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Measurement of the fuel aerosol droplet diameter is very important in the course of construction of new types of sprayers (burners, carburettors, injectors) and diagnostic processes as well. 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 the diffraction pattern of polydispersion medium is considerably more difficult. After recording the diffraction pattern I(r) of an aerosol stream one of the numerical procedures is used. The registration process and data processing are usually separated. The final result of the investigation -- average droplet diameter D(rho ) of distribution (rho) (D) -- is obtained after some time. The most useful procedure in the diagnostic is real-time measuring. The diffraction light intensity curve I(r) depends on the type of droplet diameter statistical distribution (rho) (D) and relative standard deviation d equals (sigma) /D(rho ) 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 D(rho ). The position of point [z0, I(z0)] is independent of relative standard deviation d and the type of distribution function (rho) (D) as well. After finding the value of radius-vector r0.57 that I(r0.57) equals 0.57 it is possible to evaluate average droplet diameter from relation D(rho ) equals (0.47 (lambda) f)/r0.57. After using an image converter with a rectangular optical response OR(I), the Airy picture I(r) becomes a ring. The radius of that ring depends on threshold light intensity Ip of characteristic OR(I). If Ip equals 0.57Imax, the radius of the converted Airy picture will be r equals r0.57. After introducing non-linear scale D equals f(r0.57) the image converter becomes an analogue measuring instrument. This idea has been tested with TV and CCD cameras. Making use of threshold effects of liquid crystal displays it is possible to reduce the volume of the measuring stand and its price as well.
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