foot-candles/wattage

[Louis]

this may sound like a really silly question, but I'm still pretty new to all this, and I figured it couldn't hurt to ask: is there a relationship of any kind between the wattage(/amperes/voltage) of a light source and the amount of foot-candles it emits? for instance, if you know your ASA, your shutter speed, and the f-stop you wanna use, is there a way to determine what kind of light (1K, 2K, etc.) would give you a proper exposure? I figure there must be something, but I am not aware of it.

[Louis]

re-reading my question, i need to clarify something: i know that there is a relationship between wattage and foot-candles (obviously the higher the wattage of a light the brighter it is), but i was wondering if there was a specific, mathematical relationship between wattage(/voltage/amperes) and foot-candles, one that would help a DP decide what kind of a light to use to get a proper exposure.

[Dominic Case]

In theory it's not a simple relationship. Watts measures the amount of energy consumed by a lamp.

 

Not all of that will go into light (in fact for incandescent lamps most of it goes into heat). The total amount of light given out by a point source in all directions is measured in lumens (so the efficiency of the lamp is in lumens per watt). You can also measure the brightness of the lamp in candelas.

 

Not all of the light will go in the direction that is useful. Putting a lamp in a reflector will increase the brightness (or luminous intensity) for the same number of lumens.

 

Foot candles measure the intensity with which something is illuminated by the light source. It depends on how close the lamp is to the surface. It's also measured in lumens per square meter (efectively the same thing as foot candles).

 

There's lots more, but you get the drift. There are many different aspects of light to measure, and there are many factors betwen watts and foot candles.

 

In practice, there may be some rules of thumb to relate a particular type of light source to the brightness, where those other facrtors are constant. But distance is always going to be the key factor.

[Michael Morlan]

As Dominic notes most eloquently, there is no specific mathematical relationship between wattage and fc. Lamp technology, lamp age, beam shaping, distance from instrument all alter this relationship.

 

That's why lighting manufacturers supply photometric tables for their instruments. There's even a book you can buy with lots of theatrical instruments noted within:

 

Photometrics Handbook

 

Some other examples:

 

Mole 2K Junior Solarspot

Arri Junior 5K

Kino Flo 4BANK Select

etc....

[John Pytlak RIP]

A tungsten lamp is usually rated in WATTS, at a stated operating voltage. Often the light output is given in LUMENS. Efficiencies vary, and can be calculated as LUMENS PER WATT. For example, a tungsten-halogen lamp is slightly more efficient than a lower color temperature tungsten lamp. A clear lamp is more efficient than a frosted lamp.

 

Light fixtures / luminaries vary in their efficiency in focusing all of the light (lumens) produced by a lamp where you want it. A spot concentrates the light more than a flood.

[Chris Keth]

To answer the rest of your question: DPs and gaffers just kind of get the feel of it from experience. One time you'll try something and it'll be far too much light so you'll try something else. next time, you'll skip that fixture that's too bright in favor of what worked last time. So students (like myself) tend to take a long time lighting things because we have to go through some trial and error. Once experience takes over some, things go much faster.

[John Pytlak RIP]

To answer the rest of your question: DPs and gaffers just kind of get the feel of it from experience. One time you'll try something and it'll be far too much light so you'll try something else. next time, you'll skip that fixture that's too bright in favor of what worked last time. So students (like myself) tend to take a long time lighting things because we have to go through some trial and error. Once experience takes over some, things go much faster.

<{POST_SNAPBACK}>

 

For lighting contrast, trust your eyes. But for exposure, a good lightmeter is a valuable tool.

[Paul Bruening]

Hello,

 

Lamps, housings, lenses, filters- everything effects footcandles. The newer meters often have footcandle measurements. That's the only way to really get a bead on output. Take the plunge and get a full-function light meter. You'll love yourself for it later.

[Rick Sharf]

As Dominic notes most eloquently, there is no specific mathematical relationship between wattage and fc. Lamp technology, lamp age, beam shaping, distance from instrument all alter this relationship.

 

That's why lighting manufacturers supply photometric tables for their instruments. There's even a book you can buy with lots of theatrical instruments noted within:

 

Photometrics Handbook

 

Some other examples:

 

Mole 2K Junior Solarspot

Arri Junior 5K

Kino Flo 4BANK Select

etc....

 

 

hey michael

thanks for the mole link, ive looked at the specs before but never noticed the ft. candles in the performance data. great links and i just ordered the photometrics handbook. (just figured another book couldn't hurt :lol: )

[Chris Keth]

Boy you dug deep for this thread.

[Xavier Plaza]

Hi Louis with this valuable comments ( Dominic, Michael and John ) you probably have a better idea..., i just wanna add this, there are one big difference between HMI and tungsten lights, when you talk in "K"values there is not the same 1K tungsten and 1.2K in HMI, because HMI "K" have more useful light ( footcandles). When you use a 1k tungsten you have less foodcandles... I think 5K tungsten light it is equivalent 1.2K HMI always talking about footcandles...

 

Let me know if i'm wrong and sorry for my english...

[Michael Morlan]

Xavier, you are essentially correct. HMI's are roughly five times more efficient generating usable light than tungsten.

 

The terrible truth of tungsten light sources is that over 80% of the energy being consumed by the bulb is output as infrared (read: heat.) Tungsten bulbs are terribly inefficient. Newer technologies (flourescent and HMI, for example) generate more usable visible light and waste much less of their consumed energy in the infra-red band.

 

Both HMI and flourescent, however, generate a bit of ultra-violet in the spectra. It is bad practice to work with a bare HMI bulb. You can get quite a nasty sunburn and suffer damage to your vision. Always operate HMI's with all the appropriate lenses in place.

 

Michael