DFM Dictionary - Public View/DFMC User Edit
Spatial Coherence Parameters
Spatial Coherence Factor = σ
Where σ = Sine Θ / λ
Θ = angle of incidence, λ = wavelength of light
In the figure in the upper right, a light wave has a large angle of indicdence ( Θ ) and it has a high spatial coherence. As shown in the lower right, a perpendicular light wave with an angle of incidence near zero also has a Spatial Coherence near zero since the sine of Θ is near zero. Light waves that have either a larger angle of incidence Θ and/or a smaller wavelength (λ) will have a larger spatial coherence.
In the figure shown below on the lower left, an Ideal Point Source is Spatially Coherent because all waves of light originate from exactly the same point. These waves are perpendicularly incident on the mask after passing through the Condenser Lens and provide uniform illumination with the angle of incidence Θ = 0. But any Real Source as shown on the lower right, is larger than an ideal point source. Light waves leave from different places on the real source and arrive with different angles of incidence (where Θ > 0 ) when they illuminate the mask. Spatial Coherence is a fundamental parameter of illumination. Whereas, Partial Coherence is the ratio of the two Spatial Coherence parameters, in this case, it is the ratio of the Illumination System to the Projection System.