According to the Lagrange--Helmholtz law, no optical system can ever increase the brightness (and therefore the effective temperature) of a light beam. This limitation is frequently interpreted as a consequence of the second law of thermodynamics. Actually, thermodynamics imposes no limitations of practical importance on the possibilities of increasing the brightness of light beams: the decrease of the entropy of the light beam associated with the increase of its brightness can be compensated by a loss of a negligible fraction of the energy of the beam. The general discussions are illustrated by a number of specific examples.

@article{Sobel’man:1975,
	author = {I. I. Sobel’man},
	title = {Another look at what is possible and impossible in optics},
	publisher = {Physics-Uspekhi},
	year = {1975},
	journal = {Phys. Usp.},
	volume = {17},
	number = {4},
	pages = {596-598},
	url = {https://ufn.ru/en/articles/1975/4/j/},
	doi = {10.1070/PU1975v017n04ABEH004431}
}