Densitometry - Offset Print Quality Control

Densitometry, colorimetry, and spectrophotometry are the three principal methods for measuring color in the graphic arts. A densitometer measures optical density, functioning in a colorblind manner to assess only the reflectance or transmittance of a surface. Density varies with ink film thickness, and this data can also be used to calculate dot area. Colorimeters define color numerically within various color spaces but do not measure density or dot area; they are primarily used to determine color difference using the accurate ΔE (delta E) metric. Spectrophotometry provides the highest level of color information, offering superior accuracy and versatility. Spectral analysis yields color curves and enables the calculation of density, dot area, and colorimetric data.

Among these tools, the densitometer remains the most widely used device in the pressroom for evaluating color test forms and control images. Its primary function is to analyze test forms and color bars to monitor press performance throughout a run, providing critical feedback on factors like ink density, dot gain, trapping, and print contrast. Densitometers also verify the quality of off-press and press proofs against established standards and specifications. Additionally, they can analyze incoming materials such as ink and paper. A digital color transmission/reflection densitometer is shown in Figure 3-10.

A densitometer indirectly determines the amount of light a surface absorbs by measuring the proportion of light it reflects or transmits. It operates on the principle that absorbed light equals incident light minus reflected light, then calculates density using a logarithmic equation. A reflectometer records percent reflectance compared to a standard beam, while a densitometer converts this into logarithmic density units. Transmission densitometers compare transmitted light, and reflection densitometers compare reflected light. It is critical for a company to standardize on a single make of densitometer for consistent measurements.

Color filter. Densitometers measure the process colors—yellow, magenta, and cyan—with the aid of color filters. On modern digital models, this filtration occurs automatically. Older models, however, require the press operator to manually select the correct filter for each measurement.

These devices are typically equipped with three filters: red, green, and blue. The blue filter measures reflected light in a range of approximately 400–500 nm, reading it as blue density. The green filter measures between 500–600 nm, and the red filter measures between 600–700 nm. The exact wavelength range depends on the filter specifications; wide-band filters typically measure a 50–70 nm range, while narrow-band filters measure about 20 nm. In both cases, the density reading represents an average over the measured wavelengths, not a value at a specific point.

Each filter is designed to render a specific process color as black. A blue filter makes yellow appear black, a green filter renders magenta as black, and a red filter does the same for cyan. Using the correct filter allows the densitometer to isolate a reading for each process color, even when ink films overlap.

Scanning densitometer. A scanning densitometer is a computerized quality control device that uses a densitometer to measure and analyze printed color bars. Inline models are mounted directly on the press to read density information during operation. Off-line models, which are more common, require the printed sheet to be manually positioned for scanning.

By measuring ink densities on color bars, the device indicates printed ink film thickness. The ink patches are spaced within each ink key zone on the sheet. Results are compared to target densities, and a computer monitor displays any variation. This data guides adjustments to ink fountain keys and rollers. Scanning densitometers can also provide data on dot gain, print contrast, trapping, hue error, grayness, slurring, and doubling.

Its role is complemented by increasing automation and data integration in modern pressrooms. The focus on precise color measurement supports broader industry trends in standardization and sustainable resource management.