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Monday, July 29, 2013

Density vs. Reflectance Measurement (Part 1)

A densitometer is an instrument for the measurement of the optical density of a transmitting material or an instrument which measures the negative log of the reflectance of a reflecting material.  They are widely used in the graphic arts and photographic industries for process control.  The densitometer measures the absorbance properties of individual colorants, i.e. the three primary process ink colors magenta, cyan and yellow but does not measure color in relation to the human observer as it is measured by a colorimeter or spectrophotometer.  Differences between reflection densitometers and reflectometers are as follows:

1) Readout scales - densitometers read out in density units; Reflectometers read out in reflectance units. They are related by:
                           D = - log10 R   or  D = log10 1/R 
where D is density & R is reflectance expressed in decimal form.

The densitometric scale has its value in the at it approximates the light absorption characteristics of dyes or inks that are used together in a colorant combination.  Thus the densitometer is useful in measuring the ability of cyan to absorb light in the yellow portion of the spectrum, magenta to absorb light in the green portion of the spectrum and yellow to absorb in the blue portion of the spectrum.  The logarithmic density scale, therefore, relates to amounts of colorants and not to human visual perception of color.

The second part of this discussion will include three other differences between reflection densitometers and reflectometers.