Ink density

The ink to be measured is illuminated by a light source. The light ray passes through the glazing ink layer and is partly absorbed. The non-absorbed light is reflected back from the surface of the substrate. With the aid of a densitometer, the amount of light transmitted is compared with the amount reflected back. The thicker the ink layer, the less is the amount of light that comes back.

Density measurement = ink film thickness measurement

White light consists of the three primary additive colors: blue, green and red. Filters are used to separate out one of the three colors. Each filter lets its own color through, the rest are absorbed.

Printing inks work similarly to filters. Process inks let their own color through. Thus cyan ink lets cyan light, which consists of green and blue, through and absorbs red; magenta ink lets red and blue through and absorbs green; yellow ink lets red and green through and absorbs blue.

However, to gauge the thickness of a printing ink film, only the part of the light that is absorbed by the ink is relevant. Therefore the part of the light that is unchanged as it penetrates the printing ink must be blocked. With cyan ink, blue and green are let through almost unchanged. The thicker the ink film, the greater is the absorption of the red component. To enable the red component to be measured and not be overlaid by the blue and green components, a red filter must be used. This filter only allows red light to pass and its quantity can now be measured.

The unit of measurement DENSITY is a logarithmic measurement of the amount of transmitted light that is reflected back.