Additive RGB color mixing
Study the mixing of primary colors using one smartphone screen as a source and a second phone's camera as a detector.
Activity Summary
The student uses two smartphones to explore additive color mixing: one as a colored light source, the other as a color detector.
Introduction
Why can a screen that emits only red, green, and blue reproduce all colors? Our eyes have three types of cones sensitive to red, green, and blue, and any perceived color can be produced by the right combination of these three primaries. Using one smartphone as a light source and another as a color detector, you can verify the rules of additive mixing and discover surprising effects when colored objects are illuminated by monochromatic light.
Learning Objectives
- Verify the additive mixing rules experimentally
- Measure the RGB components of mixed colors
- Observe how monochromatic lighting changes the appearance of colored objects
- Understand the difference between additive and subtractive color mixing
Instruments and sensors
Scientific instruments
- Colorimeter (RGB color detector)
Sensors
- Camera
Required Materials
- - Smartphone or tablet with FizziQ (camera used as RGB color detector) - A second smartphone or tablet (colored light source) - A darkened room - Colored objects (papers, candies) - Note: the protocol remains adaptable to any comparable color detection tool.
Experimental Protocol
Prepare two smartphones: the first as a source (color generator), the second as a detector (colorimeter via FizziQ camera).
On the source phone, display a fully red screen (R=255, G=0, B=0). Set brightness to maximum.
On the detector phone, open FizziQ and select the Camera instrument in RGB color detection mode.
In a dark room, place the source facing the detector (about 5 cm apart). Record the detected RGB values.
Repeat with a green screen (0, 255, 0) then blue (0, 0, 255). Note the RGB values each time.
Test the mixtures: yellow (255, 255, 0), cyan (0, 255, 255), magenta (255, 0, 255). Verify that the detector identifies the correct components.
Display white (255, 255, 255): all three channels should be high and balanced.
Bonus experiment: in the dark, illuminate colored objects (papers, candies) with only the red screen. Observe how their appearance changes.
Repeat with green light then blue light. Photograph the objects under each illumination.
Create a summary table: what color does each object appear under each monochromatic illumination.
Expected Results
The detector correctly identifies the primary colors and their mixtures. Under red light, red objects keep their color while blue and green objects appear black. White objects take on the color of the illumination. The RGB values for mixtures correspond to the additive rules within measurement precision.
Scientific Questions
- How does the eye perceive white from three colored lights?
- What is metamerism and why does it matter for color matching?
- Why do colors look different under fluorescent versus incandescent lighting?
- How many colors can a typical screen display?
Scientific Background
Additive synthesis creates colors by superimposing lights, in contrast to subtractive synthesis used in paints. The three additive primaries are red, green, and blue.
The fundamental rules are: **R + G = yellow**, **R + B = magenta**, **G + B = cyan**, **R + G + B = white**. They directly result from how the three types of retinal cones respond to different wavelengths.
A red object does not create light: it reflects red wavelengths and absorbs the rest. Under blue illumination, it appears nearly black because there is no red light to reflect.
**Metamerism** is the phenomenon where two colors that look identical under one illumination become different under another. It results from objects having different spectral reflectance curves.
The smartphone camera detects colors using a **Bayer filter**: a grid of R, G, and B micro-filters placed over the image sensor pixels. This mimics the trichromatic vision of the human eye.
Extensions
- Build a Newton's disc and spin it to observe color mixing
- Investigate how screen brightness affects the detected color values
- Use colored filters instead of the screen and compare results
- Explore how the distance between source and detector affects the readings
Frequently Asked Questions
The screen does not emit perfectly pure red.
Normal: screen LEDs have a broadened spectrum. Detected values will be close to (220, 20, 10) rather than (255, 0, 0).
The detector shows unexpected colors.
Ensure the room is completely dark. Ambient light adds unwanted color components.
Why does white light from a screen look the same as sunlight?
Both stimulate the three cone types equally. However, their spectra are completely different: the screen emits three narrow bands while sunlight is a continuous spectrum.
Can I use this to understand how color printers work?
Printers use subtractive mixing (CMYK), which follows different rules. This experiment specifically demonstrates additive mixing.
Detailed Description
The student uses one smartphone as a colored light source (screen set to red, green, blue, or combinations) and a second as a color detector via the FizziQ camera. They verify the additive mixing rules and explore how colored lighting changes the appearance of objects. In FizziQ, the second phone's camera is used as an RGB colorimeter, giving real-time red, green, and blue intensity readings for each illumination tested.
Ready to start?
Download FizziQ and try this activity with your students.