I use the same light as pds does and they work very well on lighting not only the dressage ring but also a considerable part of the jumping area.
The lights do have to be placed up high (much higher than 9 ft) otherwise they shine in your eyes. I have my lights way up in a pine tree and they shine down on the riding areas. Works very well and the work lights are very cheap. I got mine for $5.00 with bulbs at Habitat for Humanity. The light are guaranteed to shine for 1,500 hours.