BRIGHTNESS, WHITENESS and SHADE OF PAPER
Brightness and Whiteness in paper are the two most common misconceptions that exist in the minds of many and are used interchangeably without a second thought.
However, these two are different and describes two different aspect of a paper and certainly are NOT the same.
Brightness refers to the reflectance percentage of blue light, as measured at a wavelength at or about 457nm ie how much Blue light is reflected ( 457 nM) as a percentage from the surface of a paper. The reason brightness is measured at or about that wavelength is based on the sensitivity of the human eye to blue and yellow tints. Brightness was primarily a test to measure the effectiveness of bleaching in removing yellowness from pulp.
When paper is bleached, reflectance percentage shows highest rise in blue and violet range in the spectral curve at 457 nM range which makes brightness most suited for measuring aging paper since aging paper tends to turn yellow with passage of time.
Most white papers are in the 60 to 90% brightness range.
Brightness, as specified by the TAPPI is the measurement of the amount of reflectance of blue light (Wavelength 457 nanometers, 44nm wide).
Whiteness, although similar to brightness, is slightly different in the respect that whiteness refers or looks at the whole reflectance spectrum at equal amounts.
If a sheet is truly white, it will have a high degree of reflectance and will not absorb one wavelength of light more than another.
Color, in its simplest term, means the amount of light energy absorbed, and the wavelengths at which the light is absorbed. Therefore, if a white light is focused on a sheet of paper and all the incident light is reflected, the paper will appear white.
If some of the wavelengths are absorbed, the color of the paper will be that of the light which was not absorbed, but was reflected back to the viewer. For example, blue paper appears blue in white light because it absorbs all the other colors and reflects the blue wavelength.
Therefore when blue /violet dye is added to a paper, it reflects more light in blue wavelength which increases the brightness artificially (Sometimes even more than 100%).
It is NOT uncommon to see brightness measurements of over 100, which means that more light is reflected than that was originally shone on the paper which is is due to Optical Brightening Agents (OBA) reflecting part of the ultra-violet spectrum back in the visible spectrum. Measurement for TAPPI brightness are often in the 110 – 120 range, lower than commonly found CIE whiteness measures as the OBA’s only have a small spectrum to reflect.
The CIE have set a standard of D65 illumination (Day light) which is a standard representation of outdoor daylight under which the amount of light reflected is measured.
For a perfect reflecting non-fluorescent white paper, the CIE whiteness would be 100, however most ‘white’ paper will have CIE whiteness between 130 and 170 due to the addition of Optical Brightening Agents (OBAs) which are designed to reflect light from the non-visible range (mainly ultra-violet) back in the visible spectrum.
Shade is universal and represents the color of the paper measured on the CIE LAB model (more formally known as CIE L, a, b). The exact colour of the paper will be defined by the L,a,b value as an accurate colour shade which may or may not have an equivalent name. Viewer may state “ yellowish or Reddish or Bluish shade “ but exact colour shade can only be measured by a spectrophotometer and definied in terms of L,a,b value.
The shade of paper is almost invariably measured on the CIE LAB model (CIE L, a, b) as this model covers the full color space, whereas other models such as RGB or CMYK cover a subset of the L,a,b color space