A brief review of lighting technologies is presented. Unavoidable restrictions for incandescent
light bulbs caused by the Planck distribution and properties of the human eye are illustrated. The
efficiency and luminous efficacy of thermal radiation are calculated for various temperatures; the
results clearly show the limitations for thermal radiators. The only way to overcome these
limitations is using non-thermal radiators, such as fluorescent lamps and light-emitting diodes
(LEDs). Unique advantages of LEDs undoubtedly made a revolution in this field. A crucial element of
this progress is the blue LEDs (Nobel Prize 2014). Some experiments with a blue and a green LED are
described: (i) the luminescence triggered in a green-yellow phosphor inside a white LED by the blue
LED; (ii) radiant spectra and ‘efficiency droop’ in the LEDs; (iii) modulation of the blue LED up to
4 MHz; and (iv) the h / e ratio from the turn-on voltage of the green LED. The experiments are
suita…