A transducer is an electronic device that converts energy from one a physical form to electricity or vice versa. Microphones, loudspeakers, thermometers, moisture and pressure sensors, photocells, light-emitting diodes, and light bulbs.
Efficiency is an important consideration in any transducer. Transducer efficiency is defined as the ratio of the power output in the desired form to the total power input. Mathematically, if  
  represents the total power input and  
  represents the power output in the desired form, then the efficiency is  

No transducer is 100 percent efficient. Energy is always lost to electrical resistance as heat. Usually this loss is manifested in the form of heat. Some transducers are very efficient - aerials can be 80 - 90% efficient - but old style light bulbs are very inefficient - maybe only 20%