Hello to everyone.I saw on HQL lamps that they tend to black after longer usage.What causes this and why HPS lamps rarely have black arctube after long working hours ?

Electrodes material sputtering into the arctube, causes it to black. HPS arctubes are narrower, so they are usually blackening at the ends only, similar to fluorescent lamps. If the entire HPS arctube blackening, this means that the arctube leaking.

Electrodes material sputtering into the arctube, causes it to black. HPS arctubes are narrower, so they are usually blackening at the ends only, similar to fluorescent lamps. If the entire HPS arctube blackening, this means that the arctube leaking.

Thanks for reply.That is bad thing for such lamp.

Thanks for reply.That is bad thing for such lamp.

This is in fact virtually the only way how they wear out. After some time this blackening blocks so much light, it became uneconomical to keep the lamp vs replacing it with a fresh new one, so it became aneffective end of lamp life.

This is in fact virtually the only way how they wear out. After some time this blackening blocks so much light, it became uneconomical to keep the lamp vs replacing it with a fresh new one, so it became aneffective end of lamp life.

That is bad.I've seen that some older HQL lamps had special electrodes which cause arctube to become white as it ages.

This is still used, but then the arctube assemblies were redesigned to be smaller to boost the arc loading so the lamp efficacy. As consequence once the whitening agent gets consumed (as the rated life passes) and then the blackening goes very quickly (because of the high loading).
It is just the trade off between life and efficacy: The arctube is usually loaded so the lamp has the highest efficacy while still reaching the "standard" rated life (= 24k hours; with MVs it was when the light output drops to 50%). If the arctube is loaded more, it yields higher efficacy, but as well faster wear. Because the lifetime became rather standardized (24k hours), makers usually use any extra robustness to just boost the efficacy.
Because of the electricity costs, this actually makes a lot of sense: 24khours is perceived as a good optimum for the total cost of the light (purchase + energy + maintenance), longer life wont reduce the maintenance cost that much but it would mean higher energy costs; shorter life wont bring that much benefit in efficacy so energy cost, but would increase the maintenance cost.