Luminous Flux: The measure of the perceived power of light. It represents the total amount of visible light emitted by a source per unit of time.

Symbol and Units

• Lumen (lm): The SI unit of luminous flux. It quantifies the amount of light emitted by a source.
• Candela (cd): The SI unit of luminous intensity. It quantifies the intensity of light emitted in a particular direction.

Formula

Luminous Flux (Φv): \( \Phi_v = I_v \cdot \Omega \), where \( I_v \) is the luminous intensity in candelas and \( \Omega \) is the solid angle in steradians.

Key Points

• Luminous flux measures the perceived power of light, considering human vision sensitivity.
• The lumen (lm) is the primary unit for measuring luminous flux.
• Luminous intensity (cd) is related but focuses on the directionality of light.

Applications

• Lighting Design: Ensuring sufficient illumination in buildings and outdoor spaces.
• Display Technology: Assessing the brightness of screens and projectors.
• Automotive Lighting: Evaluating the effectiveness of headlights and signal lights.

Material Variance

• Different light sources have varying efficiencies and spectral distributions, affecting their luminous flux.
• LEDs, incandescent bulbs, and fluorescent lights all emit different amounts of luminous flux for the same power consumption.

Examples

Example 1: Calculate the luminous flux if the luminous intensity is 10 cd and the solid angle is 2 steradians:

\( \Phi_v = 10 \, \text{cd} \times 2 \, \text{sr} = 20 \, \text{lm} \)

Example 2: Convert 3000 lumens to candelas if the light is emitted uniformly in all directions (sphere with solid angle \( 4\pi \) steradians):

\( I_v = \frac{3000 \, \text{lm}}{4\pi \, \text{sr}} \approx 239.4 \, \text{cd} \)