A device for detecting particles in air; said device comprising: a flow channel comprising an inlet and an outlet, wherein the flow channel is configured to allow a flow of air comprising particles through the flow channel from the inlet to the outlet; a light source configured to illuminate the particles in the flow of air, such that an interference pattern is formed by interference between light being scattered by the particles and non-scattered light from the light source; an image sensor comprising a plurality of photo-sensitive elements configured to detect incident light, the image sensor being configured to detect the interference pattern, and wherein the image sensor is configured to acquire a time-sequence of image frames, each image frame comprising a plurality of pixels, each pixel representing an intensity of light as detected by a photo-sensitive element of the plurality of photo-sensitive elements; and a frame processor configured to filter information in the time-sequence of image frames, wherein said filtering comprises: identifying pixels of interest in the time-sequence of image frames, said pixels of interest picturing an interference pattern potentially representing a particle in the flow of air, and outputting said identified pixels of interest for performing digital holographic reconstruction on the identified pixels of interest; and discarding pixels not identified as pixels of interest from holographic reconstruction. a flow channel comprising an inlet and an outlet, wherein the flow channel is configured to allow a flow of air comprising particles through the flow channel from the inlet to the outlet; a light source configured to illuminate the particles in the flow of air, such that an interference pattern is formed by interference between light being scattered by the particles and non-scattered light from the light source; an image sensor comprising a plurality of photo-sensitive elements configured to detect incident light, the image sensor being configured to detect the interference pattern, and wherein the image sensor is configured to acquire a time-sequence of image frames, each image frame comprising a plurality of pixels, each pixel representing an intensity of light as detected by a photo-sensitive element of the plurality of photo-sensitive elements; and a frame processor configured to filter information in the time-sequence of image frames, wherein said filtering comprises: identifying pixels of interest in the time-sequence of image frames, said pixels of interest picturing an interference pattern potentially representing a particle in the flow of air, and outputting said identified pixels of interest for performing digital holographic reconstruction on the identified pixels of interest; and discarding pixels not identified as pixels of interest from holographic reconstruction. identifying pixels of interest in the time-sequence of image frames, said pixels of interest picturing an interference pattern potentially representing a particle in the flow of air, and outputting said identified pixels of interest for performing digital holographic reconstruction on the identified pixels of interest; and discarding pixels not identified as pixels of interest from holographic reconstruction.