(n.) Formation Evaluation. An image of the external or internal features of a core. External images are photographs taken under natural or ultraviolet light; natural light highlights lithology and sedimentary structures, while ultraviolet light causes hydrocarbon zones to fluoresce. Internal images are obtained using X-rays or nuclear magnetic resonance (NMR). X-ray techniques measure the attenuationof X-rays passing through the core. The attenuation depends mainly on the density. Hence the image reflects density and lithology changes, internal bedding planes, fractures and nodules. These techniques include, in increasing resolution, fluoroscopy, X-radiography and computed tomography. Most NMR images measure the quantity and relaxation time of hydrogen, and therefore give information on fluid distribution. Some NMR techniques examine carbon, sodium and phosphorous. See: nuclear magnetic resonance measurement