Visual evaluation of colours in printing and graphic arts industries is of key importance for the process of colour management and verification of print quality. In order to properly assess colour, it is necessary to ensure proper lighting conditions, as described in ISO 3664:2009 standard. Despite the fact that almost 10 years have passed since its publication, our understanding of how to use it is very average, and the preparation of workstations for print evaluation is at very different levels across the industry.
Published by Miko Przybyla in SEP/OCT 2018 issue of PRINT&PUBLISHING Magazine
The standard specifies the conditions of observation for colour evaluation and comparison of prints and prints with proofs (reference) in the entire printing industry. In order to ensure quality, the visual assessment should be carried out on appropriately prepared, adequately illuminated and periodically verified stations for visual assessment. These should be used by photographers, advertising agencies, publishers and printing houses and other people involved in the entire process. The provisions of the ISO standard specify in detail lighting conditions and set mandatory standards and levels including additional recommendations. In this article we will consider how to ensure appropriate conditions for quality control and working with colour.
Light and colour
Let’s start with a short theory of seeing and perceiving colours. We know that without lighting there are no colours. The human visual system is equipped with photoreceptors that react to light stimuli that are then processed by our brain to see objects and colours. Due to the specific structure of the human eye, our sensitivity to natural or artificial lighting changes depending on the wavelength. Thanks to this we can not only distinguish between colours but also our ability to perceive details depends on both the quantity and quality of lighting.
The amount of light falling on a given surface is determined by the illuminance and is expressed in lux. On the other hand, the quality of lighting depends on the spectral power distribution, i.e. the mixture of different wavelengths that make up the white light falling on the observed surface. It should be noted that a specific artificial lighting is used at quality assessment workstations. Its purpose is to best imitate natural sunlight, hence the most important quality parameters in printing are: correlated colour temperature (CCT), colour rendering indexes (CRI) and metamerism index (Mi) and chromaticity error. Natural daylight has a very wide range of radiation from ultraviolet through the whole palette of visible colours to infrared. Therefore, it is particularly important to verify the artificial lighting in order to ensure appropriate lighting conditions. The wider the spectral range, covering the widest possible range of colour gamut, the better the ability to distinguish between colours and details. In other words, visual evaluation and print quality will depend on the quality of lighting and observation conditions.
Lighting conditions when working with proofs
Workstations on which proofs are compared with printouts should be properly prepared and illuminated. Due to the fact that the quality of work depends on the decisions taken on the basis of visual assessment and acceptance of the order by the client, the work cannot be carried out in an accidental location. It must not be done with the existing ambient lighting, but only on a properly prepared workstation. The quality of illumination of the workplace and its immediate surroundings have a significant impact on the ability to see, work speed and certainty of recognizing the details of the surface and colour. The comfort of visual work depends on the fulfillment of three basic conditions:
- the ability to distinguish details is full
- perception is effective,
- there is no discomfort leading to excessive fatigue
In order to ensure such conditions, it is necessary to prepare a workstation with installed appropriate lighting that ensures uniform, shadow-free lighting that does not cause unnecessary reflections and minimizes the influence of ambient light. In printing houses where work is carried out 24 hours a day, variable ambient light must not affect the lighting conditions of prints. In addition, the ISO standard recommends the use of artificial light similar to daylight standard with a correlated colour temperature of 5000K with spectral distribution in the range from 300 to 780nm and strictly defined parameters. Colour coordinates (x,y 10°) with a maximum tolerance of 0.005°, colour rendering index CRI (CIE 13.3) above 90 are also specified.
Day Light overhead luminaries made by JUST Normlicht
An additional requirement for lighting quality is related to metamerism index, which for the visible range (MIvis) should be less than 1.0 and for the ultraviolet range (MIuv) less than 1.5. Depending on the substrate or material used for printing, the impression of colour may vary and it is the metamerism index that determines the lighting quality necessary for visual verification of this effect.
Natural daylight contains UV radiation, which induces the fluorescence effect of optical brightening agents (OBAs) contained in most papers used in printing. This effect changes the impression of colour, which is why the ISO 3664 standard published in 2009 introduced the obligation to use lighting containing UV radiation in order to ensure greater compatibility of assessment and impression of colour in artificial and natural light.
The distribution and level of illumination at a workstation as defined by the standard depend on the size of the workstation and must comply with the following requirements.
- Illumination level 2000lx
- Uniform distribution +/- 500 lx and +/- 250 lx is recommended.
- For workplaces smaller than 1m2, the illumination of the observation field edges must not fall below 75%.
- For workplaces larger than 1m2 the illumination of the observation field edges must not fall below 60%.
Lamps and Measurements
What must be done to meet the requirements of the standard? How should the workstations be organized and illuminated in the right way for colour assessment and how can we verify or measure whether the conditions created meet the relevant requirements?
In order to meet the detailed requirements of the standard it is necessary to use appropriate lighting equipment and appropriate measuring instruments to verify the quality of lighting.
Lamps or entire luminaires meeting the requirements should be used to illuminate the workstations. It is not enough to use any fluorescent lamps or LED tubes of 5000K colour to start working with colour. Attention should be paid to the uniformity of lighting, the angle of illumination of the luminaire and detailed requirements for the quality of light. Daylight imitating lighting products available in stores are designed to be used for general lighting purposes, including office work. This does not mean, however, that they can be used in specialist workstations. Both fluorescent lamps and modern LED lighting systems used in printing have a much wider spectral range than ordinary products and must provide a certain amount and quality of light and contain near ultraviolet radiation, which “off the shelf” products should not contain at all.
The simplest way to organize a workstation can be to use appropriate luminaires suspended above the work table. The number and length of luminaires depends on the size of the workstation.
|In order to ensure adequate protection against daylight or general lighting, which may vary with the time of day, suitable workstations can be used to provide optimized conditions for visual work. For smaller formats, smaller luminaires or boxes can be used to evaluate prints.|
Appropriately prepared and calibrated spectroradiometers with suitable technical properties should be used to verify all parameters. The measuring range of such a device should cover the whole visible and near UV range. In the case of professional multiband fluorescent lamps with a UV range, the use of simple meters or indicators does not give sufficient accuracy. It is even more difficult to measure LED light using simple indicators because measurement errors are difficult to estimate. Spectroradiometers for measuring light intensity can be additionally equipped with dedicated software supporting the process of verification and preparation of the measurement report according to ISO 3664 standard. During the compliance tests, the measuring device is placed flat on the working surface in 9 evenly distributed places throughout the field and step by step measurements are made and the results are compared with the values measured in the middle of the field.
Fig.2 Calibrated Illuminance Measuring Spectroradiometer with dedicated ISO 3664 Software interface
We should always keep in mind that lighting conditions have a direct influence on colour perception and are the most important factor influencing the quality of visual work. Ensuring the right conditions is possible with the use of high quality lighting equipment. Periodic verification of workstations with the use of appropriate instruments ensures compliance with the standard and increases the level of quality and colour management.
If you are looking for specific information on how to evaluate the lighting for printing and graphic arts industries you may visit GL Optic website here
For information regarding standardized lighting products please visit the website of JUST Normlicht here
Mikołaj Przybyła – graduate of the Adam Mickiewicz University with a major in light technology from the Poznań University of Technology, director of GL Optic Poland, member of the International Commission on Illumination (CIE) and the Illuminating Engineering Society of North America (IES).