Optical properties

Question 1

1. Which optical properties of paper do you know?

Answer 1

Brightness, whiteness, gloss, colour, gloss, florescence

Question 2

2. Please explain the Lab color system!

Answer 2

It is a way of describing every colour by 3 values
* L = lightness – represents the brightness of the colour (0 black, 100 white)
* A = red to green (positive red negative green)
* B = yellow to blue (positive yellow negative blue)

Question 3

3. What does the R457 reflectance factor in ISO brightness indicate?

Answer 3

This is measuring brightness rather than whiteness. Whiteness is the entire visible spectrum whereas brightness is but 457 wavelength as this is where blue light is reflected. If the paper if emitting blue light it will appear brighter.
This is the principle OBA work on – they absorb in the UV region and admit blue light.

Question 4

4. What factors can influence the paper brightness?

Answer 4

Opacity – when this is low light can pass through the sheet which would reduce the brightness
Fibre type – shorter fibres increase light scattering
Fibre bleaching
Fillers – titanium dioxide very bright
Refining – more scattering
OBA’s

Question 5

5. What is opacity and how can it be calculated?

Answer 5

Opacity is how much light is blocked from passing through a material. Low opacity means a lot of light can pass through
Need a calculation
(Reflectance of single sheet with black backing)/(Reflectance of a stack of sheets as a backing )

Question 6

6. How is gloss measured and which factors can have an influence on the gloss?

Answer 6

A beam of light at fixed intensity and angle projected onto the surface and the reflection is measured. More glossy will reflect more of the light
Type and amount of fillers
Refining
Calendaring
Coating