1.10.10.2: Additive mixing of light

2.1  Additive mixing of laser light

The output of laser diodes is reasonably monochromatic with the energy centred on a single wavelength. The availability of relatively cheap red and green laser pointers allows an easy experiment.

WARNING: Never look directly at a laser pointer, or shine it at someone else. Also be careful about 'specular' reflections; i.e. those from mirror–like surfaces.

First arrange a red and a green laser pointer so their outputs fall side by side on a white sheet of paper – distinct red and green dots will be seen .

If one laser is noticeably brighter than the other, its output can be reduced by passing it through some plastic or a filter (crossed polarisers can also be useful).

 

Now move the lasers so that their beams overlap; the combined dot will become yellow/orange. The scattered light from the lasers is intense and consequently their centres are overexposed in the photographs below and appear white. However the change in colour around the central beam can be clearly seen.

      

The change in colour as the beams progressively overlap can be seen more clearly in the reflections from the screen onto a second sheet of white paper; they are visible at the bottom of the photographs shown below.

                 

 

 

2.2  Additive mixing using light from torches with filters

 

In this experiment we produce sources of coloured light by passing the white light from torches (flashlights) through filters that only pass a reduced range of wavelengths.

The filters can be coloured glass, cellophane, plastic, etc.  The filters I have used for these experiments are from the glasses used for 3D effects in computer games and some films (movies). These 'anaglyph' glasses have different coloured filters for the right and left eyes. There are several versions commonly available:  red/green; red/blue; red/cyan; magenta/green. They are quite cheap; I bought the four different glasses shown in the photograph below via the internet at a cost of around A$1 each. If possible you might like to buy 2 pairs of each – you can then not only play 3D games with a friend, but also perform a wider range of experiments.

 

A schematic diagram of the experimental configuration using red and green filters is shown below. If the beams are carefully adjusted, the region of overlap should be yellow.

The torches I used were commonly available pocket versions constructed in aluminium that use 2 AA batteries and use a LED instead of an incandescent bulb. They have the advantage that it is possible to focus the beam by rotating the end of the torch. Some models also allow the light output to be reduced by 50% and/or 25% – this can be handy to allow for filters that transmit different amounts of light. However almost any torch that produces a reasonably uniform beam of light should be suitable. I found that a simple collimator – just some black cardboard rolled into a tube and fitted to the end of the torch – helped reduce stray light. In order to show clearly the experimental arrangement, the light on the screen from the torches is slightly overexposed in the photograph below – hence the overlap region appears white rather than yellow – see the later photos for correct exposure.

Here is an example using red and green filters. If the beams are carefully adjusted, the region of overlap should be yellow. 

This is what I photographed on the screen with appropriate exposure – the yellow region can be clearly seen where the beams overlap. Towards the right of the overlap region, the green beam is weaker and a shift in colour from yellow towards orange is apparent.

Although these filters work by passing a restricted range of wavelengths, the output within this range will usually still be less than teh input. Thus if the white light from one torch is passed through two successive green filters, the intensity of the green will be further reduced. In this case (shown below), the addition of green with reduced intensity  to red produces an overall colour shift towards orange.

Using the red/blue anaglyph glasses gave the following where red and blue mix additively to produce magenta.

The photograph below demonstrates how, with suitable levels of relative intensity, the red/cyan glasses can produce white light – the cyan filter passes both green and blue which can add to the red passing through the red filter

Here's what I photgraphed for the magenta/green anaglyph glasses.

If you have several sets of glasses and/or torches with adjustable levels of light output, there is a wide range of possible combinations to investigate.

If you have a third torch, you can try combining red, green and blue to determine whether they can mix additively to produce white light.

 

2.3 Additive mixing using the light from a computer screen

 

Your computer screen is capable of producing individual pixels that are a primary red, green or blue. The light from each pixel will then diverge as it travels away from the screen. If a semi-opaque screen is placed at a suitable distance, light from adjacent points on the computer screen will start to overlap. There are several possibilities for the semi–opaque screen. In the photographs below I have used tracing paper, but tissue paper or some other types of white paper or plastic used for packing can often be suitable. You can even use normal white paper, although a considerable grain is often apparent. 

 

Look at the image below through your semi–opaque paper. If necessary, increase the brightness of your screen and reduce the level of ambient room lighting.

 

The photograph below shows what happens when I press a sheet of tracing paper firmly against the above image on the screen. There is some blurring due to the paper, but no significant changes in colour

 

The photograph below shows what happens when the tracing paper is spaced slightly above the computer screen. I used a pencil, visible at the bottom of the photograph, to generate the desired spacing. A clear change in colour is now apparent at the boundaries between the colours

 

To show these colour changes in more detail, here are magnified images of the boundaries.

There is a yellow/orange region between the red and green.

There is a cyan region between the blue and green.

 

There is a magenta region between the red and blue.

You can examine additive mixing between some other colours by drawing suitable images on the screen and using this technique.

 

2.4 Additive mixing using partial reflection and transmission

2.4.1 Additive mixing using partial reflection and transmission from coloured paper

In this experiment we take advantage of the fact that a clear material will not only transmit light, but can also reflect light. This allows us combine the transmitted light from one source with the reflected light from another source. For the transparent plastic screen, I used a piece cut from a takeaway food container for the photograph shown below. The clear cover of a CD box is also suitable.

 

 

Here’s a photograph of what I saw.

The effects of adding several different pairs of colours can be investigated in this fashion. Possible sources of coloured paper include the discs that can be printed for section 3.2.

2.4.2  Additive mixing using partial reflection and transmission from the computer screen

In this version we use two coloured regions on the computer screen rather than two separate pieces of cardboard.  For the transparent plastic reflective screen, I used the clear cover of a CD box. Hold the CD cover vertically and place on the green region as as shown below.

 

 

 

 

This photograph shows what my camera saw. The relative intensity of the two colours can be altered by carefully changing the angle between the CD cover and the screen.

 

Now try adding red and blue.

The photograph below shows what my camera saw.

Finally try adding blue and green.

The photograph below shows what my camera saw.

The effect of adding different pairs of colours can be investigated by drawing additional suitable diagrams on your computer screen.

 

2.5 Additive mixing using Light Emitting Diodes (LED)

 

LED (Light Emitting Diodes) are a cheap source of reasonably monochromatic light. Several manufacturers sell units that contain individual red, green and blue LED within a single unit. The one I used for this experiment cost around A$5. If possible, use one with a “diffused” lens rather than a clear one. Otherwise use some sandpaper or abrasive on the surface of the unit to help diffuse the light from a clear unit.

Wire up the circuit below. The intensity of a LED depends upon the current; the 100k potentiometers can now be used to control the relative levels of red, green and blue light. The fixed resistors (100R and 150R) in the circuit below are to limit the maximum current through the device. Manufacturers will usually specify values for some typical voltages; otherwise start with a high value (1k is usually a good choice) and reduce it until the LED is sufficiently bright.

 

The photographs below show the unit when just the internal red, green or blue LED are used.

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The following photographs show what happened when two internal LED are used. For the red/green combination the transition from yellow at the centre to orange at the lower edge can be seen as the relative contribution of green decreases. Similar transitions in shading are apparent in the other combinations.

         

The photograph below shows what happens when all three internal LED are used. A photograph taken this close to the LED still shows some colour because of the spatial separation of the internal LED, but the intensity of the colour is much reduced 

 

 

Adjusting the relative intensities of the red, green and blue LED with the potentiometers can produce a wide range of colours by additive mixing.