Angular acceptance
Angular acceptance it a measure of how accurately the crystal has to be oriented in order to achieve maximum efficiency
Expand the mismatch Dk around the phase-matching angle qm
half-angle acceptance for critical phase matching is
dq=l/(l(n02w-ne2w)sin(2qm)
l~1m sin(2qm)~1 (n02w-ne2w)~0.04