Monitor displays & viewing conditions
Monitors, their calibration and the viewing environment all play a significant role in establishing their contribution to achieving the Standard. In this article Mark Stegman, NSW TAFE lecturer discusses their importance.
International Standards for colour monitor displays are covered within two ISO standards:
ISO 3664:2000 Viewing Conditions for Graphic Technology and Photography
ISO 12646:2004 Graphic technology – Displays for colour proofing – Characteristics and viewing conditions
ISO 3664 is for images viewed independently of any form of hardcopy. ISO 12646 is for soft proof viewing conditions and where the displayed image will be directly compared to a hard copy. The major difference between the two standards is that the ISO 3664 chromacity is D65 and ISO 12646 is D50.
- The chromaticity of the white displayed on the monitor should approximate that of D65. The luminance level of the white displayed on the monitor shall be greater than 75cd/m2 and should be greater than 100cd/m2. (When a standard says shall, the requirement must be met; should is a strong recommendation.)
- The area immediately surrounding the displayed image shall be neutral, preferably grey or black to minimize flare. The luminance of the border be preferably 3% of the white point luminance, or less.
- When measured in any plane around the monitor or observer, the level of ambient illumination shall be less than 64 lux and should be less than 32 lux. The monitor shall be situated so there are no strongly coloured areas (including clothing) directly in the field of view or which may cause reflections in the monitor screen. Ideally, all walls, floors, and furniture in the field of view should be grey and free of any posters, notices, pictures, wording or any other object that may affect the viewer’s vision. All sources of glare should be avoided since they significantly degrade the quality of the image.
This International Standard specifies requirements for uniformity, size, resolution, convergence, refresh rate, luminance levels and viewing conditions for a colour display used to simulate a hard copy proofing system.
- The chromaticity of the display should be set to D50. The luminance level should be as high as practical but shall be at least 80cd/m2 and should be at least 120cd/m2. The black point shall have a luminance that is less than 1% of the maximum luminance.
- The display resolution shall be sufficient to display an image of 1280 x 1024 pixels without interpolation. All luminance values should be within 5% of the luminance of the centre.
- The ambient illumination level, when measured at the face of the monitor, shall be less than 32 lux. The surrounds shall be no more than 10% of the maximum luminance of the screen.
Prepress workflows increasingly rely on accurate colour computer displays for image evaluation. Current RGB display technologies fall broadly within two categories: LCD and CRT. The colour point of white light from a CRT monitor is around 9000 to 10000K, while the figure for an LCD is typically 7000 to 8000K.
LCD (liquid crystal display) is a transmissive display. It operates by blocking the light from a backlight or letting it pass through colour filters. Nearly all LCD monitors today use TFT technology. A TFT monitor uses thin-film transistor technology. The active element is a Twisted-Neumatic liquid crystal.
Lightweight, compact and consuming less power, LCD panels are capable of much higher intensity (up to 250 Candelas/m2), they have higher contrast ratios than CRT (500:1 or more versus 200:1), better visual correlation of colours on reflective media with those displayed on the LCD, acceptable gamut for simulating CMYK and up to 200 dpi resolution. Fast redrawing of the display.
LCD can exhibit lower quality screen uniformity (top/down/left/right), and more shift in colour appearance when images are viewed from angles other than directly in front. Brightness of LCD displays decays over time. TFT monitors may come with dead pixels.
CRT (cathode ray tube) is an emissive display, its light created by electrons striking red, green and blue phosphors. Once the dominant technolgy, they have been largely replaced by LCD monitors.
CRT displays are a mature technology. They have sufficient colour gamut, stable colour properties, good uniformity across the monitor (top/down/left/right), smooth, analogue voltage-based reproduction behavior, and little change in colour appearance when images are viewed from an angle.
CRT monitors have limited brightness (typically 95cd/m2 as compared to light booths which can be 600cd/m2), resolution is limited to ~100dpi, they’re heavy with a significant footprint and there is limited correlation between measured reflective colours on paper with the same colour displayed on the CRT.
Contract soft proofing
Legally an approved proof is a contract – printers and customers are agreeing an acceptable reproduction can be made with designated paper and ink. It is also a communications and quality control tool. It predicts the result of the press run and provides a basis for evaluating a press operator’s efforts.
