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    Footcandles, Lumens explained
    Senior Member
    Join Date
    Aug 2007
    Can anyone explain me this in a noob-friendly manner? I mostly see visually what a light is capable of, but it would be nice to be able to read it from specifications.

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    Senior Member capitanfracassa's Avatar
    Join Date
    Jun 2006
    the theory is headache inducing, but for practical purposes:

    A lumen is a measure of visible light output or luminous flux. Essentially, it is a quantity that describes a light source. If you want more lumen you have to spend more watts or use a higher efficiency light source (more lumen per watts.) for example HMI will give you 80-100 lumen per watts. Halogen about 20 and Fluorescent about 60-70 (from memory). So a 150W HMI will give you about 13,000 lumen.

    Light disperses in space. An imaginary sphere whose center is your light source will always receive all your lumens. But as the shpere grows larger, a 1sq foot area on that sphere will be a smaller percentage of the whole surface and receive less and less light.

    A footcandle is the light level achieved by dropping 1 lumen on a square foot of surface. The further away your light source is, you need more lumens at the source to achieve the same light level at the destination. The decrease in light level is inversely proportional to the square of the distance. I.E. if you double the distance, you reduce the light level at the destination by a factor of four.

    Light level determines how much light falls on a surface. Each surface absorbs some of the light and reflects the rest. photo folks usually learn that an average scenes reflects back about 18% of the light. The light reflected back from the surface is the quantity that a video camera translates into a voltage, i.e. the level which you adjust using f stops, shutter and gain.

    If your light sources were a perfect point outputting light in all directions, you could calculate the light level from the lumen and the distance. That doesn't really work because lights have size, reflectors, fresnel lenses etc. So you need to either measure your lights or use tables or manufacturers specs.

    Manufacturers will say for example that a lamp with a fresh bulb will provide X fc at a distance of Y feet. You can translate that into f stops and shutter speed if you know the sensitivity of the camera. That works for film better than for video, but you can have a sense of how much light you need.
    Last edited by capitanfracassa; 07-05-2008 at 12:48 PM.

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