Ink Reflectance

July 24, 2020

The idea for this post started in January of this year. I was browsing my Twitter feed and saw a link to a kickstarter project about colours.

The kickstarter was for the book ‘Inks & Paints of the Middle East’ by Joumana Medlej and it had an option to include a sample sheet with the book. I made a pledge for booth and they arrived at the start of July.

I like the book. Though the reason I got it was the sample sheet. It is because I want to learn how different wavelengths of light mix into colours. So a sample sheet of various inks and paints is, I think, something that can help me do that.

Methology

To measuring colour you separate light into different wavelengths and capture how much of each wavelength there is. And then use a formula to calculate the colour. But how to measure light from a single source, to separate the light into different wavelengths, and to measure the amount of each wavelength require some equipment. As inks do not produce light by themselves, there is also needs to be a lamp that illuminates the colour sample.

I have a Thorlabs SLS201L lamp to illuminate the samples and a Thorlabs CCS100 spectrometer to separate the light into wavelengths and measure them, i.e. to measure the light spectrum.

To make sure the lamp shines from the same angle and distance for all samples and that the spectrometer measures from the same position, I used a probe holder (Thorlabs RPH-SMA). It has connections for fiber optic cables and a small opening for the sample at its bottom. This also blocks light from other sources. I connected the lamp to illuminate the sample from above and spectrometer to measure at a 45° angle. Here is a picture:

Thorlabs RPH-SMA light measuring probe with connected optical fibers.

I measured each colour sample, the unpainted paper and the lamp. Then calculated the reflectance of each sample as a fraction of the lamp spectrum.

To calculate the colour I simulated the sample being illuminated by a D65 standard light source and converted the result to red, green and blue components that images use (sRGB).

Results

I made a graph of all reflectance spectra for the colour samples and the unpainted paper. But when I looked at the result it was very chaotic. So I made it possible to tap on a colour label to show and hide it. I also added a ‘Hide all’ and a ‘Show all’ button.

Unpainted paper a Yellow Ochre Orpiment Saffron maceration Saffron decoction Safflower decoction Safflower carthamin applied thick Safflower carthamin applied diluted a Red Ochre Cinnabar Red Lead Lac from sticklac Lac from powder Sappanwood Sappanwood with alum and gum Verdigris Azurite Lapis Lazuli Indigo Lead White Hibr (tannine ink) Myrobalan hibr Sumac hibr Mulberry hibr Midâd (soot ink) Pseudo-compound (no vitriol) Compound Incomplete 1 (gall+rosewater) Incomplete 2 (gall+blue vitriol) Incomplete 3 (gall+myrtle) Golden ink Ruby ink Emerald ink Metallic paint Pistachio skin Violet Apricot Rose Frankincense Pale green Green Dark green Wild thyme Porcelain glaze Sky blue Lapis imitation Wild beast Lion Starch white Crow 400 500 600 700 0 1 2 3 4 5 Show all Hide all

I am happy with the result. Please let me know if you find something I missed or could have done differently.

References

  1. Inks & Paints of the Middle-East / Make 100, Joumana Medlej 2020. Kickstarter campaign
  2. Inks & Paints of the Middle East – A Handbook of Abbasid Art Technology, Joumana Medlej 2020. ISBN 978-1-9163830-5. www.majnouna.com
  3. Abbasid Caliphate, Wikipedia.
  4. Thorlabs CCS100/M Compact CCD Spectrometer. Thorlabs.
  5. Thorlabs SLS201L/M Stabilized Tungsten-Halogen Light Source. Thorlabs.
  6. Thorlabs RPH-SMA Compact Probe Holder Block. Thorlabs.
  7. Color Science, 2nd Edition, G. Wyszecki, W.S. Stiles 2000. ISBN 978-0-471-39918-6.

©2020 Mats Mattsson