A device for observing a biological sample is provided, including: a light source to emit a light beam at a wavelength between 1 μm and 20 μm; an image sensor including pixels defining a detection plane; a holder to hold the sample between the source and the sensor at a distance from the plane smaller than 1 mm, such that the source is configured to illuminate an area of the sample larger than 1 mm2, no image-forming optics are placed between the sample and the sensor, and the sensor is configured to acquire an image corresponding to an area of the sample larger than 1 mm2 and representative of an absorption of the beam by the sample at the wavelength; and a processor to determine a map of an amount of analyte in the sample, based on the image acquired by the sensor, the analyte absorbing light at the wavelength.