Candela and Lux: Formulas and Calculations

Candela and lux are both units used in photometry to measure light, but they quantify different aspects of it. Lux measures the brightness of a surface, while candela measures the intensity of a light source. Let’s explore their formulas and how to calculate them.

Understanding Candela (cd)

Candela (cd) is the base unit for luminous intensity. It tells us how much light a source emits in a specific direction.

Candela Formula:

Candela (Cd) = Luminous Flux (lm) / Solid Angle (Steradians)

• Luminous Flux (lm): The total amount of visible light emitted by the source.
• Solid Angle: A measure of the spread of light in three-dimensional space as viewed from the source. A complete sphere has a solid angle of 4π steradians.

Candela Calculation Example:

Let’s say a light bulb emits 800 lumens of visible light uniformly in all directions. To calculate the luminous intensity in candela:

Candela = 800 lm / (4 \* 3.14) = 63.66 cd

Therefore, the light bulb has a luminous intensity of 63.66 candela.

Understanding Lux (lx)

Lux (lx) is a unit of illuminance. It measures the amount of visible light that falls on a given surface area. In simpler terms, it tells us how bright a light appears on a particular surface to the human eye.

Lux Formula:

Lux (lx) = Luminous Flux (lm) / Surface Area (m²)

• Surface Area (m²): The area onto which the light is falling.

Lux Calculation Example:

Imagine a desk with an area of 2 square meters (m²). An 800-lumen light bulb is placed directly above the desk. To calculate the illuminance on the desk:

Lux = 800 lm / 2 m² = 400 lx

The illuminance on the desk is 400 lux.

Lux to Candela and Candela to Lux Conversion

You can convert between lux and candela, but it requires knowing the distance from the light source. Here’s the relationship, along with examples:

Lux to Candela Converter Example:

• Input:
  • Illuminance (in Lux) = 100
  • Distance from light source = 10 meters
• Output:
  • Luminous Intensity in candela = 10000

Candela to Lux Converter Example:

• Input:
  • Luminous Intensity in candela = 10000
  • Distance from light source = 10 meters
• Output:
  • Illuminance (in Lux) = 100

Lux to Candela Conversion Formula:

Here’s a visual representation of the formula:

Conclusion

In short, candela measures the intensity of a light source, while lux measures the brightness of a light on a surface. Calculations involve luminous flux, solid angle, and surface area, depending on which unit you’re working with.