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Sunday, September 22, 2013

ISO vs. TAPPI Opacity: What are the differences?

In the Paper Industry, there are two different standards for opacity measurement.  ISO 2471 and TAPPI T425 are the relevant standards here.

The differences between the two are:


ISO
TAPPI
Geometry
d/0°
15°/d
White Backing
R
R0.89
Wavelength
557nm
572nm

Opacity is the characteristic ability of a material to hide from view a non-active backing materialWhen reading a book, one prefers to read one page at a time, rather than read all of the pages at once.  Each page must have sufficient opacity to obscure the printing on the subsequent pages.  If the intended use of the paper is to allow the objects placed behind it to be seen, such as glassine, then transmittance measurements should be utilized rather than opacity measurements.  To simulate viewing conditions a ratio means of opacity measurement has been adopted by the paper industry and has been used for many years.

ISO (Printing Opacity) =  R0 / R


TAPPI (Contrast Ratio) = R0 / R0.89 

Differences in the measurements can be seen here. 

GEOMETRY

ISO 2471 utilizes the diffuse illumination and 0° viewing geometry (d/0°) which is the same as the ISO Brightness geometry.  TAPPI T425 uses 15° illumination and diffuse viewing (15°/d) originally developed by Bausch & Lomb in the 1930's.

BACKING
Both methods use a contrast between the sheet to be measured backed with a black backing (R0) and the same sheet backed with 'white'. The difference is what each uses as the 'white' backing.  ISO 2471 uses an infinite pad (R) as the white backing.  TAPPI T425 uses an 89% white body backing (R0.89).

WAVELENGTH
ISO 2471 specifies a response of YC , which has an effective wavelength of 557 nm.  In TAPPI T425, the specified response is YA , which has an effective wavelength of 572 nm.  Unlike brightness, which is measured in the blue portion of the spectrum, opacity is predominately measured in the green or greenish yellow portion of the spectrum with some response throughout the entire visible spectrum.  The objective is to measure opacity using a spectral response that correlates closely with human observation.

FOOTNOTE
Many of the geometric and spectral variables are less critical for opacity measurements than for brightness and color because opacity is a ratio measurement.  For example if a sample being measured is glossy, nearly the same amount of gloss will be present in the numerator and denominator of the ratio causing most of the gloss effect to be canceled out. Similarly the effect of fluorescence on opacity is minimized by the ratio calculated that is inherent in the determination of opacity.


This is not a full discussion of opacity, but a quick synopsis.  Keep in mind that:

TAPPI Opacity is measured using:
  • Technidyne BNL-3
  • Technidyne PROFILE/Plus TAPPI Opacity
  • Technidyne Model 425
  • Brightimeter Micro S-5

ISO Opacity is measured using:
  • Technidyne Color Touch, Color Touch 2 and PC
  • Technidyne PROFILE/Plus Color Touch
  • Technibrite TB-1 and Micro TB-1C