Daylight bulb/lamp....Newbie question!!

Matthew Rolfe

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Hello, I am trying to source the right bulbs/lamps to undertake some B&W film tests. The colour temperature suggested for these tests is 4800k and the wattage 500. Now in America there are plenty of places who supply blue daylight lamps at the correct wattage and colour, but in the uk (at 240v) I cannot find any.

What is my best option? I can find lamps at about 5000k which are high wattage but not blue coated.....do I go for this and find a lighting gel? Is the coating necessary if the colour temperature is correct? Is the extra 200k going to make a vast diffference?

Alternatively I can get bulbs at a lower Kelvin 32O0 or at 4800 at a vastly reduced wattage (100w). But this isn't the best option of course.
 

JBrunner

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It's the Kelvins that matter, not the bulb color, as a bulb that emits a specific Kelvin need not be corrected to make the specified color temp.(what the blue color on some bulbs is for, correcting a warm filament to a cooler temp, but at the expense of output efficiency for a given wattage, blue is a transmission killer with only 30% or so transmission with a tungsten source). Alternatively, you could find some hi output flo's and use a flouro-filter to round the discontinuous spectrum spikes down, but the spikes aren't really so much a factor for b&W. The 200k difference is very minor. If the 5000k bulbs are filament bulbs the addition of an extension cord will drop them 200k. Hopefully instead of just a wattage recommendation you have some illumination specifications. Output per watt varies dramatically among different types of bulbs. I'm not altogether sure why illumination/wattage specification would be really needed, as long as exposure was sufficient, and color temp was correct
 
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ic-racer

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I have used the 3200k with the appropriate blue filter on the camera.
 
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Matthew Rolfe

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Thanks for your replies.

Jbrunner...
I wasn't too sure about the bulb colours, so thanks for clearing that up. I was pretty certain that the colour didn't matter as long as the temperature was right, but I began to doubt my own knowledge...so posted this thread.
Having read your response i settled on purchasing a couple of 275 watt bulbs at just over 5000 kelvin. I originally thought that this would be the best option and you have confirmed my thoughts. Thanks! The wattage was only a rough guide to ensure decent coverage of a test card area, the emphasis is on colour temperature howether. So reducing the power input by using an extension cord will in turn reduce the Kelvin output?

Ic-racer..

What filter did you use to 'cool down' the light from 3400 to 'daylight' temperature?
 
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RMD

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I don't know whether this is any use to you,but I recently bought a 100w equivalent Natural Daylight bulb from my local 'The Range' craft store. The group has many stores dotted around the U.K.
It was a 'Craft Factory Purelite' energy saving bulb (curly variety) rated at 20w,and cost £5.
I've no idea what Kelvin it is,as it doesn't say on the packaging.
 

jeffreyg

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I don't think I would worry about the 200 kelvin difference. What you have to consider is the lens and/or the reflector on the light fixture(s) could possibly change the color temperature. 5000K is generally considered to be shadowless daylight. That would be the color temperature and the amount of light on the subject could be measured in footcandles. The amount of light can vary depending on the distance from the source as well as the number of the bulbs. It would be best to have an 18% gray card to meter off of. I don't know how you will be metering - incident or reflected?
 
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Matthew Rolfe

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Reflected metering, pentax spot meter. It is the test outlined in 'The new zone system manual' Minor White. I moved away from Ansel Adams test methods as I didn't like the idea of relying on a densitometer. Instead I wanted to evaluate test by eye, not by a machine.

Like Adams' tests it does stipulate the use of a grey card to calibrate all test procedures.

Minor White and his comrades suggested 4800 for daylight, that is why I went forr that as a starting point. But I see now that 200 Kelvin wouldn't make the slightest difference overall when taking into consideration all the other factors.

I didn't pay much attention to electronics at school, I guess it shows now....
 
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Matthew Rolfe

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Rmd....I actually looked at those bulbs in my earlier searches, but I wanted a higher wattage to evenly illuminate a larger area so went for the 275's.
 

ic-racer

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Matthew Rolfe said:
Ic-racer..

What filter did you use to 'cool down' the light from 3400 to 'daylight' temperature?
Click to expand...

80A

I use that if I need to do a quick zone 1 meter/film calibration at night or during the winter when I don't have access to good daylight target.

The blue bulbs do the exact same thing, but the filter is essentially wrapped around the bulb (and fades or darkens quickly)
 

JBrunner

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ic-racer said:
80A

...but the filter is essentially wrapped around the bulb (and fades or darkens quickly)
Click to expand...

If you are lucky enough to have them last that long!
 

JBrunner

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Matthew Rolfe said:
Thanks for your replies.

...So reducing the power input by using an extension cord will in turn reduce the Kelvin output?
Click to expand...

Yes, in the case of a bulb that uses a filament. The addition of distance and connections has the effect of lowering the voltage to the filament. It burns less bright and at a lower color temp. Most studios I work in are supposed burn 3200K, but meter in at about a real world temp of 28-2900K because of the voltage drop (caused by resistance) associated with the cables and connectors. Since the drop is more or less uniform, we tend to ignore it.

The standard Hollywood gels for color correction are CTB (towards blue) and CTO (towards orange). They come in grades 1/8, 1/4, 1/2, and "Full" from both Lee and Rosco. Full CTB will correct 3200 to about 5500 (with a 70% or so loss of transmission) or vice versa for full CTO (with a 20% or so loss of transmission). The blue filters tend to suffer from fading (because they absorb a lot of energy) unless of the highest quality (like from Lee or Rosco).

CTB and CTO stand for "color temperature blue" and "color temperature orange" respectively.
 
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Matthew Rolfe

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Thanks for the help. I have a couple of 3400k bulbs that are 500 watt so I will use a filter to bring them nearer to 5000k, like you suggest. With two bulbs at 500 watt each I can afford to lose some light transmission.

Thanks both of you for clearing up some confusion.
 
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