Prismatic Helm (also known as Prism Helm in the SNES/PS version) is headgear in Chrono Trigger. Upon completion of the Trial side-quest, the Rainbow Shell can be forged by Melchior into either three Prism Helms or one Prism Dress. If you have the Sun Stone, then he'll make some PrismSpecs (an accessory that greatly boosts damage).
Prismatic Helm can also be charmed by Ayla from the left hand of Queen Zeal (Boss).
In contrast to Prismatic Dress which can only be equipped by female characters, Prism Helm can be equipped by all party members.