the present invention is directed to an imaging doppler lidar ( idl ) system for the measurement of wind velocity for a variety of potential applications , including the measurement of complex flow dynamics from large - scale power generating wind turbines . fig2 shows a schematic illustration of the imaging doppler lidar system 10 of the present invention . the idl 10 comprises a pulsed , narrow linewidth laser transmitter 11 to flood illuminate 12 a scene 13 and a time - gated imaging receiver 16 to provide range resolution and an image of the velocity field in a measurement plane of the scene 13 . the velocity can be determined for each pixel in the image from the doppler frequency shift of the scattered laser light 14 that can be measured through a frequency discriminator 15 . the short optical pulses that illuminate the scene and the precise time - of - flight measurements of the pulse reflections from aerosols in the scene enable accurate distance information that provides the third spatial dimension normally absent when images are recorded using only ambient reflected light . the laser transmitter 11 should have an adequate energy per pulse to provide sufficient signal - to - noise at the receiver , a high repetition rate to capture transient formations in the flow , and a large enough tuning range to locate a strong absorption feature of the frequency discriminating cell . while higher pulse energy is generally desirable for increasing the minimum detectable wind velocity , simulations indicate single pulse energies of order 100 mj are reasonable . the pulse repetition frequency should be above about 10 hz to avoid washing out high velocity flow patterns , but can be much higher when integrating signal directly on an intensified , charged coupled device ( iccd ) camera . the laser pulse preferably has a pulse width of order 1 ns for sub - meter range resolution . the range resolution is determined from the mean of the pulse width and the receiver camera gate time , multiplied by the speed of light . although the beam quality of the laser is not critical , the far - field pattern of the laser must be stable and relatively uniform . the laser light preferably has a wavelength of between 300 and 1500 nanometers and a narrow linewidth no more than 200 mhz to enable high resolution of small doppler shifts . for example , the laser can comprise a q - switched solid - state laser , such as a nd : yag , er : yag , er : glass , or ho : yag laser . fig3 is a block diagram of an exemplary laser transmitter 11 . the master oscillator 21 can be a continuous wave source , providing a narrow linewidth emission that can be tuned to a desired absorption feature of a first reference cell 24 ( e . g ., an iodine absorption cell ). the output of the master oscillator 21 can be frequency doubled by second harmonic generation ( shg ) 22 and transmitted through the reference cell 24 to a detector 26 . the transmitted light through the first reference cell 24 can be held at a constant level , thereby locking the master oscillator frequency to a spectral feature of the reference cell 24 using feedback from a servo amplifier 25 . a portion of the frequency doubled light can be amplified 23 to a large energy level for illumination 12 of the target scene . this amplification can be accomplished in at least two ways . the first is to use the low - level continuous wave signal to seed a high - power q - switched slave oscillator . in this way , the output of the slave oscillator will be made single frequency and follow the frequency of the slave laser . alternately , the low level signal can be amplified using a series of pulse - pumped power amplifiers . the imaging receiver 16 preferably comprises a time - gated focal plane array ( fpa ) that uses independent time - of - flight measurements at every pixel to acquire distance information for aerosols in the scene . for example , the fpa can have a gate width of about 1 ns , a frame rate matching the laser repetition frequency , a sensor size of greater than 256 × 256 pixels , and a pixel size of less than 25 microns . the gate turn - on delay can be varied from 1 - 100 ns , allowing on - the - fly variation of the measurement plane . volumetric , vector velocity images can therefore be measured without employing beam steering optics . the system preferably has adequate sensitivity to be used with natural aerosol concentrations and cross - sections . fig4 is a block diagram of an exemplary receiver 16 comprising signal and reference fpas . optics can be used to collect the scattered light 14 and image the target onto two iccd focal plane arrays 31 and 35 . a narrow bandpass filter 34 can be used to block sun light and background signals not at the laser wavelength . one of the iccd cameras 35 can be used as a reference , imaging the scene directly reflected from a polarization independent beam splitter 33 to remove the effects of a non - uniform scattering distribution by ratioing the two signals . light coupled to the other iccd camera 31 passes through a second reference cell 15 , which is the frequency discriminator for the system , provides the signal . doppler