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The word halo is used to denote a dim disc of light or a blurred circle surrounding the images of a light source. Halos are commonly observed when looking through an ice cloud at the moon or the sun. In this case the ice particles lead to scattering. In the case of optical media dimming of the eye-for example, in keratopathy or cataract, halos are often perceived when looking at a light source at night. In this case too, the light is deviated by scattering. The essential point in the formation of halos is that light rays are directed outside the focused image. Sometimes patients with multifocal intraocular lenses (IOLs) also report seeing halos 1 2 when looking at a bright light source at night. Because of the clear media of multifocal IOLs the formation of halos must be explained by a different mechanism. Most multifocal lenses include two main focal points. The first one, comparable with the focal point in a monofocal IOL, focuses all distant objects sharply on the retina. The second focal point is generated by the same lens power plus approximately 3.5 D, depending on the lens type, to focus objects at reading distance sharply on the retina. The most common types of multifocal lenses are either diffractive or refractive. Diffractive IOLs acquire a second focal point by diffraction of light at concentric circles on the posterior surface of the lens, similar to the physical principle of the Fresnel phase plate. 3 Today, the most common diffractive multifocal IOL is the 811E (Pharmacia) made of PMMA (poly(methylmethacrylate)). In our investigations we used a refractive multifocal IOL (SA 40N, Allergan). In this lens five annular aspherical zones are incorporated in the anterior surface of the lens to obtain the multifocal function. Each zone contains continuous curves of refractive power up to 3.5 D. The central 2 mm is used mainly for the distance focus. The light distribution varies with pupil size. At a pupil diameter of 4 mm 50% is directed to the distance focus point, 35% to the near focus point, and 15% to intermediate foci. A ray diagram shows the theoretical origin of halos (Fig 1 ). Two different situations were taken into consideration, the first with an object at infinity and the second with an object at reading distance. Halos form because the out of focus image has a larger diameter than the sharp image on the retina.