Security printing process utilises specialised security paper (also called "substrate") which is manufactured with its end use in mind and is, therefore, significantly different from any standard paper. It does not contain optical brightener and has neutral reaction under ultraviolet light. It contains unique watermark impressed into the paper in the manufacturing process and visible against the light. Areas with thicker paper mass appear darker and areas with less paper mass appear lighter, making the watermark negative, positive or multitonal. The watermark can also be localised or continual. Security paper can also have chemical protective elements. Any attempt to tamper with the original data or contact with any chemical leaves visible colourful blotches. Another valuable protective element is the use of protective fibres. These are added in the paper manufacturing process and, therefore, are contained within the paper mass and cannot be imitated. They can be visible to the naked eye, under magnifying glass or react under the ultraviolet light. The paper can also contain other agents enabling either magnetic detection or detection through use of special pens.