We describe a Fourier-based method of separating bars from spirals in near-infrared images .
The method takes advantage of the fact that a bar is typically a feature with a relatively fixed position angle , and uses the simple assumption that the relative Fourier amplitudes due to the bar decline with radius past a maximum in the same or a similar manner as they rose to that maximum .
With such an assumption , the bar can be extrapolated into the spiral region and removed from an image , leaving just the spiral and the axisymmetric background disk light .
We refer to such a bar-subtracted image as the ” spiral plus disk ” image .
The axisymmetric background ( Fourier index m =0 image ) can then be added back to the bar image to give the ” bar plus disk ” image .
The procedure allows us to estimate the maximum gravitational torque per unit mass per unit square of the circular speed for the bar and spiral forcing separately , parameters which quantitatively define the bar strength Q _ { b } and the spiral strength Q _ { s } 
following the recent study of Buta & Block .
For the first time , we are able to measure the torques generated by spiral arms alone , and we can now define spiral torque classes , in the same manner as bar torque classes are delineated .

We outline the complete procedure here using a 2.1 \mu m image of NGC 6951 , a prototypical SAB ( rs ) bc spiral having an absolute blue magnitude of - 21 and a maximum rotation velocity of 230 km s ^ { -1 } .
Comparison between a rotation curve predicted from the m =0 near-infrared light distribution and an observed rotation curve suggests that NGC 6951 is maximum disk in its bar and main spiral region , implying that our assumption of a constant mass-to-light ratio in our analysis is probably reliable .
We justify our assumption on how to make the bar extrapolation using an analysis of NGC 4394 , a barred spiral with only weak near-infrared spiral structure , and we justify the number of needed Fourier terms using NGC 1530 , one of the most strongly-barred galaxies ( bar class 7 ) known .
We also evaluate the main uncertainties in the technique .
Allowing for uncertainties in vertical scaleheight , bar extrapolation , sky subtraction , orientation parameters , and the asymmetry in the spiral arms themselves , we estimate Q _ { b } =0.28 \pm 0.04 and Q _ { s } =0.21 \pm 0.06 for NGC 6951 .