Even if today the use of colour in entertainment industry is very simpler and more precise than in the past, some perplexities still remain. The impossibility of getting a wilful colour gradation and the very little difference, just perceptible and not chosen, between projected colours from identical apparatuses, are some of the many problems.

Such problematic techniques derive from a series of factors (particularly lamp, filters, reflectors) that interacting each other could lead to a not adequate final result.

To a better understanding of the origin of all this we will face in this first part the description of the processes that regulate the vision of light and of colours, so that we could better understand the correct meaning of terms like for instance Temperature of Colour, primary colours, length of wave.

Then we will go to study in a more detailed manner many key elements in the projection of the colour and of the images:the "dichroic filters" and the gobos, to understand the production process, the qualities, the limits and eventually the best way to use them.

We start now with a simple question: what is the colour?

Thinking about all the coloured things that surround us we would be tempted to say that colour is one of a body characteristics like weight or length. There is however a substantial difference between these measurements. If for example we take a book, it will be simple to weigh it definitely. But what about the colour? I can say that the book is red, I can add that it is clear red. But how can I say the determinate exact red gradation?

Besides on the earth the book weight will always be the same, instead its colour varied depending on the light that illuminates it,; in a neon light the book will be of a different red that under a halogen lamp and in the dark it will be black.

Therefore the colour is depending both on the nature of the object body and on the illumination source. In the light almost no object utters own light, so that in the dark we don't see the objects that are in the room, we can see them only when we turn on the light since emit a fraction of the same light that invests them.

The concept of colour is therefore tightly correlated to that of lights.

The light is a mixture of electromagnetic radiations. Also the waves radio or the X. Rays are electromagnetic radiation , therefore what does it distinguish these from the light? Each radiation is characterised from an own length of wave and frequency (Hz). To high frequencies correspond small lengths of wave according to the following proportion:

l= c/Hz

where c= speed of propagation in the radiation vacuum (300.000 [km] per second).

We could simply say that all the radiations move with the same speed (light speed ) but each at its own style, sometimes small and fast footsteps sometimes big falcade. The universe, the Earth has crossed in each instant from a myriad of different radiations, any visible (bright radiations), other we can feel like heat on the skin (infrared rays), of other we are definitely unaware, even if we live in (so that radio and rays range).

fig 1

The figure 1 points out the varied lengths of wave of different radiations. The lengths of wave are express in nanometre.(1 nanometre= a billionth of meter.)

The outlined zone represents the visible radiations that distinguish themselves from the other radiations because same have wave lengths between the 400 and the 800 nanometre. But what is fundamental for the man’s vision is that our eye is quite sensitive to the wave lengths of light, if our eyes were sensitive to other wave lengths we would have a totally different vision of the reality around.

fig 2

The bright radiations arrive in our eye, fig 2.a real optic tool, with a convex lens (Cornea + crystalline) and regulator of diaphragm ([iris]).

The "sensor" it is the retina, that is able to pick up radiation and to transform it in electric impulse sending then to the brain. And it is human brain that sees and interpret the electric signals like images.

We in these few lines we have described in a very simple manner as the vision happens. It could be interesting however to describe what it happens in the retina, that can transform the radiation that invests it, in electric signals.

On the back surface of the retina we have set mosaic of the cells photoreceptor. Each cell absorbs the light from an only point of image and generates , through a reaction of type photo chemistry, an electric signal that codifies the quantity of absorbed light. These cells are of two types: the cones and sticks. The sticks allow the vision with few light, the cones allow the vision in the daylight and allow the vision of colours.

Indeed the cones of three types. Each type is characterized for the presence of a pigment sensitive too long, intermediary or court visible radiation.

Till now we have considered the light like the whole of the radiations with wave length between 400 and 800 [nm], this is the white light, as it issued from the sun. But if from this "packet" of radiation we succeed in isolate only one part, for instance that with longer radiation (between the 600 and the 800 [nm]), and we imagine that only this part invests the retina, then they will be stimulated only the cones with the pigment sensitive to the long radiation and only this will cause electric signals. In this case the brain won't receive the signal of "white light" but that of a specified colour: the red.

fig 3

If we select only the radiations with intermediary wave length (between 500 and 600 [nm]) the brain will see the green and with those of short wave length (between 400 and 500 [nm]) the blue.

In figure 3 it is shown the wave lengths relative to all the visible colours.

Roughly we could imagine the retina like a tool full of keys (the cones). These keys could be only red, blue or green. If the sun light strikes the retina it is as if all the keys were pressed at once creating the message of "white light." To create the red message we need to press only the red keys.

If the pigments of colour are three how cane we see also other colours? The other colours come from a mixture of these three, so called primary colours. For example if we press the red and green keys at the same time, the message will be the yellow colour, the magenta instead is the combination of blue and red.

A question could rise now. If the sun emits light like electromagnetic radiation with wave length between and 800 nanometre what does it separate these radiations and how do we distinguish coloured objects if the sun light is white?

Returning to the initial example of the book it is red because it reflects not all the light that invests it, but only the long radiations and it absorbs the others.An object is black it absorbs all the radiations, white if it reflects all.