Light bulb efficiency. Sankey diagram showing the efficiency of an energy-saving fluorescent bulb. A fluorescent light bulb is composed of phosphor-coated glass tubes filled with inert gas and mercury vapour. Electric current excites the mercury and causes the phosphor to emit light. In a fluorescent bulb, 75% of the electrical energy is converted into light, and 25% into heat. A Sankey diagram is a flow diagram in which the width of the arrows is proportional to the flow quantity.