Using US lamps in Europe
The simplest and most reliable conversion is to purchase and install new 230 V bulbs. LED type should be fine.

 

Figure 1. Twin cable with single layer of insulation (left) and European double-insulated cable (right).

One thing to watch is the quality of cable and fittings in your lamps. I have seen North American lamps with single-insulate cable. This might be OK at 110 V but for 230 V use You will be running at higher than designed voltage and insulation should be rated accordingly. You may need to get an electrician to inspect the lamps and fittings. Since these are portable you can bring them to the electrician and avoid a call-out charge. At worst he may recommend rewiring the lamps with suitable cable.