Measurement of the fuel aerosol droplet diameter is very important in the course of construction of new type sprayers (burners, carburettors, injectors), a diagnostic process. Diffraction method is one of the most useful measuring procedures in this case. The droplet size of monodispersion medium is easy to recognize (Airy rings). Analysis of a diffraction pattern of polydispersion medium is more difficult, however. After recording the diffraction pattern I(r) of an aerosol stream, one of the numerical procedures is used. Registration process and data processing are usually separated. The final result of investigation--average droplet diameter Drho of distribution (rho) (D) is obtained after some time. The most useful procedure in a diagnostic is real-time measuring. Diffraction light intensity curve I(r) depends on the type of droplet diameter statistical distribution (rho) (D) and relative standard deviation d equals (sigma) /Drho as well. Precise analysis in nondimensional units (z equals (pi) Dr/(lambda) f and Iw(z) equals I(z)/Imax) shows the point (z0 equals 1.47, Iw(z0) equals 0.57) which may be used to display average droplet diameter Drho. The position of the point [z0,I(z0)] is independent on relative standard deviation d and type of distribution function (rho) (D) as well. After finding the value of radius-vector r0.57 equal I (r0.57) equals 0.57, it is possible to evaluate the average droplet diameter from the relation D(rho ) equals (0.47(lambda) f)/r0.57. After using image converter with a rectangular optical response OR(I), the Airy picture I(r) becomes a ring. The radius of that ring depends on the threshold light intensity Ip of characteristic OR(I). The radius of converted Airy picture will be r equals r0.57 if Ip equals 0.57Imax. After introducing a nonlinear scale, D equals f(r0.57) image converter becomes an analogue measuring instrument. This idea has been tested with TV and CCD cameras. Using threshold effects of liquid crystal displays it is possible to reduce volume of measuring stand and its price.