A ceiling-mounted visual-alarm light emitting device comprising: a base unit mounted on a ceiling and projecting downwardly therefrom; a pair of diffuse light sources mounted on opposing side surfaces of the base unit, each light source lying on a respective optical axis that is perpendicular to the respective side surface of the base unit; and a pair of lenses, each lens being mounted over a respective one of the light sources and being configured to redirect incident light emitted forwards from the side surface of the base unit by the light source, the lenses comprising: a transparent lens body having a back surface lying on the side surface of the base unit; a cavity in the lens body projecting inwards from the back surface along the optical axis, the lens being mounted over the light source so that the light source is received in the cavity; a first refractive boundary (41, 43, 48, 49) at the inner surface (41, 43, 48, 49) of the cavity; a second refractive boundary at a front surface of the lens body; and a reflector located in the lens body above the cavity for deflecting incident light emitted upwards from the cavity, via the first refractive boundary (41, 43, 48, 49), in a transverse direction towards the front surface of the lens body; wherein the second refractive boundary is arranged so that all of the light emitted from the cavity via the first refractive boundary (41, 43, 48, 49) is incident on the second refractive boundary directly or via the reflector; wherein the edges of the second refractive boundary are arranged to refract light incident thereon, emitted from the cavity via the first refractive boundary (41, 43, 48, 49), in a rearward direction both in a vertical plane and in a horizontal plane, whereby the light distribution from the pair of lenses overlaps at a predetermined distance from the base unit to give a continuous light distribution in a hemisphere around the base unit. a base unit mounted on a ceiling and projecting downwardly therefrom; a pair of diffuse light sources mounted on opposing side surfaces of the base unit, each light source lying on a respective optical axis that is perpendicular to the respective side surface of the base unit; and a pair of lenses, each lens being mounted over a respective one of the light sources and being configured to redirect incident light emitted forwards from the side surface of the base unit by the light source, the lenses comprising: a transparent lens body having a back surface lying on the side surface of the base unit; a cavity in the lens body projecting inwards from the back surface along the optical axis, the lens being mounted over the light source so that the light source is received in the cavity; a first refractive boundary (41, 43, 48, 49) at the inner surface (41, 43, 48, 49) of the cavity; a second refractive boundary at a front surface of the lens body; and a reflector located in the lens body above the cavity for deflecting incident light emitted upwards from the cavity, via the first refractive boundary (41, 43, 48, 49), in a transverse direction towards the front surface of the lens body; a transparent lens body having a back surface lying on the side surface of the base unit; a cavity in the lens body projecting inwards from the back surface along the optical axis, the lens being mounted over the light source so that the light source is received in the cavity; a first refractive boundary (41, 43, 48, 49) at the inner surface (41, 43, 48, 49) of the cavity; a second refractive boundary at a front surface of the lens body; and a reflector located in the lens body above the cavity for deflecting incident light emitted upwards from the cavity, via the first refractive boundary (41, 43, 48, 49), in a transverse direction towards the front surface of the lens body; wherein the second refractive boundary is arranged so that all of the light emitted from the cavity via the first refractive boundary (41, 43, 48, 49) is incident on the second refractive boundary directly or via the reflector; wherein the edges of the second refractive boundary are arranged to refract light incident thereon, emitted from the cavity via the first refractive boundary (41, 43, 48, 49), in a rearward direction both in a vertical plane and in a horizontal plane, whereby the light distribution from the pair of lenses overlaps at a predetermined distance from the base unit to give a continuous light distribution in a hemisphere around the base unit.