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Monday, May 23, 2016

How are Opacity and Transparency Different?

When light strikes an object, it is reflected (specular), refracted (scattering & non-specular reflection), transmitted and/or absorbed.  Parallel rays of light which are reflected or transmitted as parallel rays are said to be specularly reflected or specularly transmitted.  Light which is scattered before it is reflected or transmitted is said to be diffusely reflected or diffusely transmitted.  In general, incident light that is specularly reflected determines the gloss of an object and that part which is diffusely reflected determines its color (and brightness).  Finally, that part of the incident light which is transmitted determines the opacity and transparency.

Opacity is determined by the total transmitted light while transparency is determined by the amount of light which is transmitted without scattering.  When the light is scattered and diffusely transmitted, the object is translucent. Nearly all papers are translucent because they permit the passage of mainly diffusely transmitted light.
Transmission (without scattering)
Transmission (scattering)

Transparency is important in tracing, reproduction and packaging papers.  A completely transparent paper would be one that transmitted, without scattering, all of the light falling on it, and a clear view of an object would be had when the paper was placed between the object and the viewer, regardless of the distance between the paper and the object.  If, on the other hand, a paper has a tendency to scatter light, its transparency will depend upon the distance between the paper and any object being viewed through it.

Reference: "Properties of Paper: An Introduction" - Scott, W.E & Trosset, S. p. 84 (1989)

Thursday, May 19, 2016

New Paper Stock for Calibration Standards

Please be aware that the 80-range, brightness calibration paper for Directional (Brightimeter S-4M and Micro S-5) and Diffuse (Technibrite Micro TB-1C) instruments has changed this month.

The new paper is a neutral white, 32# text with brightness values closer to 85 brightness. This means there about 10 points of brightness between the the 60’s, 70’s, 80’s and 90’s/Fluorescent range papers. Overall the change made in paper being used should improve the linearity of the calibrations. It also provides a broader range of paper values for calibration and verification.

Questions, contact our Lab Manager - Nick Riggs.

Tuesday, May 10, 2016

Setting Tolerances (Color)

Customers ask all the time, "What is the best way to determine a production tolerances?" I'll use color as an example to help answer this question.

Generally speaking in the Paper Industry we are normally dealing with near-white colors. In that case, we often use the rule of thumb that +/- 0.3 in L*, a* or b* is visually perceptible. However, when we start to look at more saturated colors the question becomes much more difficult.

L*a*b* color tolerance
Color tolerance vs. Visual acceptability
If we set a tolerance based on L* a* b*, the color space we are looking at is cubical. However, when we plot actual visual acceptability it is more ellipsoidal-shaped.  See at the right, the black shading represents numerical acceptance, but visually unacceptability.

Obviously, there is a difference here. If we look at the ΔEcmc tolerancing which is based on ellipsoidal tolerances, this does a much better job of matching visual and numerical acceptability.  Looking at a particular cross-section of the a* b* space (below), we can see that the visually acceptable ellipses vary in size depending on the position in color space. The ellipses in the orange area of color space are longer and narrower than the broad and rounder in the green area. The shape of the ellipses are larger as the color increases in chroma (away from a*=0, b*=0).

This means that visually acceptable differences in L*, a* and b* differ depending where we are in color space.  A Δa*=0.5 would be noticeable on a near-white, but Δa*=5.0 on a red may not be noticeable.

The best advice is to give your customer a series of samples that vary from the target by different amounts. Let them visually analyze the samples and tell you which ones they would accept and which ones they would reject.  You can measure all of the samples and help define tolerances based on their feedback.  It may also be helpful to keep an archive of samples (over time) that they have rejected. This will help you hone the tolerances based on complaints, returns or rejects.

This same concept can be used for many other measurable quantities as well. Brightness, Gloss, Opacity, Smoothness, Softness, Tensile, Formation, Burst and many more parameters can have tolerances set around them. Giving your customer a say in the matter and then keeping a record of their historical actions can build a strong quality program. Also, it is important to set tolerances that are reasonable. All instrumentation has limits. Setting tolerances is a balance between what the customer sees as objectionable and what the precision and accuracy of the instrumentation.

If you have more questions about this topic, contact me at