ALL ABOUT PAPER BRIGHTNESS

The terms "brightness" and "whiteness" are often used interchangeably when considering paper. While for the regular layperson this crossover may not be detrimental, it is an important differentiation for those involved in the printing and art realms. Brightness is the measure of the amount of blue light reflected off a sheet of paper. Essentially the volume of light. Whiteness is the measure of the amount of all light reflected off a sheet of paper. Essentially the quality of light. This article will deal with paper brightness; what it is, how it is determined, and how it affects print quality. To read more, see our All About Whiteness article.

SUMMARY WHAT IS BRIGHTNESS? STANDARD TESTS PRINT RESULTS Brightness is the measurement of how much blue light (the 457 nanometer wavelength) is reflected off the surface of a sheet of paper. A percentage between 1 and 100 is determined. There are three standard tests used to determine brightness: TAPPI in North America, and ISO and D65 in Europe. The brightness of a paper will affect how the printed image appears upon it. Bright white papers provide more contrast and a wider gamut.

WHAT IS BRIGHTNESS?

When measuring brightness, a light source is directed at the paper in question, and the amount of blue light reflected off the surface is recorded. The wavelength for blue light is 457 nanometers. The type of light source varies depending on the testing method used. These methods are discussed in the next section. Brightness measurements were first developed to test the results of bleaching wood pulp. The bleach removes the natural yellowish tint of the natural pulp, and the test would be able to determine the change before and after the process. Today, most papers - at least for printing - are bleached to some extent. The measurement is based on the percentage of light reflected back, based on a scale between 1 and 100%. The higher the percentage, the more blue light reflected by the paper, and the brighter it appears. Just because two different papers have the same brightness rating, however, does not mean they will appear the same color to the human eye. Since only one wavelength (color) of light is measured, brightness does not account for the amount of other wavelengths reflected off a paper. Two papers could reflect 87% of the blue wavelength, while one reflects 50% and the other reflects 80% of the red wavelength. OBAs In general, white papers have a brightness of between 60 and 90%. However, added optical brightening agents (OBAs) can raise this number considerably. This is why you may have come across a paper with a brightness above 100. This is not possible in a natural situation, where the paper is constructed solely of wood pulp. Once brighteners are added, however, UV light is reflected in addition to the other wavelengths. The UV light going to the paper is not visible to the human eye, but the brighteners transform it into visible light. Therefore, more visible light is reflected off the paper than was absorbed onto it, making it appear brighter. Since brightening agents haven't been around for too long, their stability and archival reliability is still uncertain. It is known they may fade over time, diminishing the perceived brightness of the paper. In addition to the percentage rating, brightness may also be identified by a name such as Ultra Bright or Very Bright. These "ratings" are not standardized, however, and should not be used to compare papers.

Brightness is a paper specification Diamino Stilbenedisulfonic acid, a popular OBA

PAPER BRIGHTNESS STANDARD TESTS

Brightness is generally measured using one of three different tests. Unfortunately, these tests use such different criteria, it is difficult to reliably compare a paper with one test's rating to a paper with a different test's rating. TAPPI Brightness Test Most common test used in North America This test bases the rating on a lighting condition of: cool-white fluorescent and some filtered daylight This test accounts least for the affect of ultraviolet light (low UV content in source because light is filtered) The test is conducted with light hitting the sample from only one angle TAPPI % will be the lowest of the three tests for the same paper; lack of UV consideration means the rating is rarely above 100. ISO Brightness Test Most common test used in Europe This test bases the rating on a lighting condition of: cool-white fluorescent and some unfiltered daylight (direct sunlight) This test accounts for a medium influence of ultraviolent light (source light is unfiltered) Because of fluorescent component, % will be higher than TAPPI Rating may exceed 100, but only just D65 Brightness Test Another European test, only recently being used more regularly This test bases the rating on a lighting condition of: north sky daylight This test has the highest influence of UV light, because sunlight has the most UV of any light source Unless paper will be displayed outdoors, is not the most practical test. Rating often exceeds 100 Test Comparison TAPPI ISO D65 84 84 85 98 102 117

TAPPI Brightness Test

HOW BRIGHTNESS AFFECTS THE PRINT

Knowing the brightness of a sheet of printer paper doesn't do you much good if you don't know how that brightness will affect your printed image. These are some general guidelines, though external conditions will affect any print, so these are not hard and fast rules. The higher the brightness and whiteness ratings, the brighter and lighter the image will be. Colors will print darker on papers with a lower brightness or whiteness rating. Though colors tend to appear more vibrant on bright, white paper, this is not a definitive correlation. High brightness is more important on matte papers than on glossy papers. Coatings on glossy and semi-gloss papers are somewhat reflective, and will therefore make the colors more vibrant. Matte papers do not have this coating, so the brightness of the paper is the only factor. Though these factors cannot entirely be negated, a good print technician will know how to alter the digital image to suitably compensate in most instances.

Left to right: Photo Rag; German Etching; Enhanced Matte

At KeenART Media, we use a variety of different fine art and photographic papers. Visit our Fine Art Prints page to see the full list of papers we offer, and the Brightness rating for each paper. If you have questions about any of our fine art paper prints, or any of our other services or products, don't hesitate to contact us. Our knowledgeable staff is always happy to assist with answers, advice, or suggestions.