001) second] to femtoseconds [10-15 (0.000000000000001) second]. This characteristic allows one technique for medical optical imaging to be possible.The speed of light is approximately 300,000,000 meters per second in a vacuum. The speed of light generally decreases when light passes through a media. The ratio of the speed of light in vacuum to the speed of light in a media is referred to as the index of refraction, n (n = cv / cm). The index of refraction of dry air is 1.00029. Thus, the speed of light in a vacuum is very similar to the speed of light in our atmosphere and will be used interchangeably in this presentation. The index of refraction of breast tissue is approximately 1.54, and therefore light travels more slowly in breast tissue than in air.A laser beam directed into a breast undergoes a unique phenomenon. Light photons do not travel in a straight-line through the breast and thus individual photons require different times to travel through the breast. This phenomenon cannot be seen unless short pulses of light are used. Figure 1 illustrates this phenomenon. Figure 1a illustrates a case where the light pulse traveled straight through the breast and the pulse shape was preserved when the pulse exits the breast. This does not happen in nature. Figure 1b illustrates a pulse stretching that the light pulse experiences as the photons pass through the breast. Photons might be expected to arrive at time (t) but none actually do. It has been calculated that detection of a photon that passed straight through will occur once in 1017 years, a time older than the age of the universe. The actual time required for a photon to pass through the breast will depend on the path taken through the breast. The speed of light in the breast is estimated to be approximately 200,000,000 meters per second, or approximately 2 mm per 10 picoseconds (ps).2mm = 200,000,000 meters per second x 0.00000000001 seconds = 2 x 108 x 1 x 10-11 = 0.002 m = 2mmFor...
