Optical density is traditionally measured with filter instruments. For a given reflectance, R, the density D is found by:
D=-log(R)
The physical filter and the subsequent processing ensure that the value of R used to compute D is weighted appropriately. For example, for measurement of cyan, magenta and yellow printing densities R is weighted by a set of spectrally selective filters such as those defined in Status T.
When a colour is measured with a spectrophotometer, the resulting spectral data can be weighted by multiplying the data by the required weighting function at each wavelength and then summing the result.
Download an Excel spreadsheet illustrating this calculation for various types of density, including Status T, Status M, ISO visual density and colorimetric density.