When we enter a home or place of business we ask the occupant to point out “trouble areas” or “areas of concern”, more often than not they’ll point out the area’s most walked over (i.e. hallways, stairways, in front of couches/chairs etc.). Over a period of time you will begin to see a “lane” pattern down the middle of a hallway for example, where it’s clearly dirty. You must keep in mind that the darkness seen is partly due to dirt, which will be cleaned away, but also carpet wear, an illusion of soiling or what professional cleaners like to call: "Traffic Lane Gray".
Carpets are designed to magnify the light that passes through the fibers. Carpet color, brightness, and luster that are seen by the human eye, is a direct function of the amount of light reflected by the carpet fiber. A good way to see how a carpet reflects light is to look at it from different directions. The carpet will look darker from one direction than from another. Consider a rug pile (illustration) or the pattern left by vacuuming which reorients the carpet pile so you see light and dark regions:
Soil, besides causing a carpet to look dirty, actually damages the carpet fibers, creating improper light reflections. Fiber abrasion is created by dirt particles scratching or scraping the surface of the fibers, similar to scratches on glass. These scratches or scrapes block light reflection, causing these areas to appear dirty and gray.
Matting is another condition that will cause improper light reflection. Matting can prevent carpet fibers from returning to their original shape. As fibers lose their shape, they twist, bloom, and fray, reducing light reflection, further causing the carpet to look dirty and gray.
Remember that wear creates an illusion of soiling, which often causes frustration and misunderstanding between homeowners and carpet care experts. Even though a carpet has been cleaned, it doesn't mean that the traffic areas will look nice and clean. The fibers of the carpet have simply lost their ability to reflect light properly.
There is not a lot that a carpet care expert can do with "Traffic Lane Gray". If a ten-year-old carpet is cleaned, you have a clean ten-year-old carpet!