How can a 120 watt lamp be compatible to a 100 watt lamp, and vice versa? Also, what about 160 and 170 watt lamps?

First, understand that wattage is consumption, not output. Lamps operating at 120 wattts consume about 20% more energy but do not necessarily deliver more output (irradiance) than 100 watt lamps unless the total circuit, including ballast and lamp, is designed to elevate lamp irradiance through increasing lamp current. The primary determinant is the ballast.

100 watt ballasts will operate lamps at 100 watts at approximately 1 ampere (measure of current), regardless of the wattage rating of the lamp. Conversely, 120 watt ballasts will operate lamps at 120 watts at approximately 1.2 ampere, regardless of the wattage rating of the lamp.

It's reasonable then to understand that simply replacing 100 watt lamps with 100+ watt lamps in a 100 watt bed will not increase wattage or irradiance. On the other hand, installing 100 watt lamps in a bed employing 120 watt ballasts will result in higher wattage and irradiance due to the system design...the ballast will drive the 100 watt lamp as hard as it does the 120 watt lamp. Properly designed, the higher wattage (and current) ballast will result in about 20% increase in consumption and irradiance.

Don't take this to mean you should go to the extreme and install 100 watt lamps in 160 watt equipment. Increased wattage and irradiance will result, but you can expect severely shortened lamp life and increased end-darkening. This is due to cathode differences between 100+/- watt and 160+/- watt lamps. A 160 watt cathode is designed to operate properly at 160+/- watts while the 100 watt cathode is not.

Well-designed sunlamps, like those from Wolff System, employ robust cathodes that will operate well within a fairly large range of wattage/current. Whether your lamp is rated 100 or 120 watts, expect shorter service life due to faster degradation of the UV phosphors that results from higher current in beds utilizing higher current ballasts. Remember, one of the most important aspects of a lamp is the phosphor. Higher quality phosphors make a higher quality lamp that will perform better and last longer. Service life ratings from Wolff System is that point where irradiance has declined 30% from initial levels when the lamp is new. Service life is not physical life...a good sunlamp will rarely fail before it is due to be replaced because of normally reducing phosphor efficiency.

Your second question about 160/170 watt lamps is answered above. Just insert the different wattages in the discussion. The ballast, not the lamp, determines both consumption and output.