The article provides a short theoretical background of what the non-classical light means. We applied the criterion for the existence of non-classical effects derived by C.T. Lee on parametric fluorescence. The criterion was originally derived for the study of two light beams with one mode per beam. We checked if the criterion is still working for two multimode beams of parametric down-conversion through numerical simulations. The theoretical results were tested by measurement of photon number statistics of twin beams emitted by nonlinear BBO crystal pumped by intense femtoseconds UV pulse. We used ICCD camera as the detector of photons in both beams. It appears that the criterion can be used for the measurement of the quantum efficiencies of the ICCD cameras.
 Xiao, M., Wu, L.A., Kimble, H.J. (1987). Precision measurement beyond the shot-noise limit. Physical Review Letters, 59 (3), 278-281.
 Caves, C.M. (1981). Quantum-mechanical noise in an interferometer. Physical Review D, 23 (8), 1693-1708.
 Teich, M.C., Saleh, B.E.A. (1988). Photon bunching and antibunching. In Wolf, E. (ed.) Progress in Optics. Elsevier, Vol. 26, 1-104.
 Saleh, B.E.A., Teich, M.C. (1987). Can the channel capacity of a light-wave communication system be increased by the use of photon-number-squeezed light? Physical Review Letters, 58 (25), 2656-2659.
 Yamazaki, K., Hirota, O., Nakagawa, M. (1988). Optimum condition of squeezed state signal for optical communication with energy loss. Transactions of the Institute of Electronics, Information and Communication Engineers E, 71 (8), 775-783.
 Teich, M.C., Prucnal, P.R., Vannucci, G., Breton, M.E., McGill, W.J. (1982). Multiplication noise in the human visual system at threshold. Biological Cybernetics, 44, 157-165.
 Brida, G., Degiovanni, I.P., Genovese, M., Rastello, M.L., Ruo-Berchera, I. (2010). Detection of multimode spatial correlation in PDC and application to the absolute calibration of a CCD camera. Optics Express, 18 (20), 20572.
 Polyakov, S.V., Wareb, M., Migdalla, A. (2006). High accuracy calibration of photon-counting detectors. In Photon Counting Applications, Quantum Optics, and Quantum Cryptography, Proc. of SPIE 6372.
 Polyakov, S.V., Migdall, A.L. (2007). High accuracy verification of a correlatedphoton-based method for determining photoncounting detection efficiency. Optics Express, 15 (4), 1390-1407.
 Peřina, J. Jr., Haderka, O., Michalek, V., Hamar, M. (2012). Absolute detector calibration using twin beams. Optics Letters, 37 (13), 2475-2477.
 Loundon, R., Knight, B.L. (1987). Squeezed light. Journal of Modern Optics, 34 (6-7), 709-759.
 Haderka, O., Peřina, J. Jr., Hamar, M., Peřina, J. (2005). Direct measurement and reconstruction of nonclassical features of twin beams generated in spontaneous parametric down-conversion. Physical Review A, 71 (3), 033815-033819.
 Waks, E., Sanders, B.C., Diamanti, E., Yamamoto, Y. (2006). Highly non-classical photon statistics in parametric down-conversion. Physical Review A, 73 (3), 033814.
 Waks, E., Diamanti, E., Sanders, B.C., Bartlett, S.D., Yamamoto, Y. (2004). Direct observation of nonclassical photon statistics in parametric downconversion. Physical Review Letters, 92 (11), 113602.
 Haderka, O., Hamar, M., Peřina, J. Jr. (2004). Experimental multi-photon-resolving detector using a single avalanche photodiode. European Physical Journal D, 28 (1), 149-154.
 Haderka, O., Peřina, J. Jr., Hamar, M. (2005). Simple direct measurement of non-classical joint signal-idler photon-number statistics and the correlation area of twin photon beams. Journal of Optics B: Quantum and Semiclassical Optics, 7 (12), S572-S576.
 Breitenbach, G., Schiller, S., Mlynek, J. (1997). Measurement of the quantum states of squeezed light. Nature, 387 (6632), 471-475.
 Peřina, J. Jr., Haderka, O., Michalek, V., Hamar, M. (2013). State reconstruction of a multimode twin beam using photodetection. Physical Review A, 87 (2), 022108-022113.
 Hillery, M. (1985). Conservation laws and nonclassical states in nonlinear optical systems. Physical Review A, 31 (1), 338-342.
 Hillery, M. (1989). Total noise and non-classical states. Physical Review A, 39 (6), 2994-3002.
 Lee, C.T. (1990). Higher-order criteria for nonclassical effects in photon statistics. Physical Review A, 41 (3), 1721-1723.
 Lee, C.T. (1998). Simple criterion for non-classical two-mode states. Journal of the Optical Society of America B, 15 (3), 1187-1191.
 Lee, C.T. (1990). Many-photon antibunching in generalized pair coherent states. Physical Review A, 41 (3), 1569-1575.
 Lee, C.T. (1990). General criteria for non-classical photon statistics in multimode radiations. Optics Letters, 15 (23), 1386-1388.
 Agarwal, G.S. (1988). Generation of squeezed states of radiation at submultiple Rabi resonances. Journal of the Optical Society of America B, 5 (1), 180-183.
 Hong, C.K., Mandel, L. (1985). Higher-order squeezing of a quantum field. Physical Review Letters, 54 (4), 323-325.
 Gerry, Ch., Knight, P. (2005). Introductory Quantum Optics. Cambridge University Press.
 Scully, M.O., Zubairy, M.S. (1997). Quantum Optics. Cambridge University Press.
 Hamar, M., Peřina, J. Jr., Haderka, O., Michalek, V. (2010). Transverse coherence of photon pairs generated in spontaneous parametric down-conversion. Physical Review A, 81 (4), 043827-043836.
 Peřina, J., Křepelka, J. (2005). Multimode description of spontaneous parametric down-conversion. Journal of Optics B: Quantum and Semiclassical Optics, 7, 246-252.
 Allevi, A., Olivares, S., Bondani, M. (2012). Measuring high-order photon-number correlations in experiments with multimode pulsed quantum states.