Program:      !RGB

Status:       RiscOS compliant - ie loads directly from the file icon

Purpose:      To aid the teaching of colour addition and subtraction.

Level:        Early secondary school

Organisation: Teacher demonstration


Background:
  
    The three colour theory of vision is widely misunderstood - and
yet it can provide a very simple way of understanding such everyday
phenomena as the way a colour TV set works and why colours look
different under different kinds of light.


Theory:
  
     The human eye contains three different colour receptors, broadly
sensitive to red, green and blue light respectively. A red light will
trigger the red receptors only. a green light, the green receptors. A
mixture of red and green light will trigger both receptors. The brain
interprets this as seeing yellow. A pure yellow light (such as sodium
light) also triggers both receptors because of the overlap between
them. The eye cannot therefore distinguish between pure yellow and a
mixture of red and green. In practice, three primary colours suffice
to make nearly all the colours which the human eye can distinguish.

     When pupils (and most primary school teachers) are asked 'what
are the primary colours?', most will confidently reply 'red, blue and
yellow, of course!' This is because we are far more used to mixing
colours in the form of pigments from a paint box than with beams of
coloured light. From the painters or printers point of view, the
primary colours are magenta, cyan and yellow (these being the
physicists secondary colours). In practice, red and blue paints are
sufficiently impure as to act like magenta and cyan.


Use:
  
     Double-click on the !RGB icon in the filer window. The program is
self-explanatory.


Suggested teaching method:
  
1)   Talk about the three primary colours.
2)   Move the spots around using the SELECT button. Introduce
secondary colours. Do not at this stage move the spots over the mouse.
3)   Use the sliders to create new colours
4)   Click on 'Pigments'. The spots now become coloured transparent
discs which you can move over a white sheet. Again, overlap the discs
and discuss the consequences.
5)   Using the ADJUST button, pick up a colour from one of the discs
and use it to paint on the sheet. Mix blue (cyan) and yellow. Talk
about primary paint colours.
6)   Point out the colours on the mouse. Ask what will happen if one
of the discs is placed over the mouse. Try it.
7)   Go back to 'Lights'. The mouse is now black because there is no
light shining on it. Remind pupils of the colours on it (they are
still there). Ask what will be seen when we shine a beam of light on
the mouse. Try it.
     

Other points:
  
     Any painted colours are automatically removed when any of the
spots or discs are moved.

     Any colour, including secondary colours, may be picked up from
the spots but colours cannot be picked up from other places on the
screen.

     When in 'pigments' mode, the percentages are negative because
they refer to the % of the primary colour which has been removed.


                                                  (c) J.O.Linton 1992