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FORUMS Photography Talk by Genre General Photography Talk 
Thread started 20 Sep 2005 (Tuesday) 23:49
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Limitations of Fluorescent Lighting

 
Curtis ­ N
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Sep 20, 2005 23:49 |  #1

Note: The images in this post were replaced in December 2011, so some of the early comments relating to the original images are now irrelevant.

Occasionally a POTN member will ask about an exposure or white balance problem where fluorescent lighting is eventually determined to be the culprit. This thread will hopefully explain the difficulties associated with photographing under fluorescent light and advise on how best to deal with the problem.

Fluorescent light bulbs emit photons as electricity excites the atoms inside them. Our homes and businesses are powered by alternating current (AC). This type of current is continually reversing polarity in a cycle known as a sine wave, illustrated in this graph.

IMAGE: http://www.sprags.com/images/mainpower_sine_wave.jpg

As the current goes from zero to positive to zero to negative and back, the changing voltage causes the excited atoms in the fluorescent tubes to emit light of different intensity and different wavelengths. This results in a light source of continually changing brightness and color temperature.

In the US and Canada, the electrical current alternates at 60 cycles per second (60hz), or one cycle every 1/60 second. During each cycle, two pulses of light are emitted, one during the positive portion of the cycle and one during the negative portion.

If your camera's shutter is open for less than 1/60 second, light from only part of the cycle will be recorded by the image sensor, and you never know what part of the cycle you'll catch. At 1/240 shutter speed, for example, you may catch a pulse of light at its peak intensity and highest color temperature, resulting in an overexposed, blue-toned image. Or you might catch the middle of the cycle where the light output is dim, resulting in an underexposed, orange-toned image.

"No worries!" you say. "I shoot RAW, so I can adjust the exposure and white balance with my RAW converter to make it right." Sorry, but it ain't that easy. The problem is that at fast shutter speeds, the second shutter curtain begins to close before the first curtain is fully open (this is why your camera has a maximum shutter speed for flash). As the shutter curtains follow each other across the frame, different parts of the sensor are exposed at different times. So different parts of the image will be illuminated by light of varying intensity and color temperature. This makes for some truly ugly images, and even if you're a Photoshop master, it's no easy fix! Adding more fluorescent fixtures won't help, either, since they are all powered by the same current and therefore synchronized with each other.

These four images of a sheet of white paper are from a four-shot burst at 1/320 shutter.
IMAGE: http://performancephoto.smugmug.com/photos/i-GsKNHft/0/XL/i-GsKNHft-XL.jpg

So when you're faced with shooting under fluorescent lighting, here are your options:
1) The fastest shutter speed you can use is 1/120 (1/125 is close enough). Theoretically this will catch precisely 1/2 of a cycle, with all parts of the image illuminated by the full spectrum of output by the lamps.
2) The second fastest shutter speed you can use is 1/60, to catch a full cycle. This should give consistent results. Use Tv or Manual mode. You want to avoid 1/100 or 1/80.
3) Multiples of 1/2 cycle, i.e. 1/40 (1.5 cycles), 1/30 (2 cycles), 1/20 (3 cycles), etc. We're now in tripod territory, but anything slower than this should also work fine.
4) Overpower the fluorescent light with flash if you can. Set your shutter at flash sync speed, get as close to your subject as possible, and stop down until the maximum flash distance is not much more than your subject distance, forcing the E-TTL flash metering to maximize the light output.
5) Since fluorescent tubes vary in color temperature, a custom white balance is recommended, using the same shutter speed guidelines listed above.

Here is a four-shot burst of the same sheet of paper at 1/60 shutter.
IMAGE: http://performancephoto.smugmug.com/photos/i-sKCc4g6/0/XL/i-sKCc4g6-XL.jpg

A caveat to non-US readers: In Europe, the current alternates at 1/50 second. The above speeds would need to be adjusted accordingly (1/100, 1/50, 1/25, etc). If you are familiar with AC frequency standards in various parts of the world, feel free to add to this thread with that information.

