Passing through the lens, light is partially reflected from glass surfaces, their faces, and inner parts of the lens housing. Part of the reflected light overlays image as scattered light distributed evenly over the whole frame area. The scattered light causes additional unwanted lighting of objects. If there is no scattered light, a lens would give an image with relative brightness of different areas matched with brightness of corresponding object areas. Brightness ranges of the object and its image could be same. But it is not so in real life.
Light scattering in modern coated lenses is about 1-2%, sometimes 3% (non-coated lenses may have light scattering of 10-15%). But even this small value may affect image quality significantly in some cases. A scattered light overlay shadow parts of the image and makes them brighter that decreases overall image brightness range.
Larger overall brightness range leads to higher effect of scattered light. Image contrast drops in bright and dark areas not evenly. Bright areas almost not affected. But in dark shadows scattered light produces noticeable effect.
So, scattered light decreases overall image contrast and causes non-uniform rendering of brightness in shadows and at bright spots. On the color film, light scattering may cause unwanted toning of shadows with bright color of the object since such color will dominate in the scattered light spectrum.
Taking into account large impact of the scattered light on the image quality, it is necessary to use lenses with as small light scattering ratio as possible, and protect lenses against side light with matte boxes or lens hoods.
Light scattering value is determined by the coefficient that should be calculated as relation of the image brightness of the black object in front of the evenly bright background to the brightness of the background. Measurements should be done at fully opened diaphragm and at other aperture values. When the aperture is stopped down, light scattering is increased a bit first due to reflection of the light from film by the back surface of the aperture blades, but then light scattering decreases.
Keep in the mind that mentioned above technique of the light scattering calculation allows finding light scattering of the lens itself. In real life, actual value of light scattering is bit higher due to extra light scattering by the movie camera parts located near the film channel, and by the film surface. Final result is affected by total amount of the scattered light regardless of the scattering nature.