ADDITIVE MIXING OF COLOURS

A human being’s perception of colour differs significantly from a human being’s perception of sound. While the human ear is capable of distinguishing several different sound frequencies over a wide range, human vision has a tri-chromatic nature, and the human eye is only sensitive to light in the wavelength range from 400 to 700 nm. A human’s colour vision derives from a response of three photo-receptors contained in the retina of the eye. Therefore, blue, green and red are the basic colours for our visual perception, and other colour impressions are obtained if a combination of photo-receptors is stimulated to various extents. Or in other words, for human beings all other colours are obtained as a mixture of fundamental colours.

Experiments concerning our perception of light and colour can be carried out with a Spektra^TM spectrometer. The reason is that the spectrometer has a tri-colour LED as its light source. The red-, green- and blue-light emitters can be switched on and off independently, and their light intensity can be regulated.

A simulation of the experiments that can be carried out with the spectrometer’s light source in the real world is shown on the left-hand side of the picture. If any of the position bars is slid to the right the corresponding colour intensity is enhanced. The simulation is not an entirely realistic representation of the colours that someone would observe with the light source of the Spektra^TM spectrometer, but the same conclusions can be drawn in both cases concerning the effect of the additive mixing of colours.

Suggested experiments

Experiment 1:
Set the position bar for the green to the far right-hand position and start enhancing the blue colour. When the maximum intensity of the blue is reached, start reducing the intensity of green. What colours are observed during the process?

Experiment 2:
With the blue at the maximum intensity, start adding the red. When the maximum intensity of red is reached, start reducing the intensity of the blue. What colours are observed during the process?

Experiment 3:
With the red at the maximum intensity, start adding the green. When the maximum intensity of green is reached, start reducing the intensity of the red. What colours are observed during the process?

Experiment 4:
What has to be done to obtain the perception of white?
What is required for us to perceive a yellow colour?

Experiments demonstrating additive colour mixing using spectrometer  SPEKTRA^TM

Similarly, the experiments demonstarting additive mixing of colours can be carried out using spectrometer SPEKTRA^TM. In this case, instead of bars, knobs are used. Light emitters  are swiched on and off by swiches which are in front of the knobs. (See the picture of SPEKTRA^TM.) Observe the lights source (LED), which is positioned inside the measuring chamber.

Developed and prepared by: Nataša Gros*, Danica Dolničar**, Margareta Vrtačnik**
*University of Ljubljana, Faculty of Chemistry and Chemical Technology
**University of Ljubljana, Faculty of Natural Science and Engineering