White light to lasers – collimated, coherent, Lambertian; light comes in all colors, shapes and distributions. One source defines light as “the electromagnetic radiation that may be perceived by the human eye.” Light can also be defined as the form of radiant energy acting on the retina of the eye to make sight possible. The eye is remarkable in its ability to see color as well as handle more than six orders of magnitude in brightness automatically.

Humans only see the light from a portion of the optical spectrum. The rest of the optical spectrum is in the ultraviolet (UV) or in the infrared (IR). Several animals use their IR capability to hunt. Insects use UV reflection to locate pollen sources.

The main characterization of light is by its wavelength, most often specified in nanometers (nm). We can also define a frequency to each wavelength by dividing the speed of light by the wavelength. The human eye is most sensitive to light at a wavelength of 555nm, which is equal to 5.4 x 10¹⁴ Hz. This high frequency (and its inherent bandwidth) is what makes light such a good information carrier in optical fibers.

The range of light wavelengths we see is called the Visible Spectrum. It covers a range from about 400nm to 555nm to 750nm (violet to green to red). The near-UV wavelength range just shorter than the visible is the range from which we most often want to protect our eyes and skin. Varying amounts are present in sources like high power quartz halogen lamps, fluorescence lamps, the sun and lasers used in medicine. The range just longer than the visible, the near-IR, is most commonly used in IR remote controls and is the dominant wavelength in the night sky spectrum. Even though we can not see these near-visible wavelengths our technology advancement over the past few centuries has led to the development of electro-optical sensors to detect and measure them, new light sources to produce them, and optical fiber to transmit them.

Light comes in all colors. If it is only one color or wavelength, it is called monochromatic, like a laser. If it is a blend of many colors, it is called polychromatic, such as the sun. For a good reason there is a very good match between the sun and the eye. Nocturnal animals usually have peak eye sensitivity shifted toward the near infrared where the night sky illumination peaks.

Two additional properties of light that are important in many areas of optics are the spatial distribution of the light and the coherence of the lightwaves. The spatial distribution describes the direction(s) that the light is traveling. If it appears uniform in all directions, that is it has uniform brightness from any viewing direction, it is called Lambertian. The other extreme is collimated light, which essentially travels in only one direction such as a laser. The coherence of light refers to the way the light waves are ordered or phased with each other. Common incandescent lamps are incoherent (random phase) while lasers are coherent (in-phase).

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