It's called a laser power meter (surprising, I know ). They are not often sold retail because they are in such low demand in the consumer market. However, you can purchase one from J. BAUER Electronics (the LaserBee 2.5W USB, or whatever), or you can buy a more professional grade used in labs from Ophir or Coherent. However, the LaserBee is going to be the least expensive type (the ones from Ophir or Coherent are $1000+, whereas the LaserBee is up from $130 or so).