Why are lights rated by their power consumption?

[grantsmith]

Why are lights rated with their power consumption rather than the amount of light they produce?

 

i.e. a 1k pup will produce less light than a 1k hmi but they are both rated 1k, different lenses and reflectors will produce a different amount of light but are both rated (in wattage) the same.

 

Am I missing something really obvious? For me it seems it would be much easier knowing how much light was comming out of each lamp

 

Thanks

[Michael Collier]

Well quantifying light output is hard to do and misleading, given the vast number of variables. Rating power consumption helps if your planning to run in a limited power environment.

 

The differences should be a matter of experience. You should know about how powerful each unit is, given its design type and power rating (a 650 fressy will be about the same power between arri, altman, mole, etc) Plus output is generally ballparked (its what scrims are for) if you install a 2K you can easily scrim it down to a 1K or a 500w equivalent output. Then small adjustments in position and angle can finetune everything.

 

The main reason is because it works. Its an easy system and everyone can agree on it. a lumen, fc, photon reading of output is a difficult system to understand, especially when the output can change between flood and spot. All that needs to be understood is that effeciencies vary between types (tungsten, HMI, led, Floro).

 

Now if you need to balpark a large instalation or setup and can't afford to under-estimate, thats where your ASC handbook comes in, along with manufacter litterature. If you know a 650 fress can throw x fc in x diameter circle, you can start to estimate just by lumen or fc/sq foot how many lights you need, and can calculate for lightloss due to scrim, diffusion, and gel combinations.

 

In general though I don't find myself calculating much more than required ambient light levels for large sets/locations. If I know that a room is 40ftx40ft and needs to reach an f4 at 200asa, I know that I need x number of lights placed to make sure I can at least pull an exposure. This is done mainly to plan for electrcity (I rarley have a spark, so I have to check that my lighting will stay under 2Kw per circut availible, less if there is a long cable run) Then you can spend time shaping the light where its needed and important (faces and backgrounds)

 

other than that you shouldn't need to plan units out so minutley. Where a 1K with CTB would work, so would a 2K HMI with scrim/nd/diffusion.

[timHealy]

Why are lights rated with their power consumption rather than the amount of light they produce?

 

i.e. a 1k pup will produce less light than a 1k hmi but they are both rated 1k, different lenses and reflectors will produce a different amount of light but are both rated (in wattage) the same.

 

Am I missing something really obvious? For me it seems it would be much easier knowing how much light was comming out of each lamp

 

Thanks

 

It becomes a matter of physics. Lights use electricity and draw power. Power can be quantified in a few ways but a 1000 watt light draws 8.3 amps at 120 volts. If you have a 20 amp curcuit and have a 5000 watt light you'll need to fiqure out another way to get that light on. For simplicity and adding a safety value, many electricians use 10 amps per 1000 watts. So a 10k draws 100 amps. If you have ten 10k you'll need a 1000 amps from a power source at 120 volts.

 

the formula watts = amp x volts works and you can use it to work with your lighting needs and power sources. Also in this equation you will know what kind of cable runs you will need from zip cord(lamp wire) to 400 to 4 ought.

 

Best

 

Tim

[Jon Kukla]

For simplicity and adding a safety value, many electricians use 10 amps per 1000 watts.

 

Yes, but only if in a country using 120V. Important to note.

[timHealy]

yes I mentioned I was using 120 volt examples twice.

 

best

 

Tim

 

PS that last line was suppose to read "from zip cord to 4 ought"

Edited August 30, 2006 by heel_e

[JD Hartman]

Depending on the brand of the globe and the type (tunsten, halogen, HMI) the lumens will vary dramically. Not so much a factor with a 650W, but starting from a halogen 2K on up, you can get more lumens per watt by choosing a globe from a specific manufacturer. That is why some Gaffers specify globes from Ushio, OSRAM , Radiac, etc. So as already stated, watts (power consumption) is the only unifying factor among different fixtures.

[Kristy Tully]

Can you imagine being a fourth electrician, just starting out and having to remember the 1K light actually needs 1.4K power, and will blow a circuit it it's with the 650w light (that really needs 1.2K power). It was hard enough for me on my first juicing job to not blow circuits with the obvious names. Let the more experienced members of the team, the gaffer and the DP, understand how much light each unit gives off under different circumstances. There are some older 1200 hmi electronic ballasts that need 20amps of power. There was a time when I was a gaffer in NY and they'd always show up on location jobs where there wasn't 20amp circuits. Just 15amps, that would blow. That was fun to explain to the DP, who had ordered them to be able to plug into the wall.

[timHealy]

Can you imagine being a fourth electrician, just starting out and having to remember the 1K light actually needs 1.4K power, and will blow a circuit it it's with the 650w light (that really needs 1.2K power). It was hard enough for me on my first juicing job to not blow circuits with the obvious names. Let the more experienced members of the team, the gaffer and the DP, understand how much light each unit gives off under different circumstances. There are some older 1200 hmi electronic ballasts that need 20amps of power. There was a time when I was a gaffer in NY and they'd always show up on location jobs where there wasn't 20amp circuits. Just 15amps, that would blow. That was fun to explain to the DP, who had ordered them to be able to plug into the wall.

 

Hey Kristy,

 

Your math is a bit off. In the US with 120 volts, 1000 watt lamps draw 8.3 amps not 1.4. But experienced electricians just round up to 10 amps per 1000 watts.

 

Just remember West Virgina which looks like WVA which means watts = volts * amps or just use a little algebra to figure out the variable you don't know.

 

I don't doubt your experience but your 1.2 par should have worked on a 15 amp curcuit. In residential homes and apartments there may be several outlets shared on one curcuit so perhaps the light was not the only thing plugged in, or as you mentioned, the ballast or head may have been old and not working properly. Your 650 watt light really should have a draw of only 6 amps. Any chance there was a larger bulb in it? With tungsten lamps one can put in larger bulbs (If it fits and the wiring can hold the draw) but with HMI's one cannot.

 

The best outlets to use are the hospital grade outlets which are 20 amp curcuits and I believe they have a little green dot on the exterior of the outlets surface. But unfortunately they don't typically get installed in homes.

 

It sounds like some of the jobs you had was so low budget that not only did they not have a gaffer on a tech scout, they did not even have a DP who looked at the panel to see what kind, and the conditon of the breakers, let alone open the box and see what size tricos you would need for your plan b.

 

best

 

Tim

Edited September 2, 2006 by heel_e