Understanding how light interacts with pigments helps artists
mix colors more effectively. This guide explains two key
ideas—reflectance and color
perception—and shows what happens when you mix
Cadmium Red (PR108) and Ultramarine
Blue (PB29) using real reflectance data.
💡 Core Concepts
1.
Reflectance (What Light the Pigment Reflects)
Reflectance tells us how much light a
pigment reflects at each color (wavelength).
It’s measured in nanometers (nm). For
example, blue is around 450 nm, and red is around 650 nm.
Devices like FORS (Fiber Optic Reflectance
Spectroscopy) create a graph showing how much light is reflected
at each wavelength.
These graphs help us understand how pigments behave under
light.
2. Color Perception (What
We See)
Color perception is how our eyes and
brain interpret reflected light.
It depends on:
The pigment’s reflectance curve
The light source (e.g., sunlight vs. LED)
How human eyes process light (based on the CIE 1931 color
matching functions)
The environment and surrounding colors
Scientists use color models like sRGB
and CIELAB to describe how we see
colors.
📈 Reflectance Curve
Examples
Cadmium Red (PR108)
Reflects mostly red light in the 620–700 nm
range
Reflectance: around 60–70% in the red
zone
Absorbs blue and green almost entirely (under 10%)
Ultramarine Blue (PB29)
Reflects mostly blue light in the 420–480
nm range
Reflectance: about 40–60% in the blue
zone
Absorbs red and green light (under 15%)
🎨 Mixing
Cadmium Red and Ultramarine Blue
How to
Estimate Reflectance of a Mixture
To estimate the light reflected by a mixture, multiply the
reflectance values of both pigments at each wavelength:
This shows how much light passes through and reflects back
from both pigments.
Reflectance Graph
Below is a graph showing how Cadmium Red (PR108), Ultramarine
Blue (PB29), and their mixture reflect light across the visible
spectrum:
Reflectance Data Table
Wavelength (nm)
PR108 (%)
PB29 (%)
Mixed Result (%)
400
2
40
0.8
450
3
60
1.8
500
4
50
2.0
550
6
30
1.8
600
25
15
3.75
650
60
10
6.0
700
65
5
3.25
What the Mixture Looks
Like
Both pigments absorb in different areas, so the mixture
reflects very little light.
The result is a dull, dark purple or
grayish-violet color.
This happens often when mixing complementary
colors—they cancel each other’s reflected light.
Kubelka-Munk K/S Graph
Below is a graph showing Kubelka-Munk K/S values for PR108,
PB29, and their mixture:
What Are K/S Values?
The Kubelka-Munk K/S value is a way to
describe how strongly a pigment absorbs and scatters light. It’s
used in pigment science and color matching to predict how paints
behave when mixed or layered.
The formula is:
\[
K/S = \frac{(1 - R)^2}{2R}
\]
Where:
R is the reflectance (a value between 0 and
1)
K is the absorption coefficient
S is the scattering coefficient
What K/S Tells You
Higher K/S values mean the pigment absorbs
more light at that wavelength.
Lower K/S values mean the pigment reflects
more light (and looks brighter).
K/S curves help artists and conservators understand how a
pigment will interact with others.
Example
PR108 (Cadmium Red) absorbs blue/green
light strongly, so its K/S values are high in that range.
PB29 (Ultramarine Blue) absorbs red/yellow
light, giving high K/S values in those zones.
When mixed, their K/S values add up—resulting in
broad absorption and a dark, muted color.