We can't always control the lighting conditions we shoot in. But by understanding how both cameras and fluorescent lights work, we can maximize the quality of the images we capture in those conditions.

"If you're not having fun, your pictures will reflect that." - Joe McNally
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robertwgross
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Sep 20, 2005 23:59 |  #2

To make fluorescent lighting matters trickier, some tubes have "good phosphors" (the tube coatings that glow) and the spectrum is full. Other tubes have "bad phosphors" and the spectrum is not full. Some tubes are worn out and getting dim, or else the control circuit allows them to flicker visibly. For some strange reason, you tend to find all of these in churches where weddings are held, so the wedding photographer must check this stuff out in advance. If fluorescent lighting is there, then the slow shutter techniques are relevant and overpowering the fluorescent light is generally the way to go.

The graph at the top has a confusing vertical scale.

---Bob Gross---




  
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Curtis ­ N
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Sep 21, 2005 00:02 as a reply to  @ robertwgross's post |  #3

robertwgross wrote:
The graph at the top has a confusing vertical scale.

You're right. I'll fix it.


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SkipD
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Sep 21, 2005 06:41 |  #4

Curtis, what is the cause of the gradient in brightness in your example shots? I fully understand the color and brightness differences but not the gradients in each frame. I suspect is has to do with the angle at which the paper was lit.

Thanks.


Skip Douglas
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Curtis ­ N
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Sep 21, 2005 08:15 as a reply to  @ SkipD's post |  #5

SkipD wrote:
what is the cause of the gradient in brightness in your example shots?

Maybe I should have explained it, or maybe I should have setup the shots with more even lighting. The sheet of paper was taped to the edge of a fluorescent shop light and hanging below it. So the high side of the paper was closer to the light and therefore brighter.

Edit 9/22/05: The original images that Skip referred to have been replaced by shots taken under even lighting. Any variation in brightness across the frames is indeed caused by the fluorescent light phenomenon referred to in the original post.


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Sep 21, 2005 08:46 |  #6

Good thread, Curtis!
"4) Overpower the fluorescent light with flash if you can." On a big room, I'd try to just fill the light on the important area & let the rest of the room go a touch darker (& greener). It helped to isolate the subject. Other times I'd use two lights, like this overexposed (cull) example:
Simple "every-day-emergency" location lighting


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Curtis ­ N
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Sep 21, 2005 23:38 |  #7

Bob, Skip, Frank, thanks for your insights and input.

I replaced the original images with new ones taken with more even lighting. Hopefully that will help avoid confusion.

I'm still hoping some of our members in the other hemisphere will provide information on AC frequencies in other parts of the world.


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SkipD
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Sep 22, 2005 07:39 |  #8

This thread should be a STICKY thread. Pictures say far more than words can, and so many folks getting into photography don't understand flourescent lighting at all. Good job, Curtis.


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Sep 22, 2005 07:45 as a reply to  @ Curtis N's post |  #9

Curtis -

Excellent post.

We had a recent POTN query on halogen lighting which uses tungsten filaments but delivers a brighter light with a continuous spectrum. The same type of variation was noted at high shutter speeds. I think the problem is generic to any lighting source powered by AC, including your humble incadescent bulbs.

Since you're already set up, perhaps you could check standard incadescent light bulbs also!

This is the post I'm referring to: https://photography-on-the.net/forum/showthre​ad.php?t=99429

My guess is that color temperature varies over the AC power cycle when using other AC driven light sources. With fluorescent, the problem is simply uglier and much harder to correct for because of the underlying discontinuous spectrum.


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SkipD
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Sep 22, 2005 08:00 as a reply to  @ maderito's post |  #10

maderito wrote:
We had a recent POTN query on halogen lighting which uses tungsten filaments but delivers a brighter light with a continuous spectrum. The same type of variation was noted at high shutter speeds. I think the problem is generic to any lighting source powered by AC, including your humble incadescent bulbs.

