We strive to deliver consistent high-quality print. To do this we go through rigorous certification and calibration to conform to commercial printing standards.
A device called a densitometer is used to measure the ink density of the printed sample. This is measured against the PDF your book was printed from. The difference between the two is expressed as the "Delta E" value.
In order to speak on the same terms for color evaluation, you can reference this chart
|<= 1.0||Not perceptible by human eyes.|
|1 - 2||Perceptible through close observation.|
|2 - 10||Perceptible at a glance.|
|11 - 49||Colors are more similar than opposite|
|100||Colors are the exact opposite|
This is a way to speak about color based on empirical data, rather than perception of the human eye. The readings are based on reproduction on gloss stock.
Factors such as image resolution and the paper stock used will affect the Delta E.
We strive to be at a Delta of 5 or better, but in many cases up to an 8 would be deemed an acceptable commercial standard.