Brightness levels - and what it actually means
Quick facts
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LOWER BRIGHTNESS LEVELS |
HIGHER BRIGHTNESS LEVELS |
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General Overview:
Brightness was originally introduced as a visual aid for controlling the amount of pulp bleaching.
The measurement of brightness is expressed as a percentage, and it's a measure of light reflectance of a specific (short) wavelength of blue light, which is compared with an ultimate reference brightness standard (IR3 standard). In other words, it measures the amount of light reflected from the surface of the paper - and the higher the brightness value, the brighter the paper is.
Scientifically, the measurement is taken as the reflectance of blue light with a wavelength of exactly 457 nanometers (nm), +/-44 nm. However as only the short wavelengths are measured (blue) the measurement ignores shade or the longer green and red wavelengths. This means that two samples with identical brightness values can look very different when compared side by side.
Optical Brightening Agents:
The beginning brightness range for a base paper pulp is from 0-100 but during the papermaking process, optical brightening agents (OBA's) are frequently added to improve a paper's brightness. OBA's use fluorescence to absorb invisible radiation from the ultraviolet part of the light spectrum and re-emit the radiation as light in the visible blue range. This addition of blue reflection causes the sample to appear whiter to the user and gives a higher ISO brightness and D65 brightness value. Put simply, brightness is a more narrow measurement of light reflectance than whiteness. On very bright sheets, this can create a situation where there is more reflected visible light from the surface of the paper than the light source emits, resulting in a measurement in excess of 100. To increase brightness measurement you need to increase the OBA and the UV energy in the light source.
Although brightening products are widely used in a large range of paper products there are some products within the industry which do not allow any fluorescence or OBA content at all. In these cases it is necessary to extinguish or "quench" the fluorescence which may be present in trace or moderate amounts. There are various quenching agents available, one of which is GloQuench PAS which claims to be 20% to 40% more effective than most other quenching agents available in the market today.
Test Methods include:
Brightness can be measured by a variety of methods. On any given paper, the brightness number given by each of these methods will be different. Papers may be sold on TAPPI, ISO or D65 Brightness.
Both the Technical Association of the Pulp and Paper Industry (TAPPI) and the International Organisation for Standardization (ISO) have industry standards for measuring brightness. While the utilisation of either the TAPPI or ISO standard helps to ensure consistency in the brightness measurements indicated on product specification sheets, these standards utilise different methods of measuring reflectance within the blue spectrum.
As such, these two standards cannot be correlated or used interchangeably. These differences in brightness measurement standards, along with the impact of shade, are the reasons why two papers with an identical brightness measurement can look visually different.
TAPPI T-452 Brightness (N. America standard) - quantifies the brightness of paper where the environment is a mixture of cool-white fluorescence and some filtered daylight. The ultra-violet light component of TAPPI brightness is the lowest of any standard and calculates against a single angle of light hitting the sample, whereas ISO and D65 measurements check light from all angles. This means the TAPPI Brightness numbers rarely exceed 100%.
ISO 2470 Brightness (European standard) - quantifies the brightness of paper where the environment is a mixture of cool-white fluorescence and some unfiltered daylight. The fluorescence present in a paper sample (which accounts for a significant contribution to its brightness) will be stimulated more using this standard versus TAPPI, therefore it will yield a higher brightness result. This is in part also due to the fact that ISO measurements are diffuse, as measurements check light from all angles. ISO Brightness numbers can exceed 100% by a small amount.
D65 Brightness (Standard illuminant) - quantifies the brightness of paper where the environment is viewed in Western Europe/Northern Europe at approx midday sun (which has the maximum amount of ultraviolet light available to interact with the fluorescent component of brightness). There is no actual D65 light source, it is simulated but it attempts to portray standard illumination at open-air in various parts of the World. This provides Brightness numbers that easily exceed 100% due to the highly magnified fluorescent component - as well as the fact that the measurements are diffuse and checked from all angles (unlike the TAPPI standard which is a single angle) and so the ratings are always higher than TAPPI or ISO.
X-Rite Brightness - combines the measurement of TAPPI Brightness with measurement of colour in L*a*b*, XYZ and several other recognised formats. The instrument uses 0/45 degrees measurement geometry (which is standard in several ANSI, TAPPI and ISO test methods)
Examples of HP Paper Brightness:
HP Natural Tracing Paper: 61% (Test type unlisted)
HP Premium Vivid Colour Backlit Film: 86% (TAPPI)
HP Premium Gloss Photo Paper: 87% (ISO)
HP Durable Semi-gloss Display Film: Bigger than 92% (X-RITE)
HP Collector Satin Canvas Paper: 95% (TAPPI)
HP Professional Matte Canvas Paper: 97% (TAPPI)
HP Matte Polypropylene Paper: Greater than 101% (ISO)
HP Durable Banner with DuPont Tyvek: 102% (ISO)
HP Universal Bond Paper: 110% (ISO)
HP Bright White Inkjet Paper: 113% (ISO)
HP Coated Paper: 113% (ISO)
