fluorescent light flicker

I am doing a bit of B&W film testing using fluorescent lighting. It suddenly occurred to me that in the USA the

lights will flicker at 120 Hz, twice the line frequency. At slow speeds this shouldn't matter, but at higher

shutter speeds (perhaps 1/60 or 1/125 and beyond) there will be uneven lighting at various points in the film

plane due to this flicker. This could throw off the film testing somewhat if the flicker is very prominent. If

the phosphors have a long phosphorescent lifetime then the flicker will be negligible. However, if the phosphors

have a short phosphorescent lifetime then the flicker will be prominent.

 

Does anyone know how bad the fluorescent light flicker really is and whether it is prominent enough to affect

film testing at high shutter speeds?

All light flicker. In the US we use 60 cycles/second. I remembr that when I was in Mexico in the 1960's they were on a 50 cycles/second and incandescent light bulbs had a definite flicker. Fluorescent lights should pose no more of a flicker problem than incandescent lights.

 

Paul

If fluorescent lights filcker badly then it's usually due to the starter/choke getting old. The persistence of the phosphor should be enough to smooth out the flicker but I have known offices etc. where the flicker has been so bad as to give me a blinding headache. If the flicker is significant then it should show up as banding on a neg. Try shooting a perfectly plain surface under fluorescent light at, say, 1/30th and then at 1/500th, process the negs and look for uneven parallel bands of density.

Banding would only show with a focal plane shutter, if it does at all. A leaf shutter (between the lens elements) would not be prone to the effect.

Frank,

 

You're right. I assumed that we were dealing with a focal plane shutter. Thanks for clarifying that.

Many modern fluorescents (including most compact fluorescents) have electronic ballasts that operate at high frequency, typically around 20 to 60 kilohertz. Flicker with these will be negligible, and at too high a frequency to affect typical photography.

 

The 120Hz flicker will only show up with older magnetic ballasts.

An incandescent bulb has a small amount of flicker at 60 hz because it takes some time for the filament to heat and cool. Thermal inertia, you could call it. The cold filament has a much lower electrical resistance than when hot. A filament at start up may draw as much as 10 times more current than after it is hot. As you may have seen when shutting off a fluorescent lamp, the phosphor continues to glow for a short time after the current flow stops. If you have access to an oscilloscope and a silicon phototransistor, you can observe the fluctuation in illumination of various light sources.

Patrick makes a couple of good points. The filament really makes a difference. That's why when choosing a lamp to modulate with sound you have to choose one with a filament that rapidly cools. Otherwise slow cooling will produce distorted sound. Of course, LED's and lasers eliminate that problem. A cool way to "hear" incadescent flicker is to connect a phototransistor to the microphone input of a small amplifier. Direct the phototransistor at the light source and you will hear the 60 HZ (USA) hum. Fluorescents and other light sources have an interesting sign too. Sometimes you can tap the bulb when listening a get a ringing sound due to the filament vibrating.

On fluorescent afterglow: that can be an issue if you have fluorescent lamps in the darkroom. Some may glow long enough to fog film when it is being loaded in a tank.

Back to the original question- I tried a series of photos with a focal plane shutter of a 40 watt fluorescent tube: 1/60, 1/500, 1/1000, 1/2000 and 1/4000 sec. No flicker. However, YMMV depending on make of tube.

I once had a problem with fluorescents that would not turn off completely. It was due to a nearby radio and/or TV transmitter. Made me wonder about the safety of living there. I no longer do, but I moved back to my home state of West Virginia when I retired from NASA. Sometimes nearby high voltage power lines can cause the problem.

Yes, RF emissions from radio transmitters and electric fields from power lines can light fluorescent tubes. A favorite activity of extremely bored youth from my teenage years was to try to get close enough to the substation to make the fluorescent lamp in you hands light up. For real fluorescent lighting fun, though, try a tesla coil.

One further thought on the original question- rather than flicker, perhaps the unusual spectrum of fluorescent lamps may be more of a problem than flicker. Look at a plot of a film's spectral sensitivity and compare it with that of various lamps. If the lamp's output dips sharply at the wavelength that corresponds to the wavelength that the film is most sensitive to, then possibly the film might be overexposed.

It is true that fluorescents may have non-uniform spectra, though I think there are some that are close to daylight now. There is not much likelihood that a panchromatic film will have a uniform spectral response. Differences in color temperature and/or spectrum are not important to overall exposure if the meter and the film have the same spectral response, but two different sources of the same color temperature may make different images of a color chart on film .