Tuesday, January 03, 2006

Video\Photo: Ambient Light Impact

You don't have to be shooting video for very long before you begin to realize that room lighting, windows and even the time of day outdoors can really change the color fidelity of your video.

Here's why.

Today's video cameras capture images in three colors-- Red, Green and Blue. These separate color channels are combined to form a final image. To produce true white the red, green and blue values must be equal in intensity.

The light that a camera captures is either reflected light or direct light. The bulk of the image is usually reflected light. But, not all light sources are created equal. Every light source is comprised of wavelengths of light at various intensities. The wavelengths generated by different light sources can be measured with something called the Spectral Power Distribution Curve.

Here is a sample of a normal incandescent bulb:

Notice that the highest power output for this type of bulb is in the red and yellow wavelengths and that there is very little blue light generated. What you can't see is that most of the light generated by incandescent bulbs is in the form of heat. When this light is reflected off of a white card and is captured by the CCD in your video camera the result is a very orange image. That's because the blue capture cells might have a value of 1o%, the green a value of 50% and the red a value of 100%. To make the captured image look white, the blue value must be boosted 1000% and the green value must be boosted by 200% just to match the value of red.

This is called "White Balancing" and it can be performed automatically or manually in most video cameras.

If you White Balance your camera manually and take it outside, everything that you shoot will appear to be blue. That's because daylight has a very different Spectral Power Distribution Curve than incadescent bulbs. Here's what it looks like:

Notice that daylight has a lot more blue than it does green or red. So, if you boosted the blue value 1000%, the green value by 200% and left the red value alone as we did with the White Balance for incandescent then the blue value would far outweigh all the other values resulting is a very blue overall image. To correctly White Balance for this light source we actually have to boost the red values and leave the blue values as is.

So, you can see why a person standing beside a window with one half of their body lit by incandescent bulbs and the other side lit by the daylight coming through the window would never be able to be entirely color corrected. One side or the other would show up incorrectly.

But, there is another type of bulb that really gives videographers and photographers fits. That's the flourescent bulb. It has an entirely different pattern to it's Spectral Power Distribution Curve. And, to make matters worse there are several different types of flourescent bulbs commonly in use. Here's a typical 'Cool White' bulb:

The most notable thing about flourescent lights is that the bulk of their light output is in the form of spikes at particular color wavelengths. Look at that massive green spike. That's why when you shoot in a room having standard Wool White flourescent bulbs that faces tend to look greenish. Again, you can color correct this using White Balance; but, if you have mixed lighting in a room something in the image is going to suffer.

In recent years, the manufacturers of flourescent lighting have developed tri-phosphor 'Daylight Balanced' bulbs that generate total light output that mimics the great outdoors. While these bulbs don't have the continuous power distribution of natural sunlight, it doesn't really matter much, since your camera's CCD doesn't capture light continuously either! It can't directly capture magenta wavelengths. It synthesizes them by combine blue values and red values. Here is the Spectral Power Distribution Curve for Daylight balanced flourescent bulbs.

All of these images came from GE's web site. It's well worth visiting the site and comparing the Spectral Power Distribution Curves of all the various type of lighting you might encounter.


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