Luminous Intensity

The Source of Light: Understanding Luminous Intensity

Luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle. This might sound complex, but it essentially quantifies how bright a light source appears to the human eye when viewed from a specific angle. It is a measure of the 'strength' or 'concentration' of light in a single direction, rather than the total amount of light emitted by the source. This is one of the seven base quantities in the International System of Units (SI), highlighting its fundamental importance in the science of light measurement, known as photometry.

Imagine a flashlight. The total light coming out of it in all directions is its luminous flux (measured in lumens). But the bright, focused spot it creates on a wall is a result of its high luminous intensity in that forward direction. A bare light bulb might have the same total lumen output, but because its light is spread out in all directions, its luminous intensity in any single direction is much lower. The SI unit for luminous intensity is the candela (cd). The concept is critical in designing and specifying directional light sources like spotlights, automotive headlights, LEDs, and lasers, where controlling the direction and intensity of the light is paramount.

Relevant Formulas and Concepts

• Definition of Luminous Intensity (Optics/Photometry): Luminous Intensity (Iᵥ) is the luminous flux (dΦᵥ) per unit solid angle (dΩ): Iᵥ = dΦᵥ / dΩ.
• Relationship to Luminous Flux (Lumens): The total luminous flux (Φᵥ) from an isotropic light source (one that emits equally in all directions) is its luminous intensity (Iᵥ) multiplied by 4π steradians (the solid angle of a sphere): Φᵥ = Iᵥ × 4π.
• Relationship to Illuminance (Lux): The illuminance (Eᵥ) on a surface is related to the luminous intensity of the source (Iᵥ) and the distance (r) by the inverse-square law: Eᵥ = Iᵥ / r².

A Deep Dive into Common Luminous Intensity Units

• Candela (cd): The SI base unit of luminous intensity. The modern definition is complex, linking it to a specific frequency of light and radiant intensity. An older, more intuitive definition was based on the light produced by a standard wax candle, and one candela is roughly the luminous intensity of a single common candle.
• Hefnerkerze (HK): An obsolete German unit of luminous intensity. It was defined in 1890 and was based on the light produced by a specific type of lamp burning amyl acetate, known as a Hefner lamp. It was used in Germany, Austria, and Scandinavia but was replaced by the candela. One Hefnerkerze is about 0.903 candelas.