Have you ever noticed how complicated the topic of light can be in physics? Particles, waves, EMF, refraction...

    On this page, we will tackle a specific element of light: luminous intensity. To convert between units like the candle or lumen steradian, try our free luminous intensity converter.

    Below, we keep it simple and answer a few questions to cover the basics of luminous intensity. Keep reading to discover what luminous intensity is, the ways it manifests, and how to measure it.


    What is luminous intensity?

    Luminous Intensity is the quantity of visible light. It is the amount of light that emits in a unit time per unit solid angle.

    The unit for the quantity of light emitting from any source for a moment of time (which is the luminous power or flux) is the lumen.

    Using visual sensation is one way to evaluate the lumen. The sensitivity of the human eye is greatest for light when it possesses a wavelength of 555 nanometres (10-9 meter.)

    At this wavelength, there are 685 lumens per watt of radiant power or flux. This is also known as luminous efficacy. At other wavelengths, the luminous efficiency is less.

    What's the unit for luminous intensity?

    The unit of luminous intensity is one lumen per steradian, which is the unit of solid angle. There are 4 steradians around a point enclosed by a spherical surface.

    Did you know that a steradian is useful in three-dimensional geometry, and only for spheres?

    Another unit of luminous intensity is also called the standard candle, or candela (which is one lumen per steradian.)

    Using the candle as a measure of luminous intensity is a pretty arcane way of doing things. Back in the day when candles were the main source of light for people, this is the method they knew.

    Still with us? Ok, so the next question is...

    What are examples of luminous intensity?

    Light Bulbs

    Since candles aren't used anymore, and we've switched to light bulbs, those bulbs are now rated for luminous intensity by the amount of illumination given at a distance.

    In SI units (the International System of Unit), the standard of one lumen per square meter is called '1 lux'.


    If we're measuring luminous intensity for flashlights, most of which have focused direction, we could have a one-candela (i.e. candlepower) flashlight which happens to have a beam that illuminates one-square-meter of distance.

    So then that square meter will have an illumination of 12.57 lumens, as compared with one lumen for the old-fashioned candle. This is why it's not easy and simple to compare the output of a flashlight with the output of a candle.

    Measuring Luminous Intensity

    When measuring how powerful a light source, is we need to consider its purpose- for example, a half candlepower flashlight will be more powerful than a traditional candle for camping at night.

    The candle has more candlepower (a higher candela rating) at its source, but the flashlight is way more focused and useful.

    The Moon

    A final weird fact about luminous intensity that puts it into perspective: A full moon on a clear night makes the Earth illuminated by about a quarter of 1 lux at the Earth's surface.

    In comparison, an average living room will have an illumination of about 50 lux. And direct sunlight results in anything up to 130,000 lux on the Earth's surface.

    Fun fact: the luminous intensity of the Sun is measurable on other planets using a simple math equation called the inverse square law.

    How do you convert between similar units?

    First, what is a conversion factor? It's an arithmetical multiplier for converting a quantity expressed in one set of units into an equivalent expressed in another.

    Converting a candela to lumen steradian is easy. 1 unit = 1 unit, so 100 lm/sr = 100 cd.

    Another example would be converting to lux. One steradian measured with a diameter of one meter gives a surface of one m2. So, with a measuring distance of 1 meter, the values for candela (lumen per steradian) and for lux (lumen per m2) are exactly the same.

    You can convert to lux from candela if you know the measurement distance.

    What is the formula for luminous intensity?

    The formula for luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle. It's based on the luminosity function, a standardized model of the sensitivity of the human eye.

    It looks like this on paper:

    l = r 2 · i / cos θ


    • r represents the distance in meters
    • i stands illuminance in lux
    • θ is the angular span of the light ray

    If you're not in the mood to do longhand formulas, you can get your hands on a photometer. It's an instrument that will do all the work for you and give you a direct measurement for light intensity.

    Final thoughts?

    You can always use a handy calculator to do the hard computing for you. Unless you're a fan of pulling out a pen and some paper to work out problems the old fashion way, we've got you covered.

    Want to check out a cool calculator to convert for luminous intensity? Try our converter and save yourself some time!

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