Saturday, 4 August 2012

Fluorescent Light

Photography is nothing but “capturing light”. That’s the reason why choosing the light carefully becomes a matter of paramount importance in photography.

Fluorescent is one of the toughest light sources to tackle. It is when faced with the task of taking professional quality photos under fluorescent lighting that you see most amateur photographers sweat. Let us find out why is a fluorescent light not ideal for photography and if left with no other alternative how to get the best out of them.

fluorescent light photography
Photo By IImungo

At around the 4000 K mark, long tube fluorescent lights are designed to offer diffuse, unobtrusive light. But when you are actually shooting under fluorescent lights, you may have noticed weird things happen with your images such as wide shifts to both exposure and colour temperature values from frame to frame; with all camera settings remaining the same. Yes it is true; this happens and the reason is …

A fluorescent strip light is a partially evacuated glass tube coated with phosphors “materials that fluoresce”. Inside of the tube there is mercury and when electricity passes through the tube, the mercury vapour emits strong ultraviolet light. The phosphors coating on the tube of which we mentioned earlier absorbs these ultraviolet rays and emits visible light. The exact nature of light depends on many factors like the precise combination of chemicals in the phosphors coating and that varies from manufacturer to manufacturer. Not only that as the chemical properties of materials change with age, the nature of light it emits also changes.

So Instead of generating a continuous spectrum of colours; what it actually does is produce a mixture of spikes and lows of quite distinct colours. Usually a combination of green and orange/magenta light (with distinct spikes at 430, 550 and 610nm in cheap tubes). And now we have a source generating unpleasant light and on top it flickers.

Why does Fluorescent Lights Flicker.

Fluorescent lights emit light when electricity passes through the tube and activates the atoms inside. Our household electric supply is Alternating Current (AC) which travels as a sine wave continuously reversing polarity.

sine wave graph
Sine Wave Graph

As the current goes from zero to positive to zero to negative to zero to complete a wave; there is a definite change in voltage and that is reflected on the atoms inside our fluorescent tube. They emit light of different intensity and wavelength corresponding to the changing voltage. This is the reason for the fluorescent light’s constant change in light intensity and temperature.

In the United States and Canada the electrical current alternates at 60 cycles per second (60 Hz), or one cycle every 1/60 second. With each cycle, two pulses of light will be emitted. One pulse emitted during the positive portion of the cycle and one pulse emitted during the negative portion. If you live outside of US or Canada, you need to find out the speed at which current alternates and make adjustments accordingly.

Now what determines the colour and brightness of the image captured is your camera’s shutter speed. If the shutter is open for less than 1/60 second, light from only part of the cycle will be recorded by the image sensor, and that could be any part of the cycle either bright or dim. Operating at 1/240 shutter speed, chances are; you may catch a pulse of light at its peak intensity and highest colour 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).

fluorescent light photography
Photo By VSZ

The problem gets more complicated with faster shutter speeds. When you are shooting above your camera’s maximum sync speed (1/250 for most cameras) the second curtain actually begins to close before the first curtain has reached the other end. And our light is still changing in brightness and temperature. Result will be an image of which different parts are illuminated by light of different brightness and colour temperature. You don’t expect them to be pretty, do you?

So if you want consistent results under fluorescent lights, try shooting at a shutter speed of 1/60th (to capture a full 60 Hz wave) or 1/30th (to capture two complete waves.). Can’t guarantee that your results will be pretty, but at least they will be consistent. – Readers from nations other than U.S and Canada – kindly make adjustments corresponding to the speed at which current alternates in your country. Use these shutter speeds in combination with custom white balance setting and you could get the most out of the situation.

The good news is most new generation fluorescent lights you encounter now use electronic ballasts that operate at 10 kHz or more, which means the flicker is too fast to be a problem for normal shooting. Under magnetic ballasts, you can use a lower shutter speed and/or bracket and this will usually solve the problem.

In the next article we will discuss about Photography - Types of Light - Available Light - Street Light

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  3. Effect of Location on Light
  4. Seasonal Quality of Light
  5. Night Photography