Among other things, a diluted sample is generated based on mixing a small sample of blood with a one or more diluents. A thin film of the diluted sample is formed on the surface of a contact optical microscopy sensor. Red blood cells within a portion of the thin film of the diluted sample are illuminated using light of a predetermined wavelength. One or more images of the diluted sample are acquired based on illuminating the red blood cells within the portion of the thin film of the diluted sample. The acquired one or more images of the diluted sample are then processed. The mean corpuscular hemoglobin in the red blood cells within the portion of the thin film of the diluted sample is determined based on processing the acquired images of the diluted sample.