A lens for covering an array of light emitting diodes mounted to a printed circuit board in a visual alarm device, the lens being a dome lens extending upwardly in an axial direction about a center axis and outwardly in a radial direction, the lens comprising: a first portion which is a bottom portion, the first potion forming a first annular ring about the center axis, the first portion including a first inner surface, a first outer surface and a bottom surface, wherein the first portion is oriented so that a first tangent to the first inner surface is at a first angle to the center axis, wherein the first angle is less than forty-five degrees, a second portion which is an intermediate portion, the second portion being continuous with the first portion, the second portion forming a second annular ring about the center axis, the second portion including a second inner surface and a second outer surface, wherein the second portion is oriented so that a second tangent to the second inner surface is canted toward the center axis, a third portion which is a top portion that is continuous with the second portion to enclose the lens, the third potion forming an annular solid disk about the center axis, the third portion including a third inner surface and a third outer surface, wherein the first inner surface, the second inner surface and the third inner surface together form a lens composite inner surface that is aberration-free and continuous; wherein the first outer surface, the second outer surface and the third outer surface are each convex and together form a lens composite outer surface that is an aberration-free and continuous dome; and characterised in that the lens includes a fourth portion which is a base portion that extends axially below the first portion, the fourth portion being cylindrical and having a fourth outer surface that is continuous with the first outer surface, and a fourth inner surface that extends downwardly from the bottom surface of the first portion, wherein the fourth inner surface and the bottom surface are configured to be positioned against the PCB. a first portion which is a bottom portion, the first potion forming a first annular ring about the center axis, the first portion including a first inner surface, a first outer surface and a bottom surface, wherein the first portion is oriented so that a first tangent to the first inner surface is at a first angle to the center axis, wherein the first angle is less than forty-five degrees, a second portion which is an intermediate portion, the second portion being continuous with the first portion, the second portion forming a second annular ring about the center axis, the second portion including a second inner surface and a second outer surface, wherein the second portion is oriented so that a second tangent to the second inner surface is canted toward the center axis, a third portion which is a top portion that is continuous with the second portion to enclose the lens, the third potion forming an annular solid disk about the center axis, the third portion including a third inner surface and a third outer surface, wherein the first inner surface, the second inner surface and the third inner surface together form a lens composite inner surface that is aberration-free and continuous; wherein the first outer surface, the second outer surface and the third outer surface are each convex and together form a lens composite outer surface that is an aberration-free and continuous dome; and characterised in that the lens includes a fourth portion which is a base portion that extends axially below the first portion, the fourth portion being cylindrical and having a fourth outer surface that is continuous with the first outer surface, and a fourth inner surface that extends downwardly from the bottom surface of the first portion, wherein the fourth inner surface and the bottom surface are configured to be positioned against the PCB.