Maybe shields are an intense gravity field, and when a blast hits it, the photons run into a wall when the acceleraion of gravity is suddenly several hundred thousand G's and the blast is seperated into the individual photons and rendered useless. Particle weapons are not high energy, only a fast-moving projectile, and are ripped apart more easily. When the object/blast is ripped apart, the pieces are spread evenly among the field, and cause stress on it and its ability to absorb additional blasts. The field slowly bleeds off the photons/particles back into space, and regains some of its ability to shield. Em weapons don't get absorbed, but affect the shield generator's ability to maintain a strong field. Instead of overloading the field with excess particles, it decreases the strength itself.
Boosting power to the shield system increases the field's ability to bleed off photons/particles.
If you know the exact strength of the field, you can make sense of the scrambled light waves and "see" through it. That's how radar works. For outgoing weapons, the fire control is synchronized with the forward shield, which opens small holes in the shield directly in front of the cannon/missile tube, which are just large enough to let the shot out, and are open for only a fraction of a second. Scoring a hit through one of these holes is almost impossible because it requires that the shot be timed to hit just as the hole is open, in the right place, at the right angle.
High-power weapons like beams are so powerful and concentrated that the shield cannot rip apart the photons before they are already through the shield.
Ionized particle weapons like the Kayser and plasma-based weapons like the ones on cap ships are both energy AND particles, so it takes a lot out of the field to isolate all of the blast, and decreases its absorbtion ability immensely(sp).
Ships like the Ursa and Boanerges have strong fields that can absorb more photons/particles than ships like the Pegasus. However, some ships can bleed off the absorbed particles at a high rate.
The reason why the Lucifer's shields were impenetrable was because not only was the field extermely strong, but it could bleed off absorbed energy faster than it absorbed it. Whether or not Lucy's shield could block beams is unknown, but a strong enough field COULD stop a beam. Of course, to get through a field that strong, one only has to make a more powerful, more concentrated beam.
Is there anything that I missed that I could theorize on an explanation?