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Old January 23rd, 2004, 07:53 PM   #1
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explain light wattage

Can one of you experts explain what X wattage translates to? I mean how much does 5W, 10W, 20W, 50W, and 100W light? If you want to light someone 10-15ft away, how much power do you need?

Thanks,
Vincent
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Old January 23rd, 2004, 10:23 PM   #2
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Vincent,
Well, it actually doesn't work that way. Watts are units of power/power consumption. They are not directly applicable to incident illumination, which is what you are asking.

A forum like this is far too limited to offer any meaninful primer tutorial. My recommendation: get a good book on basic lighting. John Jackman's book is an excellent place to start.
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Old January 24th, 2004, 01:37 AM   #3
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Look at http://www.lowel.com
and click onto the catalog and then the lighting systems catalog. Each light model has a "Lamp beam data" box as well as a performance graph.

The output varies with the type of lamp, type of reflector, area of beam . Is the light indirect (bounced off of an umbrella or through a soft box) see http://www.photoflex.com. Often a light will be filtered through a fabric baffle for difusion (as in a soft box or umbrella) or through a scrim (wire screen)to cut the light down.

Safety screens will also effect light output. Some lamps use a clear safety with almost no loss while a Lowel wire mesh safety cuts light transmision by 30%.

GE has an "HIR" lamp that will give more light for less wattage. From the GE BS and Propaganda sheet "The exclusive GE HIR is the most efficient halogen lamp, offering 35% more lumen per watt "

It does work, I have one. It enables me to get 1000 watts worth of light out of a 650 watt lamp. The result is cooler operation and I can plug several lights into one circuit without overloading breakers or fusing. Most of the time you won't need high wattage output for DV though.

Look at the Ushio catalog for bulb types, wattage and the resultant light output. Save the PDF file, the UShio cat is the easiest to read and mirrors the Sylvania and GE book. The lamp designations are the same through all of the manufacturers.

http://www.ushio.com/files/entcatalog3.pdf
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Old January 24th, 2004, 09:51 AM   #4
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One thing to keep in mind when thinking about light is the "law of inverse squares". This might be stated as "the intensity of illumination from a light source is inversely proportional to the square of its distance".

In other words, you must consider the AREA of what you're lighting, not the distance from the light. To keep it simple, consider a 100 watt floodlamp at a distance of 10' from a wall. Let's say it covers an area on the wall that is 10' wide x 10' high. Now move the lamp 20' from the wall. You are now covering a 20' wide x 20' high area. If you wanted to maintain the same level of brightness at this distance you would need to use a 400 watt lamp, not a 200 watt lamp.

Why? Think about it... the area of of first example is 10 x 10 = 100 square feet. The second example is 20 x 20 = 400 square feet. To maintain the same amount of lumens per square foot you need 4 times the wattage.
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Old January 24th, 2004, 10:37 AM   #5
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Thank you for the explanations. I guess there is no clear answer or, more specifically, it is hard to compare the amount of light output by different light manufacturers.

I was looking at the portable lights from Bescor and their website indicates 10, 20, 50, and 100W lights. It's hard to figure out exactly how much area/distance you can light with these outputs. Then it gets more confusing when you try to compare it with lights from other manufacturers. Do you get the same light output from 20W light from CompanyA as 20W light from CompanyB? Answer seems to be no.

Thanks.
Vincent
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Old January 24th, 2004, 11:41 AM   #6
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If you read through the lowel catalog you'd find some lower wattage examples in their I lights. It'll give you a better feel ,that's all.
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Old January 30th, 2004, 12:35 PM   #7
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Adding to Boyd’s excellent explanation of the inverse square law, you also have to consider the falloff.

If you need to light your subject only, a lower wattage source close could be used. Because of the IS law objects in the background would be many f-stops lower. However, if both your subject and background objects require illumination, using a higher wattage source further from your subject can provide the same light falling on the subject, while lighting background objects 1 or 2 f-stops lower, depending on there distance of course.

Bear in mind that anything in front of your subject will be blown out.
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Old January 30th, 2004, 01:22 PM   #8
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<<<-- Originally posted by Vincent Im : Do you get the same light output from 20W light from CompanyA as 20W light from CompanyB? Answer seems to be no.

Thanks.
Vincent -->>>

Well, yes and no. If you're comparing exactly the same TYPES of light, then yes, 20W means 20W and they should be the same.

But no, because if there's anything different, the results could be drastically different light output. For example, a 20W HMI will put out 4x as much light as a 20W tungsten. A 20W focused spotlight will give much, much more intense light than a 20W softlight. A 20W fluorescent will put out as many lumens as a 20W HMI, or as an 80W tungsten bulb -- but the fluorescent's going to have dramatic dropoff over a short distance, whereas the HMI might still be lighting something up 50 feet away.

There really is no short answer -- you've got to go through the process of reading and educating yourself so you know what the variables are... or, alternatively, go to a trade show like NAB or DVExpo, see the products, put 'em through their paces, and decide based on that.
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