Luminous Intensity Converter

Luminous intensity measures how much light a source emits in a specific direction, independent of distance — one of the seven fundamental base quantities in the International System of Units (SI). Unlike luminous flux (total light output in all directions) or illuminance (light landing on a surface), luminous intensity describes the raw output of a source aimed at a particular point, which is exactly why it's used to rate flashlights, headlights, and LEDs that project light in a focused direction.

Our Luminous Intensity converter supports the international candle (the modern candela's direct predecessor and numerically equivalent for most practical purposes), the decimal candle and pentane candle (French photometric standards), and lumen/steradian, which is dimensionally identical to the candela since it's the ratio that defines luminous intensity in the first place. It also includes several now-obsolete national standards: the German candle, UK candle, hefner candle (based on a specific German amyl-acetate lamp), carcel unit (a French oil-lamp standard), and bougie decimal — each historically defined by a physical reference light source before the candela was standardized internationally.

To use the converter, select your starting and target units from the dropdown lists and enter a value — the result is calculated instantly to up to 12 significant digits. For example, converting 1 hefner candle to modern candela gives exactly 0.9 candela, a conversion needed when interpreting older German photometric literature or antique lighting equipment specifications against modern standards.

These historical candle-based units matter because a great deal of lighting and photometric literature predates the candela's 1948 international standardization — older engineering handbooks, antique lamp specifications, and historical scientific papers may reference the German, UK, or hefner candle rather than the modern SI unit, and getting the conversion factor right is essential for correctly interpreting that data. Modern applications of luminous intensity include LED and flashlight beam intensity ratings, automotive headlight regulations (which specify minimum and maximum candela in different directions), and stage and architectural lighting design, where luminous intensity data feeds into lighting simulation software.

All conversion factors used in this tool are drawn from historically documented photometric standards and the modern SI candela definition, so results are dependable whether you're working with a contemporary LED datasheet or translating a historical lighting reference. The converter is free, requires no account, and works equally well on desktop and mobile devices.

Luminous intensity is also the starting point for calculating luminous flux — multiply intensity by the solid angle a source radiates into, in steradians, to get total lumens — so this converter pairs naturally with the intensity-to-flux calculations engineers perform when specifying a light source's total output rather than just its directional brightness.