Friday, 11 March 2011

Sunlight, daylight and finding the right lightbulb



This article looks at the available sunlight and different types of artificial light, and how they differ. The aim is to give you an overiew to make a better choice for whatever light you need. Read it in conjunction with the slides above.

Sunlight and daylight on a sunny day. these provide that optimum light for most of us, whether to prevent SAD (Seasonal Affective Disorder), grow plants or for photography. Living in the UK (cloudy for two thirds of the time) or at night, we have a wide range of artificial lights that attempt to provide an acceptable alternative. And most of the time our light-bulbs, fluorescent tubes and even LED's are adequate. However, in certain situations, the type of light is critical. For me it is with photography, for others it may be whether they suffer depression in the dark winter months and want to try light therapy.

The way to understand the quality of the light we see, we can describe it in terms of colour, through the rainbow sequence of violet, blue, green, yellow, orange to red; If a light produces all colours in equal proportion, we see it as white. We can describe different white lights in terms of is a light more bluish or yellowish. To measure the colour of light more accurately, we can look at the amount of light produced over the full range of colours, in what is called the spectrum of light. Scientists measure the colour in wavelengths; visible light extends from violet at about 400nm (1/000 000 04 metres!) to red at 800nm. However, you can get a visual impression of how light spectra differ in the charts above.

Sunlight
Our ideal is to try and match sunlight. The sun produces an almost even spread of light from violet to red, with a tendency to produce more light towards the blue end of its spectrum. In fact, the spectrum of sunlight is similar to that of a black body heated to 5250 deg C - that fits, the sun is hot!

Incandescent Light Bulb
Until recently, the most familiar lightbulb was the incandescent light bulb. In it, a tungsten filament is heated until it glows. They typically reach temperatures of 2000 deg C to 3000 deg C (below the melting point of tungsten at 3422 deg C) The light has less blue and increasing amounts of yellow and red. Taking a photo without correction under such light gives a yellow cast to the picture. This can be corrected physically by using a blue or daylight filter, that cuts out some of the red and yellow light. For continuous lighting (in contrast to flash, see below), this is actually a very good solution with one proviso, that you can deal with the heat generated by the bulbs. More than 90% of the energy is used to produce heat!

Halogen Light
The smaller Halogen bulbs are a bit more efficient and the light is less yellow. However they work on the same principle (heating a tungsten wire) but in a halogen gas atmosphere. This means that they can be run hotter to give a whiter light. Most of the energy still goes to producing heat!

Compact Fluorescent
The main household replacement to the incandenscent light bulb is the compact fluorescent or energy saver bulb. They are much more energy efficient for the ss vapourto ame brightness, using only a third of the energy required by an incandescent bulb. They work by using electricity to make a gas glow. The glow is mainly in the ultraviolet, dangerous and invisible. So the inner parts of the tubes are coated with a phosphor. When the phosphor is lit by the UV light, it absorbs it and then emits the trapped energy as visible and safe light.

There is a benefit - and a snag with this process. The benefit is that by choosing a suitable phosphor, you can get a white light with your preferred tint (warmer yellow or daylight). The snag is that the light emitted is not in a continuous spectrum, but in a range of peaks. However, you can obtain daylight bulbs that work sufficiently well for photography and the eye. The reason is that the colour detection by the camera and that of the eye are based on observing Red Green and Blue light peaks (hence the use of the term RGB in photo-editing programs).

Special fluorescent tubes provide light at wavelengths that are optimal for plant growth, either in the Greenhouse or in the aquarium.

Light Emitting Diodes
A future growth area is the use of white LEDs (Light emitting diodes). These are actually blue LEDs that shine on a phosphor coat which then re-emits light at different colours. Most white LEDs emit light in two peaks, one at the blue end of the spectrum and the other wide peak around the yellow end of the spectrum. Again, for photography and a better match for our eyes, you need to establish that the white LED chosen actually gives sufficient peaks of red, green and blue light.
Sometimes this information is not easy to find!

Xenon Camera Flash
The last light source that is easily accessible, to photographers at least, is the Xenon flashlight. The light is created in a sudden discharge over a fraction of a second. It emits a continuous spectrum that mimics that of sunlight very well!

So what lights do I use as a photographer?
If I do not have any control, I still prefer sunlight as my light source. Next is light from incandescent bulbs, because the white balance setting in my camera can most effectively compensate for this. Fluorescent lighting is a nightmare as it can have colour tinges that the automatic colour settings on my camera do not correct for.

Where I can control lighting I use two different sources.
  1. For continuous light, I use daylight compact fluorescent bulbs for photography. They do not generate as much heat as the old incandescent halogen bulbs.
  2. Otherwise, I use Xenon flashlights as you can get cheap but reliable ones for manual operation , as slave flashes in addition to a branded one for your camera.
If you do not take light sources seriously, this can seriously impact on your photography, plant growing, aquariums or combating SAD!

This article gives you an introduction into different light types and their spectra, so that you can make more informed choices when looking for a suitable light, when daylight is not available!

What lights are you using? What lights do you need? Go and find out

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