the human perception of brightness, making it crucial in lighting design and display engineering.

📌 Key Points

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Luminance is also commonly called 'photometric brightness'.
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It is the only photometric quantity that describes the light reaching the eye from a surface.
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The SI unit $cd/m^2$ is the standard measure used for display brightness and monitor specifications.

✅ Advantages

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Useful for calculating visual comfort in glare analysis
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Essential for evaluating the brightness of flat panel displays and signboards

❌ Disadvantages / Limitations

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Complex to measure compared to illuminance
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Requires knowledge of the viewing angle

🛠️ Applications / Uses

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Lighting system design for workspaces
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Display and monitor brightness calibration
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Automotive dashboard and HUD design

📄 Additional Information

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Option A (Illuminance) is measured in Lux ( $lm/m^2$ ).
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Option B (Luminous flux) is measured in Lumens ( $lm$ ).
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Option D (Luminous intensity) is measured in Candela ( $cd$ ).

📊 Diagram / Illustration

✅

C is correct — Candela per square metre ( $cd/m^2$ ) is the standardized SI unit for the quantity known as Luminance.

Core Concepts Used

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Photometry Luminance vs Illuminance Projected Area

💡 EXAM TIP

Remember: 'Luminance' relates to the source surface (cd/ $m^2$ ), while 'Illuminance' relates to the incident light on a surface (Lux).

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