A device for measuring the particle content of air, the device comprising: - a laser source for generating laser light and for directing it to a target area, - an air channel for directing the air subjected to the measurement as an air flow through the target area, - a light detector located adjacent to the target area for detecting bursts of light generated as the particles entrained with said air flow scatter said laser light in the target area, and - a light trap between said laser source and said target area for reducing stray light propagating to the target area, the light trap comprising at least two intermediate walls which are substantially transverse in relation to the propagation direction of said laser light, each of the intermediate walls having an aperture for passage of a desirably limited amount of the laser light through the given intermediate wall , characterized in that - said at least two intermediate walls are part of a same piece manufactured as one continuous block, such that each of said apertures is delimited along the full perimeter of the aperture by a continuous portion of said piece manufactured as one continuous block, - said at least two intermediate walls include a first intermediate wall and a second intermediate wall , which are located in this order from the laser source towards the target area, - the aperture in the first intermediate wall is conical in a cross-section taken along the optical axis of the laser light, such that it is larger from the side of the laser source than from the side of the target area, and - the aperture in the second intermediate wall is conical in a cross-section taken along the optical axis of the laser light, such that it is larger from the side of the target area than from the side of the laser source . - a laser source for generating laser light and for directing it to a target area, - an air channel for directing the air subjected to the measurement as an air flow through the target area, - a light detector located adjacent to the target area for detecting bursts of light generated as the particles entrained with said air flow scatter said laser light in the target area, and - a light trap between said laser source and said target area for reducing stray light propagating to the target area, the light trap comprising at least two intermediate walls which are substantially transverse in relation to the propagation direction of said laser light, each of the intermediate walls having an aperture for passage of a desirably limited amount of the laser light through the given intermediate wall , characterized in that - said at least two intermediate walls are part of a same piece manufactured as one continuous block, such that each of said apertures is delimited along the full perimeter of the aperture by a continuous portion of said piece manufactured as one continuous block, - said at least two intermediate walls include a first intermediate wall and a second intermediate wall , which are located in this order from the laser source towards the target area, - the aperture in the first intermediate wall is conical in a cross-section taken along the optical axis of the laser light, such that it is larger from the side of the laser source than from the side of the target area, and - the aperture in the second intermediate wall is conical in a cross-section taken along the optical axis of the laser light, such that it is larger from the side of the target area than from the side of the laser source . <EOS>