The long period (3-5 years) required to complete exposure tests under natural I conditions of weather is a handicap to the paint manufacturer who needs to know quickly the properties of his products, particularly new or modified formulas.

A considerable number of instruments have been devised to provide artificial weathering each claiming to reproduce effectively the effects of the normal weathering.

The, as yet, uncontrolled variations in natural weather considerably limit the extent to which artificial 'weatherometers' can be taken as a precise guide to the performance of a paint under all varieties of climatic conditions. The very sub-standard paints will be rapidly eliminated by the artificial tests, but as the quality improves the effects of finer differences between one formulation and another are difficult to detect. The exact numerical assessment by artificial test is virtually impossible.

In brief, weatherometers are means of providing alternating conditions of light, humidity and temperature. A popular type consisting of a large circular chamber or drum, around which the panels are fixed.

The drum revolves slowly around a carbon arc lamp and the panels are sprayed intermittently with distilled water. The carbon arc is chosen as the light source which best reproduces the kind of light emanating from the sun on a cloudless day; it is also a means of heating the panels.

The water spray simulates rainfall, by which the panels are also cooled. The continually repeated cycle of heating, cooling, wetting and drying and light irradiation causes an eventual disintegration of the paint coating.

The number of cycles which the paint can withstand before showing signs of weakening is taken as a measure of durability.

The correlation of this artificially obtained figure and natural weathering is very approximate but a figure of 250 hours in a weatherometer has been cited as being equivalent to one year exterior exposure (UK).

More recently the carbon arc of weatherometers is being replaced with a xenon arc which more nearly represents natural daylight, particularly if it's rays are filtered so that the appropriate cut-off of UV radiation is made and infra-red excess is absorbed.

These lamps giving a more realistic exposure to the artificial form of sunlight.