JTT and JDD bulbs are not exactly the same. JTT (Japan Tubular Tungsten) bulbs have quartz or glass envelopes for high-temperature resistance. JDD (Japan Double-Dome) bulbs are also tungsten filament bulbs.They have an additional outer glass envelope to help improve safety by containing shattered glass if the bulb breaks. If you are using them for food heating, JDD should work fine as long as the wattage (150W), voltage, and base type match your old JTT bulbs.