In this second part we will see the elements that allow to get varied colour effects through projector. Starting from the illuminating source, the lamp we introduce an important concept: the ghostly distribution of energy.

If we imagine the energy emitted from the sun for each wave length we will get the here below graphic: the ghostly distribution of the sun.

fig 4

The light appears white when his spectral composition is next to that of the sun.

Each bright source produce radiations according to its own spectrum, to lines or continuous, different from that one issued from the sun. Depending on the form of the light spectrum the light can appear yellowish rather than bluish.

The sun radiation is equal to that one of a black cover heated to 5700 °C temp. The spectrum form of the black body is exclusively produced by temperatures, the higher it is the narrower curve and moved toward shorter wave lengths. At low colour, it corresponds to a red to high temperatures correspond lights towards blue.

The normal lamps have for instance a temperature of colour about 3200 °K, while the fluorescent lamps could also reach temperatures of 6500°K.

The colour temperature is a value that gives an immediate information about the type of light but it is surely more complete to check the spectrum. The figure 5, for instance shows the spectrum of a lamp of 1200 watt, diffused in many apparatuses.

fig 5

We can immediately notice that the greatest issues are between 400 and 500 [nm], therefore the radiation of the blue predominate.That does not absolutely mean that turning on this lamp see a blue light. If the lamp only is turned the eye will see simply a white light . Only comparing with the light emitted by a different source, the eye will notice a different light, more bluish or

Always by the same spectrum people notice that this lamp emits also not visible radiations, and 400 [nm]. This is the zone of the Ultraviolet rays, the UV, very harmful to the retina. That is the reason why it is dissuaded to look at lights of this type without glasses of protection.

Many projectors climb on beyond the lamp a reflector. This as per the can have three effects:

1)      assembling to only one direction the bright emitted by the lamp , increasing its bright intensity

2)      correcting the Colour temperature of the same lamp accordingto the cases

3)      decreasing the heat element that travels together with the light: the infrared rays.

The parabolic form reflectors can be in shiny metal (in this case there is not heat demolition) or in treated glass.

We follow the bright bundle beyond the lamp and the reflector, up to the wheel colour, constituted by a succession of coloured glasses: the filters dichroic. These can filter the light, allowing only to a part of the radiation to pass through and to send back the remaining, as shown in figure 6.

fig 6

Beyond the filters, the white light turns in coloured.

What are these filters and how do they select the light?

Only few people know their true nature, many, (among them also internal employees) , believe that same are produced by glass on which has been applied a coloured film or a varnish. Surely it is true that there is glass resistant to high temperatures, but what has been applied is nor a coloured film neither a pigment.

Oxides of titanium and silicon layers are lain on the glass. The light crossing through this coat suffers from a selection, some radiations can cross through all the layers, others are rejected.

This effect is determined by the little thickness of layers which can interfere with the light wave lengths. Indeed the basic physics principle of the dichroic filters is exactly the optic interference.

This multicoat takes the name of coating.

The dichroic filters can produce different colours depending only on the different thickness of the deposed layers.

Only a small part of the light, around 8%, passing through a dichroic filter is "lost." This small percentage is due to the passage of the light through two different elements:air and glass. Each passage determines a loss of around the 4% and if we consider that the light suffers from two of these passages (one in entrance and one in exit) here it is the 8%. That happens each time that the light passes through a transparent glass of each thickness.

The high bright result is the reason that has made now irreplaceable the use of dichroic filter in the professional entertainment. High resolution is translated in bright colours even with a not powerful source.

Other important characteristic is the ability of these filters to resist for long time to high temperatures generated inside of the projectors.

Eventually the characteristic that has given its name to these filters: the dichroism, is the ability to change colour depending on the angle of light incidence: if we look at a filter sat in front of a bright source we will see its transmitted colour. Tilting the filter the colour gradually changes. That is always due to the interference with the bright radiations which crossing the layers obliquely, proceed on a longer path, modifying the final result between transmitted and reflected radiations.