Solid Light Barrier

Contrary top popular belief, the phrase "energy shields" does not refer to a single continuous line of development. While the electromagnetic fields used by Covenant infantry and SPARTAN-II MJOLNIR are effective, systems used by Forerunner technology and limited Covenant systems make use of a vastly different mechanism - solid light. A phase transition induces a state change in photons, resulting it the photons behaving like particles rather than as a wave, and creating solid and almost invisible plane. While UNSC technology has a very poor understanding of the systems, reverse-engineering has produced some significant results - the energy shields introduced on UNSC starships after 2553 use solid light barriers for defence, deemed appropriate given the Covenant's expansive knowledge of electromagnetic shields; even earlier, the MJOLNIR Mark IV/Vajra II V-II/B, V-II/D and V-II/E used solid light barriers for defence, though the technology for smaller-scale versions of the system were never perfected and were eventually discontinued by 2557.

History
When the Covenant attacked Harvest, the UNSC assembled the single largest space fleet it had assembled since the Interplanetary War to respond to a threat of such magnitude. There are several reasons for this: for one thing, the UNSC hoped that the sheer numbers of the fleet would force the ship to back down, and perhaps return the new alien civilisation to the negotiating table - a futile effort. Mostly, however, it was because the alien ship, in wiping out Battlegroup Heracles, had demonstrated that it used advanced directed energy weapons and a potent energy shield system for defence. When Cole's fleet engaged the enemy warship engaged the ship, they lost thirteen ships before the fleet managed to lower the enemy ships shields and inflict significant damage to it.

From then on, the primary advantage the Covenant would possess was their energy shield technology. For the most part, their DEW systems were about as effective as UNSC shipborne weaponry, and their users certainly had an inferior understanding of their own weapons; likewise, the advantages Covenant ships enjoy in slipspace technology were largely ignored by their ship captains. The single most important advantage the Covenant enjoyed was virtual immunity from anything the UNSC could hurl at their ships. Tactics would eventually be found to penetrate or circumvent the shields, but their effectiveness could not be denied, and ONI tasked Project TEMERITY, a sub-section of Naval Special Weapons, to study captured systems and attempt to reverse-engineer them for UNSC use.

The history of energy shield technology from then on becomes long and mixed. Project MJOLNIR would go on to develop powerful and efficient electromagnetic shields, able to repel bullets and debris, and capable of interfering with plasma weaponry, though the shields would only see use on the MJOLNIR Powered Assault Armour series. Others experimented with a mixture of superheated plasma held in place by a magnetic field, similar to Covenant weaponry, but creating a soliud barrier rather than a hurtling projectile. But in the end, it was the Solid Light Barriers developed by Project TEMERITY that might be claimed to have had the largest impact on UNSC shield technology.

While most projects preferred to focus on recovering, studying and reverse-engineering Covenant equipment, the often poor quality of the equipment from battle damage hindered most investigations. TEMERITY instead chose to direct their attention to anomalous technology present on a number of worlds within UNSC space; besides being the primary target of almost every Covenant attack, the technology predated both the UNSC and Covenant by hundreds of thousands of years, and was still centuries more advanced. Recovering a number of relics from Covenant hands, TEMERITY made significant gains, some of them quite startling. In particular, they developed the SLB-77X, a prototype solid light barrier generator. Equipped to the UNSC corvette Raven, the barrier, a single pane of solid light, proved more than sufficient for reflecting debris, and proved effective against most DEW types. The SLB-78X would be six generators working in tandem to create a three-dimensional "box" around the Raven, effectively protecting the ship from all sides. The power requirements for the system proved enormous, impractically so, and it was not until advances were made in that area in 2542 that the system was deemed a viable alternative to the MJOLNIR-driven alternative system. Rather than a solid box, the SLB-82X would create a layer of solid-light that closely hugged the external hull to reduce the ship's sensor signature, with small panes able to be deactivated in the barrier layer to allow hull-mounted weaponry, such as the 50mm CIWS system and Archer missile pods, to fire at enemy ships. In 2552, the introduction of the SLB-83A had begun with the first field prototypes installed in the Ajax-Class Heavy Frigates. Less than a handful of ships equipped with the systems would survive the Fall of Reach, and of these most were destroyed during the Battle of Earth, but the series would be revived in the form of the SLB-85C, introduced in 2554 and made standard on all post-war ship designs.