I don't think that lamps which emit the light by heating a filament are going to exhibit the same problem as flourescents because the heat of the filament (and thus the emitted light) can't change nearly as fast as the fluctuating voltage in the AC power. Flourescent light isn't generated by heating, and that light source definitely does change at twice the AC power cycle rate.

I think the problem that was described in the other thread was really caused by the mixing of flourescent light with that from the halogen lights.


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Curtis ­ N
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Sep 22, 2005 08:19 as a reply to  @ SkipD's post |  #11

SkipD wrote:
This thread should be a STICKY thread.

Well, I did add it to my post in the Photography Tips & Tutorials List. So hopefully we can direct people to it when the need arises.

I agree with Skip's opinion on tungsten lighting. When you turn an incandescent or halogen bulb on or off, you can tell that the filament takes a second or two to warm up and cool down. Larger, higher wattage filaments are slower. It's unlikely that the temperature changes significantly within a 1/60 second cycle.


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Sep 22, 2005 08:25 as a reply to  @ SkipD's post |  #12

SkipD - You certainly could be very right. Curtis could prove the point one way or the other! I've read that incadescent light does vary its temperature color over time. What you say makes a lot of sense too. So now I'm very curious to know. Remember - it would be an unsual shot that uses high shutter speeds under available incadescent light. So it's not a problem that would come up very often.

Addendum:
I just did a high speed burst of a white sheet of paper illuminated by a halogen light (tungsten filament) with shutter speed = 1/400. On review, the image and the histogram changes across the burst.


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SkipD
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Sep 22, 2005 08:42 |  #13

I had to try it too..... I just did a series with my 20D at 1/400 shutter speed under a halogen desk lamp (small 50-watt halogen bulb). Absolutely no difference in brightness, color, or appearance of the histogram in a half dozen exposures.


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Sep 22, 2005 09:04 |  #14

I've read that incadescent light does vary its temperature color over time.

"over time" is the key. Generally, it does as the filaments age & the lamp surface is coated with crap in the air. This can take months.
There's another more important factor, too. 120v & 60 cps is a "non-standard". Government regulations mandate that an average per 24-hour period be met. With varying loads during the day, there can be a wide variation in the power that you're getting in any given hour. The 24-hour average is usually made up in the last hour of the day! Wish I could remember the source for that info.
Do I worry about it? No. I have better things to do, so I use CustomWB & RAW & get on with the job.


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PacAce
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Sep 22, 2005 09:05 as a reply to  @ SkipD's post |  #15

SkipD wrote:
Curtis, what is the cause of the gradient in brightness in your example shots? I fully understand the color and brightness differences but not the gradients in each frame. I suspect is has to do with the angle at which the paper was lit.

Thanks.

The reason for the gradient in brightness when shot at 1/320 is because the full frame is not exposed at the same time. The curtain travels across the frame exposing just part of the frame as it goes from one end to the other. At 1/320th of a second, the frame is exposed to about 1/5 of a complete cycle or just short of a quadrant's equivalent (see Chris' graph). Depending on the timing, the shutter could be opening when the flourescent light is at or near the peak and going towards zero (Chris' first pic in burst), going from just past 0 towards the peak (2nd pic) or going from one side of 0 to the other side of 0 (3rd pic). If the timing were such that the cycle was approaching the peak when the shutter was closed, the frome would be mostly grayish blue with a hint of brown at both side of the frame.

When the shutter speed is at or below sync speed, the frame is fully exposed so the gradient is less dramatic. However, there will still be a slight gradient because the curtains still have to travel from one end of the frame to the other to open up and it takes a finite amount of time to do that and during this time, the flourescent light is continually changing in intensity.


...Leo

  
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Limitations of Fluorescent Lighting
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