When light leaves a flashlight or a light fixture, it spreads out and becomes wider as the distance increases. Beam spread is an industry term for the amount of light diffusion, or spreading of the beam. Manufacturers use specific measurements of beam spread including beam and field angles, and publish bulletins with these data for comparison.
Beam angle is the value that shows the angle of beam spread from the middle of the beam. This is measured by drawing an imaginary line out from the center of the light bulb, which is the center of the light beam. The angle is observed where the light intensity at the outside of the beam has dropped to 50 percent of the maximum light. The beam angle will be narrow for a spotlight designed to light smaller areas, and will be wider for a floodlight used for lighting larger areas.
Field angle is a standard measurement of the angle from the imaginary centerline of the beam to a point where the light intensity is low. Manufacturers normally check the field angle when the light intensity has been reduced to 10 percent of maximum. This angle is sometimes called the cut-off angle, but the intent is to show the effective lighting area of a light bulb or fixture design.
Beam spread values use angles rather than distances. When lighting designers are placing fixtures in a room, they use lights that provide a specific light intensity, normally measured in lumens. Lights can also be chosen for their color temperature, which will provide light with a different color in a range from yellow through white to blue. Distances of the light fixtures to walls and floors may vary greatly, but the designer uses the beam and field angle data to place lights for the best coverage.
Stage lights used for theatrical performances have published beam spread data, but many of these lights can also be adjusted manually. Lighting hung over the stage may have adjustable doors that will block light from selected areas, creating light and dark areas for dramatic effect. Spot lights used for highlighting a performer or part of the stage have lenses that can change the beam from narrow to wide, as well as changing the shape and color of the beam.
Lasers have very narrow beam spread by design, because the light beam is aligned or oriented in a single direction by the barrel or tube shape of the laser. Beam spread can occur for two reasons, diffraction and diffusion. Diffraction occurs when the light beam is turned to a slightly different angle from passing through the lens. Diffusion is a gradual spreading of the light from dust, humidity and other particles in the air, which causes the light to be reflected very slightly. Laboratory lasers are often run in very clean rooms and in vacuum chambers to eliminate air and dust and minimize diffusion.