When examining the picture, use an adequate light table or picture monitor. Do not examine x-rays, for example, against a window. The background lighting should be dim. Always compare present abnormalities with findings in earlier x-rays if they are available. Was the abnormality present already in the earlier x-ray? Has it become larger or declined? Interpretation of the chest x-ray is not simple. Ask for interpretation by a radiologist if necessary. A chest x-ray that appears normal at first sight may hide significant pathology, e.g. behind the heart, mediastinum or diaphragm. Erroneus interpretation of an x-ray image is often associated with problems of perception. Many normal structures (e.g. cross-sections of blood vessels) and/or their superimposed projections may also cause false positive interpretations. The radiation dose from a chest x-ray is rather low (postero-anterior x-ray about 0.03 mSv, corresponding to 3 days' natural background radiation, and lateral x-ray about twice as much).