SPAD stands for Special Products Analysis Division (a division of Minolta). As the name implies, this is not directly related to optical transmission through a leaf. Although SPAD units have come into common use, they are not easily extrapolated to other methods of measuring chlorophyll and plant health. The arbitrary SPAD units can be converted to standard units of optical transmission (CCI) by the equations shown on the graph, which were derived by measuring color filters and leaves with both meters.