shifted light experiences a change in transmission , which appears on the iccd as a brighter or darker signal relative to an unshifted signal . the frequency discriminator 15 can be a single - edge spectral absorption filter that preferably has a line width and absorption depth compatible with the velocity range of interest . the frequency discriminator should have a sufficiently large field - of - view , and have a relatively short length and large diameter . for example , for 532 nm laser emission , an iodine absorption cell can be used as the frequency discriminator . an iodine absorption cell is basically an evacuated glass container with a controlled vapor pressure of iodine gas within . the iodine concentration determines the absorption per unit length at the laser wavelength . the iodine absorption spectrum is a rich forest of deep , very narrow lines starting at about 480 nm and ending near 640 nm . the absorption spectrum provides a very stable reference spectrum superimposed on the scattered laser light being observed . fig5 shows the scattering geometry at the target aerosol at the scene . the measured velocity direction bisects the angle between the transmitter { right arrow over ( k )} t and receiver { right arrow over ( k )} r direction vectors , in the scattering plane . the doppler shift δν is diminished by the cosine of the angle θ between the direction of the wind velocity { right arrow over ( v )} and the direction of the measured velocity component . fig6 shows a method to determine the velocity using frequency discrimination provided by an iodine absorption cell 15 . two cameras 31 and 35 ( signal s and reference r , respectively ) measure counts n s and n r per pixel proportional to the scattered light intensity at frequency ν . after removing dark levels n d , s and n d , r , the images are ratioed , pixel by pixel . three ratio images γ ( x , y , ν l ), γ ( x , y , ν u ), and γ ( x , y , ν b ) are recorded at each pixel ( x , y ). the first is with the laser tuned to the 50 % relative transmission point on the side of an absorption line of iodine , ν l , providing a measure of the change in transmission due to doppler shifts of the scattered laser light . the second image is at a frequency of high transmission , ν u , well removed from the absorption line , which allows for correction of intensity variations in the image not associated with doppler shifts . the third is also taken near the 50 % transmission point , ν b , but is taken of a uniform , stationary scene without doppler shifts . the last image allows for removal of spatial variations of laser frequency , if present . in addition , simultaneous transmission measurements using single detectors are taken from the first reference iodine cell ( shown in fig3 ) to correct for frequency drift of the laser during data collection . ratios of these images are designated by γ r in the equation . t ( ν ) is the transmission of the iodine cell at frequency ν and dt / dv is the slope of the transmission line at the measurement frequency . the doppler shift δν d can be calculated , to first order , from the various ratio measurements using the equations in fig6 . fig7 is a schematic illustration of the imaging geometry . the laser transmitter 11 illuminates a region of diameter d t at the image plane of the scene 13 . each fpa sensor 31 and 35 records an image of the same plane 13 , with a one - to - one mapping of the sensor pixel size x pix to a resolution element of size x res in the image plane . the spatial resolution of the lidar system at the target plane is determined by the magnification m rec of the imaging system , number of pixels n pix in the fpa , and diameter d t of the illuminated area . for a given range r from the receiver 16 to the image plane , the spatial resolution x res is often in excess of what is required . it is advantageous to perform pixel binning on the camera to increase signal - to - noise at the expense of some spatial resolution . radiometric calculations indicate that noise equivalent velocities below 1 m / s are achievable for a backscattering coefficient of about β ˜ 1 × 10 − 4 m − 1 sr − 1 . this is consistent with aerosol particle densities in the 1 - 10 micron size of about 1000 - 5000 particles / cm 3 . fig8 is a schematic illustration of a full scale implementation of an idl system . the laser transmitter 11 can be ground based and the laser beam can be directed upwards 12 a or steered 12 b to the desired scene 13 using a tower - mounted remote steering mirror 36 . in the latter case , the ground can be used as a backstop for the laser illumination . multiple receivers 16 can be located at ground locations to provide a three - dimensional , vector image of the wind velocity field in the scene 13 . the present invention has been described as an imaging doppler lidar . it will be understood that the above description is merely illustrative of the applications of the principles of the present invention , the scope of which is to be determined by the claims viewed in light of the specification . other variants and modifications of the invention will be apparent to those of skill in the art .