Optical properties and means of visual assesment

CIE Whiteness (ISO 11475):

What The whiteness of the substrate, expressed in a percentage, is derived from the CIELab values and in fact is a diffuse reflection of light on all waves of the visible light range.
Why

Higher whiteness gives higher contrast.

How

Whiteness according to CIE is determined with an Elrepho, using a D/65 light source, UV included, and a viewing angle of 10°.

Calculation

Report the average of 10 significant values and the standard deviation of both front and back.

Note

Other light sources are D50/2° incl. UV and C/2° excl. UV according to ISO 11476.

Andere lichtbronnen zijn D50/2° incl. UV en C/2° excl. UV (ISO 11476)


CIE Brightness (ISO 2470):

What The percentage of white as a direct reflection, measured in the blue range of the visible spectrum.
Why

Brightness has impact on readability and colour intensity.

How

Determined with an Elrepho, using a D65/10° or D50/2° light source and a R457 filter. Since the wavelength of 457 nm is exclusively blue, the measured value reflects a whiteness excluding all wavelengths in the red and green range. D/50 measurement is especially suited for those substrates to which no extra OBA’s (Optical Brightening Agents) and/or colouring agents have been added. D/65 is used for those substrates with a certain amount of fluorescence, that promotes whiteness.

Calculation

Report the average of 10 significant values and the standard deviation of both front and back.


Shade CIELab en Delta E (ISO 5631-2):

What

Reflection of colour is referred to as a combination of tint, saturation and dark/light value. The CIELab method measures shade, where
L* = the lightness (100% is perfect white, 0% is perfect black)
a* = colour on the red – green axis (-a = green, +a = red)
b* = colour on the blue – yellow axis (-b = blue, +b = yellow)

Delta E76 (∆Eab) is a value for fluctuations in the shade between more sheets of the same production run. A shade is consistent if the shade differences are not visible, i.e. ∆Eab is lower than 1.

Why

CIELab and ∆E values are important for reproducing the same printed matter more than once (e.g. corporate image) and nowadays especially for standardisation according to ISO 12647-2.

How

CIELab values are determined with an Elrepho, using a D/65 light source, UV included, and a viewing angle of 10°.

Calculation

Report the average of 10 significant values of both front and back.

Note

1: spectrophoto meters in agreement with ISO 13655 as used in the graphical industry have no scattering device for diffuse reflection of light; it shines the light directly onto the substrate’s surface under 0:45 or 45:0 degrees.

2: ∆Eab < 1: hardly noticeable
∆Eab between values 1 and 2: visible to the trained eye
∆Eab between values 2 and 4: visible to the eye
∆Eab value > 4: complaint worthy
∆Eab value > 6: different colour

3: The light sources C and D50 are described in resp. parts 1 and 3 of this ISO standard. These differ in light source and viewing angle, however they do not differ in other aspects of the method described above.

 


Shade CIELab en Delta E (ISO 13655):

What

Reflection of colour is referred to as a combination of tint, saturation and dark/light value. The CIELab method measures shade, where
L* = the lightness (100% is perfect white, 0% is perfect black)
a* = colour on the red – green axis (-a = green, +a = red)
b* = colour on the blue – yellow axis (-b = blue, +b = yellow)

Delta E76 (∆Eab) is a value for fluctuations in the shade between more sheets of the same production run. A shade is consistent if the shade differences are not visible, i.e. ∆Eab is lower than 1.

Why

CIELab and ∆E values are important for reproducing the same printed matter more than once (e.g. corporate image) and nowadays especially for standardisation according to ISO 12647-2.

How

CIELab values are determined with a Techkon Spectrodens Advanced, using M0, M1 or M2, viewing angle 2°

Calculation

Report the average of 10 significant values of both front and back.

Note

∆Eab < 1: hardly noticeable
∆Eab between values 1 and 2: visible to the trained eye
∆Eab between values 2 and 4: visible to the eye
∆Eab value > 4: complaint worthy
∆Eab value > 6: different colour

Opacity (ISO 2471, TAPPI T 519):

What Degree of opacity, expressed in a percentage. Paper that lets through a lot of light is transparent, paper that lets through hardly any light is opaque. The higher the value, the opaquer.
Why

Of importance when printing two-sided. A difference of 1% is visible to the naked eye in case of opaque papers.

