D50 is the standard lighting condition of the printing industry. It has a degree of whiteness of 5,000 Kelvin, which is the colour that glowing metal at 5,000 Kelvin would cast.
With the ISO 3664: 2009, the D50 standard lighting condition has undergone a change. It now contains a higher UV spectrum. Since this change had not been clearly communicated to all commercial printers and prepress businesses, even today there are many D50 lights with old neon pipes, causing confusion in matching colour. In one printing studio, two D50 light units might be next to each other, that have different UV components and thus, for example, the optical brighteners in modern uncoated papers are visually completely different.
D65 is standardized viewing condition with 6500 K color temperature. It is the default setting for most monitors and is mainly used in color matching etc. for websites, so in media that are considered primarily to be viewed on the monitor.
In the printing industry, the standard light is D50 according to ISO 3664: 2009 standard, a warmer light than D65 with 5000 Kelvin color temperature, which is also used for color matching on the press and in the evaluation of proofs.
Delta-E is a unit for the colour distance between two colours. The larger the number, the greater the “distance” between the colours. The main idea is that a Delta-E colour distance – in any direction – looks the same to the human eye. So if one colour is brighter by 5 Delta-E than another, to the human eye it looks just as distant from a colour that is redder or bluer by 5 Delta-E.
The most common method is Delta-E76 or CIE 1976 / CIE76.
This method uses the Euclidean distance, between the LAB colour values of the colour.
The calculation formula is: Square root of (L-distance squared) + (a-distance squared) + (b-distance squared)
Example: How far apart are the colours in Delta-E76:
Colour 1: LAB 90/65/55
Colour 2: LAB 87/65/58
square root of [((90-87) = 3 squared) + ((65-65) = 0 squared) + ((55-58) =-3 squared)]
Square root of [(9) + (0) + (9)]
Square root of (18)
So CIE76 is quite easy to calculate, but the Delta-E76 calculation has its drawbacks, as it has been shown that the same Delta-E76 colour distance from the eye is not really seen as equally spaced, so newer calculation methods were established.
Other common methods are…
Delta-E00 is currently the most common method used in proofing and printing technology, Delta-E CMC is widely used in textile printing.
Densitometers measure ink density and optical density.
Incident light densitometers measure prints or photo prints, transmitted light densitometers measure films for printing plates or screen printing, slide films and negative films.
Important for proofing: Densitometers can indeed measure colour densities, but two identical colour densities do not necessarily have the same colour impression. Densitometrically measured values from a proof cannot be compared with the same values on an offset print, a print cannot be adjusted densitometrically to the values of the proof.
Densitometers from Techkon are very common in pressrooms.
A design grid is an auxiliary construction and helps to place the texts, pictures and graphics in the layout. It serves to standardize the document.
A Digital Proof (Contract Proof) is an ISO certified test equipment for the graphics industry. A digital proof simulates the colour of offset printing or gravure printing colour accurate binding within the tight tolerances of ISO 12647-7. Today, a digital proof is processed via a RIP and then produced with pigment inkjet printers on special proofing papers.
The proof data is converted to colour separations, then reassembled into a composite image to simulate overprinting and trapping correctly. The data is then passed as a composite image to a usually more than 8-colour inkjet printer, which prints the data. In addition to the proof data, a digital proof must carry a Mediawedge by UGRA / FOGRA or IDEAlliance to be legally binding. Thanks to the standardized mediawedge, the print shop is able to check the proof for correctness. Since many printers don’t have the neccessary metrology at hand, digital proofs are often directly provided with a test report, which documents the accuracy of the measured values of the media wedge directly on the proof.
Older proofing methods as chromalin or Kodak Approval etc. are today hardly any more found in the market.
In the ISO 12647 “Contact Proofs” (ISO 12647-7) are the highest standard of proofs, but the term “Validation Prints” (ISO 12647-8) is also defined. A “Validation Print” is distinguished in that it is less accurate in colour, but therefore can also be produced on laser printers. Compared to the contract proof, a Validation Print has higher colour variations and is NOT automatically legally binding – only after prior consultation. A real “proof”, ie a real contract proof according to ISO 12647-7 is currently not only the most colour accurate and best option, but also the only legally binding proof.
Digital proofs (correct: contract proofs) are an ISO-certified test equipment for the graphic arts industry. Digital proofs simulate the colourfulness of offset or gravure printing in a colour and legally binding manner within the narrow tolerances of ISO 12647-7. Today, they are almost exclusively calculated using a RIP and then produced with inkjet printers on special proof papers.
The proof data is converted into separations, then reassembled into a composite image to correctly simulate overprinting and trapping. The data is then transferred as a newly created composite image to an inkjet printer, usually with more than 8 colours, which prints the data. In addition to the print data, digital proofs must also carry a UGRA/Fogra media wedge in order to be colour-consistent and legally binding. Thanks to the standardised wedge, the printer is able to check the proof for correctness. Since many printing houses do not have this measuring technique at hand, the proof is often provided directly with a test report that shows the correctness of the measured values of the media wedge directly on the proof.
Earlier methods such as Chromalin etc. are no longer available on the market today.
ISO 12647 defines the highest standard of contract proofs, or “proofs” (ISO 12647-7), as well as the term “validation prints” (ISO 12647-8). Validation Prints are characterised by the fact that although they are less accurate in colour, they can also be produced on laser printers. Compared to contract proofs, however, they accept much higher colour deviations and are only legally binding after prior consultation. Real “proofs”, i.e. true contract proofs according to ISO 12647-7 are currently not only by far the best variant in terms of colour but also the only legally binding proofs.
In the case of a dot proof, the halftone screening of the final print is simulated in the proof. This screening shows possible moiré or other disturbing effects, that by dot proofing can be seen in advance on the proof.
Two different variants are possible. Firstly, for the production of printing plates in the RIP screening, imposed 1Bit data files are combined in the proofing software and then proofed. Secondly, some proofing systems can simulate the printed screen without using real 1-Bit Data. Screen angle, dot form and LPI are then specified within the proofing system, and the system then simulates the printing screen.
Perfectly suited for halftone proofing proofing systems are like the Kodak Approval system. Due to their high resolution, printing and proofing details are superior to common inkjet systems. Due to the high price in acquisition and in consumables, the long processing and laminating time and the small color gamut of these systems, they have never really got a large markt share outside of the US.
In particular, in recent years the halftone proof is much less common in the Proof practice and can be found today mostly only in the range of proofs within a printing house. Proofing Service providers today mainly focus on inkjet proofs without screening, because they have much larger color spaces and lower the cost of a proof compared to systems such as the Kodak Approval lower by up to 90%. The higher modern printing screen and other screening methods such as hybrid screening and frequency-modulated screens also decreased noticeably the risk of unwanted Moiré-patterns in print during the last couple of years. The focus is now more on the color accuracy of the proof and the reproduction of spot colors, as in the simulation of dots and screening.