A B C D E F G H I J K L M N O P Q R S T U V X Y W Z
photochemical Action
The photochemical action is the process in which the activation energy necessary for a molecule change, from a chemical point of view, is furnished by the absorption of a photon. For each incident photon there is a chance to have a permanent change (the effects of the photochemical action are generally irreversible) in the molecule state. To check the photochemical effect means to check the whole chain of chemical transformations, which come from the light, but whose speed depends from many factors: the temperature, the damp and many polluting agents.
The typical consequence of the photochemical effect is the discolouring caused by the sun rays. The light exposure makes the fibres fray and the crack of the surfaces, as well as the yellowing of some pigments.
The sensibility of an object gives an indication of the extension of the damage that will result because of affixed H exposure. The most common approach and the most approved, furnished by the ICOM (International Council of Museum) [ICO97], classifies the objects in three categories according to their sensibility.
opening Angle of the luminous bundle
In the fixture lighting systems used to give a concentrated light in a small area, it is often used to indicate the opening angle of the luminous bundle.
It represents the angle where the luminous intensity is the 50% of the maximum one. In some cases (particularly for instruments with very concentrated bundle) the opening angle is the one where the luminous intensity reaches the 10% of the maximum one. The datum has usually the indication of the percentage which reports (50% or 10%).
Even if the indication of the opening angle, is useful in many cases, it is a very synthetic information and often restrictive: the systems with the same opening angle can also have distributions of different intensities. For a more complete description of the photometrical behaviour of system it is necessary to consider its photometrical curve.
Anomalies in the vision of colours
There are anomalies in the vision of colours both of cerebral and congenital origin. The cerebral ones can verify when the subject has a damage in the zone of the brain appointed to the recognition and the analysis of colours. The most common are the congenital anomalies for the vision of colours, characterized by an abnormal ability not to give some colours and by the impossibility to distinguish colours that appear different for a normal subject. These problems are due to a strange behaviour of the photo-pigments of the eye . The subject can for instance have no cone receptor, or only one, or two types compared to the three of the people who have a normal vision. The different names of these anomalies vary according to the number and of the type of the absent photo-receptors.
In Italy these kind of anomalies are called daltonism. Since they are connected with the genetic anomalies of the X chromosome, they regard the 4% of men and the 0,04% of women.
There are different tests to recognize these anomalies in the vision of colours.
Astigmatism
It is possible to speak of astigmatism when the eye has different powers according to the ocular meridians.
This means that the person, who has the astigmatism, will clearly see direct lines in a certain direction, and not well focused those direct in another direction.
