This Replacement for Philips 278168 MS400/BU/PS is a 400W pulse start metal halide HID lamp designed for powerful, reliable performance in commercial and industrial lighting applications. Featuring an E39 mogul base and ED37 clear glass bulb, it installs easily into compatible fixtures using M155 (also compatible with M128 and M135) ballast systems. Delivering approximately 42,600 lumens of 4100K bright white light, this lamp provides strong, high-quality illumination ideal for warehouses, gymnasiums, retail spaces, and large-area lighting. The base-up (BU) operating position ensures proper performance in approved fixtures. With a rated life of up to 20,000 hours and an 11.5" overall length, this HID lamp offers long-lasting durability and reduced maintenance for demanding lighting environments.