As mentioned earlier, there are many inconveniences in determining the lightfastness of pigments under natural light. These include fluctuations in the intensity of sunlight, changes in air humidity, and the need to conduct experiments in sunny weather, often with temperature changes and so on. All of these factors are often obstacles to rapid testing of light fastness, especially in areas with very few sunny days. Therefore, in the determination of the lightfastness of pigments, attempts have been made to use artificial light that imitates the sun.
The intensity of light emitted by artificial light sources should always be constant, and the chemical effects caused by the light should be similar to the effects of sunlight.
Until now, in testing the light fastness of pigments, mercury quartz lamps and arc lamps that can emit a large amount of ultraviolet light are mainly used. The intensity of the light emitted by such lamps is actually fixed. changing.
The main disadvantage of mercury-quartz lamps is that after a certain period of use, the ultraviolet light emitted by them will gradually decrease in intensity. Generally, when the quartz lamp is lit for 1000-1500 hours, its light intensity can be fixed. After that, the lamp holder (burner) must be replaced. An electric arc with a common carbon pole can also be used as an artificial light source, but the ultraviolet light intensity of this arc is lower than that of the electric arc 56 with an iron core carbon pole inside. The use of arc lamps with carbon electrodes with iron cores not only emits light from outside the industry, but also emits light in the visible part of the spectrum. Therefore, in order to extract ultraviolet light, the light of this lamp must be passed through a set of filters made of colored liquid placed in a quartz vessel or placed in a glass vessel that can transmit ultraviolet light. constituted. These filters absorb all visible light and pass only ultraviolet light.
When preparing the optical filter, copper sulfate solution that can absorb red light and yellow light and nitrosodimethylaniline solution that can absorb green light, blue light and purple light can be used, and special colored glass can be used.
When conducting general tests, artificial light sources without filters can be used, while for more accurate tests, it is recommended to add filters.
A 750-watt filament lamp was also used when determining lightfastness; its light was filtered through water and copper sulfate solutions. Now, there are still various types of incandescent lamps made of glass that can pass through ultraviolet rays, and they are also widely used. In this case, the filter is not necessary.
The further improvement of this lamp is that in addition to its glow line, there is an auxiliary mercury lamp head to extend the ultraviolet part of its spectrum. These lamps emit all the rays of sunlight (ultraviolet, light, and heat); the undesirable short-wave light emitted by pure quartz lamps is absorbed by the outer glass bulb.
High-pressure mercury lamps with a high light output should also be proposed. Under the condition of consuming the same electric energy, the light they emit can be 2~2.5 times larger than that of ordinary incandescent lamps.
These lamps are used with aluminum reflectors; they emit high, sunlight-like luminous intensity and UV exposure. The degree of heating of the surface when illuminated by these lamps is much lower than that of incandescent lamps.
This lamp is available in various wattages.
The service life of this lamp is 2000 hours on average.
