A smoke detector comprising: a cover which fits on a base; a circuit board; a chamber layer and a chamber cover wherein the chamber cover fits on the chamber layer, the chamber layer fits on the base between the cover and the base; a light source for generating a beam of light and arranged to transmit the beam along an optical axis; a sensor for receiving light scattered from the beam off the optical axis by smoke; and a light trap comprising one or more walls that define a volume for receiving light that passes unscattered from the light source past the sensor along the optical axis, where said volume comprises: a first end closest on the optical axis to the light source; and a second end furthest on the optical axis from the light source; wherein said volume is open at said first end but is otherwise closed, and wherein at least one of the walls is sloped from the first end to the second end, such that the light trap progressively narrows from the first end axially towards the second end, and wherein the open, first end encompasses substantially the whole of the solid angle of the beam, wherein said light trap, when viewed from the side of the smoke detector, comprises walls which extend above and below a direction parallel to or coincident with the optical axis and walls which extend alongside said direction, and wherein the walls of the light trap that extend above and below the said direction slope towards one another in the direction from the first end to the second end, wherein the light trap is a cone, wherein the cone is a right circular cone, wherein the cone comprises an upper portion and a lower portion, wherein the upper portion comprises legs which hook into holes formed in the chamber layer thereby securing it to the chamber layer, wherein the lower portion is formed integrally with the remainder of the chamber layer, wherein the upper portion is detachable from the lower portion. a cover which fits on a base; a circuit board; a chamber layer and a chamber cover wherein the chamber cover fits on the chamber layer, the chamber layer fits on the base between the cover and the base; a light source for generating a beam of light and arranged to transmit the beam along an optical axis; a sensor for receiving light scattered from the beam off the optical axis by smoke; and a light trap comprising one or more walls that define a volume for receiving light that passes unscattered from the light source past the sensor along the optical axis, where said volume comprises: a first end closest on the optical axis to the light source; and a second end furthest on the optical axis from the light source; a first end closest on the optical axis to the light source; and a second end furthest on the optical axis from the light source; wherein said volume is open at said first end but is otherwise closed, and wherein at least one of the walls is sloped from the first end to the second end, such that the light trap progressively narrows from the first end axially towards the second end, and wherein the open, first end encompasses substantially the whole of the solid angle of the beam, wherein said light trap, when viewed from the side of the smoke detector, comprises walls which extend above and below a direction parallel to or coincident with the optical axis and walls which extend alongside said direction, and wherein the walls of the light trap that extend above and below the said direction slope towards one another in the direction from the first end to the second end, wherein the light trap is a cone, wherein the cone is a right circular cone, wherein the cone comprises an upper portion and a lower portion, wherein the upper portion comprises legs which hook into holes formed in the chamber layer thereby securing it to the chamber layer, wherein the lower portion is formed integrally with the remainder of the chamber layer, wherein the upper portion is detachable from the lower portion.