Context :

Aims : We study the variation in the magnetic field strength and the umbral intensity of sunspots during the declining phase of the solar cycle no .
23 and in the beginning of cycle no .
24 .

Methods : We analyze a sample of 183 sunspots observed from 1999 until 2011 with the Tenerife Infrared Polarimeter ( TIP ) at the German Vacuum Tower Telescope ( VTT ) .
The magnetic field strength is derived from the Zeeman splitting of the Stokes- V signal in one near-infrared spectral line , either Fe i 1564.8 nm , Fe i 1089.6 nm , or Si i 1082.7 nm .
This avoids the effects of the unpolarized stray light from the field-free quiet Sun surroundings that can affect the splitting seen in Stokes- I in the umbra .
The minimum umbral continuum intensity and umbral area are also measured .

Results : We find that there is a systematic trend for sunspots in the late stage of the solar cycle no .
23 to be weaker , i.e. , to have a smaller maximum magnetic field strength than those at the start of the cycle .
The decrease in the field strength with time of about 94 G yr ^ { -1 } is well beyond the statistical fluctuations that would be expected because of the larger number of sunspots close to cycle maximum ( 14 G yr ^ { -1 } ) .
In the same time interval , the continuum intensity of the umbra increases with a rate of 1.3 ( \pm 0.4 ) % of I _ { c } yr ^ { -1 } , while the umbral area does not show any trend above the statistical variance .
Sunspots in the new cycle no .
24 show higher field strengths and lower continuum intensities than those at the end of cycle no .
23 , interrupting the trend .

Conclusions : Sunspots have an intrinsically weaker field strength and brighter umbrae at the late stages of solar cycles compared to their initial stages , without any significant change in their area .
The abrupt increase in field strength in sunspots of the new cycle suggests that the cyclic variations are dominating over any long-term trend that continues across cycles .
We find a slight decrease in field strength and an increase in intensity as a long-term trend across the cycles .