Wednesday, May 13, 2009

Temperature of Light - part 2 (Artificial Light Sources)

Now that we've covered natural sunlight, let's take a look at some common artificial light sources and compare them against sunlight. The following chart shows three light sources, sunlight, incandescent, and a daylight fluorescent. I haven't yet found a chart for a studio strobe light, but it should look quite similar to the daylight fluorescent, but without the sharp peaks (we discuss that a few paragraphs from now).
The chart below shows light from beyond ultraviolet down through infrared. The visible spectrum is shown as a rainbow fountain fill from violet to red. This chart is only an approximation of the characteristics for these light sources. The colors shown are not a perfect representation, but are intended only to mark where visible light starts and stops. The numbers along the x-axis are the wavelength specified in microns and the y-axix is amplitude in unspecified units.

Looking at the chart, one of the first things to notice is that the sun's emissions (dotted black line) are strongest over the visible spectrum. This of course should be no surprise whether you are an evolutionist, a creationist, or anything in between. Our eyes are optimized for the light that is most prevalent on earth and this chart confirms this as fact.

Incandescent Light
The oldest continuous artificial light source of which I am aware, is the incandescent light bulb (dashed red line). Although it is a continuous spectrum light source, it is interesting to note just how little of this light's output falls in the visible spectrum. Its output curve is slanted heavily toward red and contains only a small amount of blue or violet. Even its strongest color within the visible spectrum falls far below its main concentration of output in the infrared. Most of the energy from an incandescent bulb simply gets expended as heat. This, along with a spectral content that is not a good match for sunlight makes the incandescent light source far from the ideal photographic light source. The amount of power required for good illumination will tax the wiring of most any building and the heat generated will cause our poor subject to perspire (or pass out).

For years photographers used incandescent lights with special coatings that improved the spectral content. Nonetheless, these lights require immense amounts of power to achieve reasonable lighting levels and they get extremely hot, to the point that many, myself included, consider them unsafe and a bad choice considering the available alternatives.

Daylight Balanced Fluorescent Light
The daylight balanced fluorescent bulb is one of the wonders of our modern technology and actually does a nice job of lighting. This continuous output light source outputs a continuous spectrum of light that closely mimics the visible spectral output of the sun. The intensity tends to be a bit low, but you can see from the chart that it has a basic curve that is similar to sunlight. The balance of emissions for a properly designed daylight bulb is close enough to a sunlight that you can use the sunlight setting on a digital camera's white balance with very good results. Note that very little emission falls in the infrared, which is indicative of the excellent efficiency for this type of lighting. As light bulbs go, these operate with very little heat.You will note one small anomaly with this light source. There are two large peaks in the output, one at about 430 microns and another at about 540 microns. Fortunately, our eyes tend to average out the light's emissions and so we don't notice these peaks. Because one does not encounter pure colors in real life, the peaks do not significantly impact the light balance for a digital camera.

Overall, my view is that a daylight balanced fluorescent light source is well suited to photography. The negatives for this light source are that the intensity degrades and the spectral output tends to shift over the life of the bulb. Also, the intensity cannot easily be varied, so most light fixtures that are based on this technology, use arrays of bulbs that get switched on or off to obtain a desired output level. This of course limits the number of settings and increases the expense.

Studio Strobe
A studio strobe outputs a very bright pulsed light with a continuous spectrum that closely matches daylight. On account of this, it is one of the best artificial light sources for photography. Because it is pulsed, the energy required is very low compared to a continuous light source. The power output for a studio strobe can be accurately controlled over a broad range of power settings. The significant disadvantage is that the photographer cannot see the lighting until after making the exposure and processing the resultant image.

To allow the photographer to visualize the lighting, most studio strobes come equipped with incandescent modeling lights. These are much lower power than would be needed for most purposes, but allow one to at lest see where the shadows fall. Typical wattages are around 150 – 250 W. They tend to generate a fair amount of heat, as is common for all incandescent lights. In the future, we will likely see then incandescent modeling lamp replaced with LED based modeling lamps.

For more information on artificial lighting sources, visit the National Lighting Product Information Program website.

In the next postings I will begin to demonstrate the effect of using the light sources that have been discussed and how one can effectively achieve a proper white balance whether shooting raw or jpeg.

Until then,


Continue on to "Temperature of Light - part 3 (An Introduction to Histograms)

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