How

Determined with an Elrepho, using a C/2° light source. It is in fact the ratio of reflection of a stack of the paper (considered to be 100% opaque) and one sheet of this paper, measured over a black cavity.

Calculation

Where:
R∞ is the reflection value of a stack of the paper (considered to be 100% opaque)
R0 is the single sheet value measured over a black cavity

Report the average of 10 significant values and the standard deviation.


Transparency/Transmittance of light (ISO 22891):

What

Ratio between light reflected by a sheet of paper and light reflected by that same sheet over a black cavity, expressed in a percentage.

Why

In cases you should be able to see through paper, e.g. tracing paper, specific wrappings, or paper for certain (creative) applications.

How

Determined with an Elrepho, using a C/2° light source. The reflection is measured over a white working standard of known values. Subsequently this measurement is repeated over a black cavity.

Calculation

Transparency is calculated conforming the Kubelka-Munk analysis.

Report the average of 10 significant values and the standard deviation.


Delta UV (ISO 2470):

What

An indication of the amount of present Optical Brightening Agents (OBA’s) in the substrate, expressed as a percentage.

Why

Some applications should have none or as least as possible optical brighteners added, like paint or ink colour samples or machine readable transaction forms (giro credit slips)

How

Determined with an Elrepho. Brightness is measured using a D65/10° light source and a second time on wavelength 420 nm (total UV blockage).
The Delta UV is the difference between both values.

Calculation

If, in the case of fluorescent samples, measurements are made with a filter with a cut-off wavelength of 420 nm placed in the light beam, it is possible to determine the ISO brightness of the non-fluorescent substrate and thus to calculate the contribution of the fluorescent whitening agent to the ISO brightness:


Where:
R457 = brightness at 457 nm (red/green cut-off)
R420 = brightness at 420 nm (UV cut-off)

Report the average of 10 significant values and the standard deviation.

Note

OBA not added is not the same as OBA free. Not added means that there may be some leftover OBA from a previous making on the paper machine that did have OBA. OBA free means the substrate is produced on either a dedicated OBA free paper machine or an additive is used make the paper optically dead.


Gloss converging beam 45°/75° (ISO 8254, TAPPI T480):

What

Paper gloss is the reflection of light on the paper surface in a certain angle, expressed in GU (Gloss Units).
Printing gloss is measured after 24 hrs drying of the ink, using a 75° angle.
Delta (∆) gloss is the difference between the both.

Why

Depending on the application. For a printing form with hardly any text and many photographs, often a gloss finish of the paper surface is chosen. For a combination of text and photographs one often chooses a half matt or matt finish. Photographs are then enhanced by applying varnish or lacquer.
Printing gloss is also influenced by the speed the mineral oils in the ink are being absorbed by the paper.

In each case a printer will be most interested in Delta gloss, a paper manufacturer in paper gloss (though the latter may vary in adding certain chemicals to create a good Printing - and therefor a good Delta gloss.

How

Gloss is measured with a gloss meter on front and back in both MD and CD.
In case of printing gloss only the printed side(s.)

Calculation

Where:
GUU = gloss unprinted
GUP = gloss printed

Report the average of MD/CD of 10 significant values and the standard deviation.

Note

Usually an angle of 75° is used for coated and SC paper and 45° for other uncoated paper.
In practice most paper manufacturers only use 75° gloss meters.


Gloss converging beam 20°/60°/85° (ISO 8254, TAPPI T653):

What

Paper gloss is the reflection of light on the paper surface in a certain angle, expressed in GU (Gloss Units).

Why

Depending on the application. For a printing form with hardly any text and many photographs, often a gloss finish of the paper surface is chosen. For a combination of text and photographs one often chooses a half matt or matt finish. Photographs are then enhanced by applying varnish or lacquer.

How

Gloss is measured with a gloss meter on front and back in both MD and CD.

Calculation

Report the average of MD/CD of 10 significant values and the standard deviation.

Note

Measurement at 20° is especially designed for high gloss surfaces, such as cast coated substrates.

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