Soft proofing was seen as convenient for checking content, but not a substitute for hard-copy contract proofs. With major advances in colour management software, ICC profiling and hardware technology, soft proofing is becoming a viable option. Vendors are offering SWOP certified, monitor based proofing solutions. These need to meet gamma, white point and colour targets
Viewing conditions for graphic technology
Viewing conditions – Graphic technology and photography. This International Standard specifies viewing conditions for images on both reflective and transmissive media, such as prints (both photographic and photomechanical) and transparencies, as well as images displayed in isolation on colour monitors. It is not applicable to unprinted papers.
Although colour measurement devices are used extensively in the graphic arts process, visual viewing remains an important part of colour evaluation. Light, object and observer all influence colour perception. The ISO viewing standard has two parts – P1/T1 for graphic technology critical comparison and P2/T2 for photographic display.
ISO 3664 includes critical comparison between transparencies, reflection photographic or photomechanical prints and/or other objects or images.
The comparison is usually either between the original and its reproduction or between different copies of a reproduction, such as samples from a press run or multiple photographic prints. The images being compared may be on the same media (reflective or transmissive), or on different reflective media (including photographic or photomechanical prints and press proofs or off press proofs). The images may even be between transmissive and reflective media such as that pertaining when a transparency is compared to a proof of its printed reproduction. The high illumination levels specified, permit more critical evaluation of colour and tone gradation in higher density areas.
To comply with this standard, the values specified shall be achieved at the surface of viewing. The specified relative spectral power distribution applies to the illuminated surface, rather than to the source (or lamp), because the light from the source may be modified by reflecting and transmitting components of the apparatus. The required relative spectral power distribution may be obtained from a mixture of light from different sources.
Colour rendering index (CRI)
The CIE general colour rendering index of the viewing surface shall have a value of 90 or higher (the modifying effects of the reflector, baffles, and diffuser can reduce the lamp’s CRI rating).
Extraneous light, whether from sources or reflected by objects and surfaces, shall be baffled from view and from illuminating the print, transparency, or other image being evaluated. In addition, no strongly coloured surfaces (including clothing) should be present in the immediate environment. Walls, ceiling, floors and other surfaces which are in the field of view, shall be coloured a neutral matt grey, with a reflectance of 60% or less.
Manufacturers of viewing apparatus shall specify the average number of hours during which the apparatus is expected to remain within specification.
Illumination and illuminance
The illumination at the plane of viewing shall approximate that of CIE standard illuminant D50. It shall have a metamerism index of less than four.
The illuminance shall be 2000 lux ± 500 lux at the centre of the illuminated viewing surface area. For a viewing area up to one metre square, the illuminance at any point within the illuminated area shall not be less than 75% of the illuminance of the centre.
The surround and backing shall be neutral and matt, The surround shall have a luminous reflectance between 10% and 60%. For many applications, a mid-grey of 20% reflectance is convenient and is recommended where no other condition is defined. However, whatever value is selected, it is important when images are being compared that the surrounds for each are similar. The surround shall extend beyond the materials being viewed on all sides by at least 1/3 of their dimension.
The luminance at the centre of the illuminated surface of the transparency illuminator shall be 1270cd/m2 ± 320cd/m2. Any departures from uniformity shall be gradually diminishing from centre to edge such that the luminance at any point within the luminous area is not less than 75% of the luminance.
The surround shall be at least 50mm wide on all sides. It shall appear neutral compared to the source and shall have a luminance that is between 5% and 10% of that of the surface of the image plane of the illuminator in the direction of observation. The previous version of this standard specified an illuminated surround. This is no longer applicable. The dark surround has therefore been incorporated for all assessment conditions in this International standard. In practice this condition may be met by using an opaque black mask.
Luminance: the amount of visible light that comes to the eye from a surface.
Illuminance: the amount of light incident on a surface.
Reflectance: the proportion of incident light that is reflected from a surface.
It can be relatively difficult to get a spectral power distribution close to D50 from a fluorescent light. A characteristic flaw of a fluorescent tube is distinct spikes in greens and purples of the visible spectrum. A filtered tungsten halogen bulb design may achieve a better spectral match to the D50 reference illuminant.
A factor in the quality and stability of light sources, is the hours used and number of times it is turned on and off. A usage meter can assist in monitoring the effective life expectancy.
A UV light that can be activated with other light sources can determine the presence of optical brighteners in papers.
A dimmer switch can assist in adjusting from the P1 standard (2000 lux) to the P2 (500 lux) specifications.
A copy holder that adjusts the angle of view will reduce the problem of reflected glare.
The phenomenon by which two materials that match under one circumstance, appear different to different viewers or under different lighting. Metameric mismatch occurs when tristimulus values are the same but spectral characteristics are not.
Metamerism visual checkers use two metameric colours that can behave differently under certain light conditions.