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To convert a measurement from candela (cd) to Hefnerkerze (HK), you use the conversion factor that 1 candela is approximately equal to 1.107 Hefnerkerze.
Example:
Convert a luminous intensity of 100 cd to HK.
100 cd × 1.107 HK/cd ≈ 110.7 HK
Answer: A luminous intensity of 100 cd is equal to approximately 110.7 Hefnerkerze.
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.
Iᵥ = dΦᵥ / dΩ.Φᵥ = Iᵥ × 4π.Eᵥ = Iᵥ / r².Luminous flux (lumens) measures the total amount of visible light produced by a source in all directions. Luminous intensity (candela) measures the 'brightness' or concentration of that light in a specific direction. A spotlight can have a very high candela rating because it concentrates all its lumens into a narrow beam, while a bare bulb with the same lumen rating will have a low candela rating because its light is spread out.
Think of a standard wax candle. The luminous intensity of that candle in any particular direction is approximately one candela. This was the original basis for the unit.
It is a base unit because it cannot be derived from the other six base units (meter, kilogram, second, ampere, kelvin, mole). It represents a fundamental aspect of how we perceive the physical world (the brightness of light) and is the foundation for all other photometric units like lumens and lux.
LED manufacturers often specify the luminous intensity of their products in candelas (or millicandelas, mcd) to indicate how bright the LED is when viewed from directly in front. A high-candela LED is highly directional and appears very bright when looked at straight-on.
Just as a regular angle measures a 'slice' of a 2D circle, a solid angle measures a 'cone' of 3D space. The unit for solid angle is the steradian (sr). The total solid angle of a sphere is 4π steradians. Luminous intensity is luminous flux per steradian.