1. Lab 13: Subtractive Color: Filters & Paint
Experiments with Filters:
Color Samples & Paints (“Pigments”)
Mixing Paint
Spectra of Light Sources

2. B G R
λ(nm)
Additive primary colors: Red, Green and Blue (RGB)
Subtractive primary colors: Cyan, Magenta and Yellow (CMY)
Intensity
400
500
600
Wavelength (nm)
700
Graphs are based on RGB

3. Absorption of light Magenta filter absorbs green light Subtractive
Screen
Filter
White Light
Magenta filter absorbs green light
White Light B G R M
Subtractive
 material
Additive
 perception

4. Filters (Pigments) Absorb their complementary hue
Transmit (reflect) their own hue
Transmittance (reflectance)
Fraction of incident light passed through (reflected back) B G R Y M C
Transmittance 1
Ideal Magenta filter
Reflectance B G R Y M C 1
Ideal Yellow pigments

5. Real Filters
Real filters transmit a fraction of incident light B Y G M R C
Transmittance 1 B G R
Intensity
100
100 %
60 % B G R
Intensity
100
0.6 TM
R (600 – 700 nm)
G (500 – 600 nm)
B (400 – 500 nm)
Spectral Ranges

6. Hue - use color triangle
0.2
0.4
0.6
0.8 1
fraction of red (r)
fraction of green (g) B Y G M R C
Transmittance 1
r = 0.6/1.2 = 0.5
g = 0/1.2 = 0
b = 0.6/1.2 = 0.5

7. Caution Probe Plastic Filters Brushes Spills?
Misaligned probe  ask your TA to reposition
Polished probe tip  do not scratch (it's a $3000 research grade device!)
Plastic Filters
Wrinkle when heated under the halogen lamp
Set – Freeze – Remove
Place the filter on the filter holder
Freeze the screen using “snapshot ” feature
Remove the filter
Brushes
Wash them thoroughly for next section
Spills?
There paper towels on the sink counter

8. Paint + = 2 parts 1 part Reflectance Use ideal paint reflectance,B Y G M R C
Reflectance 1
2 parts B G R Y M C
Reflectance 1 B G R Y M C
1 part
Reflectance 1 + =
Use ideal paint reflectance,
NOT from spectrophotometer!
Now find hue using color triangle

9. Hue – Color triangle again
0.2
0.4
0.6
0.8 1
fraction of red (r)
fraction of green (g) B G R Y M C
Reflectance 1
r = 0.5/1.5 = 0.33
g = 1/1.5 = 0.67
b = 0

10. What colors are the patches?
Retrieved on May 10, 2005 from (
Light source: Sodium lamp (Yellow)

11. Light source: white light
Retrieved on May 10, 2005 from (
Light source: white light

12. Incident Light Sources
Continuous Spectrum
White light sources
Discrete Line Spectra
Fluorescent light
Discharge lamps (gas filled tubes)
Spectral lines specific to gas species
Neon, Mercury, Water vapor, and other gases available