Electrical Science: Illuminance, Luminance & Luminous Intensity

Understanding Illuminance, Luminance, and Luminous Intensity

  • Lighting as Specialization

    • Lighting is a specialized area within electrical installations.

    • Designers and installers often rely on specialists for lighting design.

    • Electricians must have basic knowledge for effective installation and maintenance of luminaires.

Laws of Illumination and Quantities

  • Key Laws Explaining Light Behavior

    • Two primary laws explain how light behaves when emitted from a luminaire:

    • Inverse Square Law

      • Describes how the intensity of light decreases as the distance from the source increases.

    • Cosine Law

      • Relates the angle at which light strikes a surface to the intensity of illumination.

Key Lighting Terms and Units

  • Luminous Flux

    • Unit: Lumens - ϕ - lm

    • Symbol: F

    • Lumen (lm) is the SI unit for measuring luminous flux.

    • This measures the total amount of visible light emitted by a source.

  • Practical Importance of Luminous Flux

    • Luminous flux accounts for the sensitivity of the human eye to different wavelengths of light.

    • It is crucial for designing effective lighting in various environments:

    • Home and Office Lighting

      • Ensures adequate illumination, contributing to comfort and performance while reducing eye strain.

    • Automotive Lighting

      • Ensures visibility and safety through adequate lighting in vehicles.

    • Street and Public Area Lighting

      • Engineers calibrate designs to enhance safety; important for visibility at night.

    • Emergency and Safety Lighting

      • Ensures visibility of exit and warning signs during low-light conditions or emergencies.

  • Luminous Intensity

    • Unit: Candela (Cd)

    • Symbol: I

    • Definition: "The amount of light emitted per solid angle or in a given direction."

    • For reference, a typical candle has a luminous intensity of approximately 1 Cd.

  • Illuminance

    • Unit: Lux - E

    • Symbol: E

    • Definition: "The amount of light falling on a surface."

    • Measured as lumens per square meter (lm/m²).

  • Luminance

    • Unit: Candela / metre squared

    • Symbol: L

    • Definition: "Luminance is the luminous intensity projected on a given area."

    • Example:

    • A table illuminated by light emitted from a candle reflects light at a specific angle to the observer’s eye; the amount of light received at that angle is the luminance.

Luminous Efficacy and Efficiency

  • Luminous Efficacy

    • Unit: Lumens per watt (lm/W)

    • Symbol: K

    • Definition: A measure of the efficiency of a lamp or luminaire.

    • It is the ratio of luminous flux to the power consumed, expressed in lm/W.

  • Efficiency vs. Efficacy

    • Efficiency: The ratio of power input to power output, measured in the same unit.

    • Efficacy: The comparison of power input and output in different units, specifically the light output in lumens compared to power usage in watts.

  • Summary:

    • Efficiency discusses the effectiveness in output versus input in the same measurement context, while efficacy provides a comparative measure across different types of units involved in lighting performance assessment.