Dye lasers sometimes use "axial pumping," which consists of a hollow, annular shaped flashlamp, with the outer envelope mirrored to reflect suitable light back to the center. The dye cell is placed in the middle, providing a more even distribution of pumping light, and more efficient transfer of energy. The hollow flashlamp also has lower inductance than a normal flashlamp, which provides a shorter flash discharge. Rarely, a "coaxial" design is used for dye lasers, which consists of a normal flashlamp surrounded by an annular shaped dye cell. This provides better transfer efficiency, eliminating the need for a reflector, but diffraction losses cause a lower gain. The output spectrum of a flashlamp is primarily a product of its current density. After determining the "explosion energy" for the pulse duration, (the amount of energy that will destroy it in one to ten flashes), and choosing a safe energy level for operation, the balance of voltage and capacitance can be adjusted to center the output anywhere from the near infrared to the far ultraviolet. Low current densities result from the use of very high voltage and low current. This produces broadened spectral lines with the output centered in the near-IR, and is best for pumping infrared lasers such as and . Higher current densities broaden the spectral lines to the point where they begin to blend together, and continuum emission is produced. Longer wavelengths reach saturation levels at lower current densities than shorter wavelengths, so as current is increased the output center will shift toward the visual spectrum, which is better for pumping visible light lasers, such as ruby. At this point, the gas becomes nearly an ideal "greybody radiator." Even higher current densities will produce blackbody radiation, centering the output in the ultraviolet. Xenon is used extensively because of its good efficiency, although krypton is often used for pumping neodymium doped laser rods. This is because the spectral lines in the near-IR range better match the absorption lines of neodymium, giving krypton better transfer efficiency even though its overall power output is lower. This is especially effective with Nd:YAG, which has a narrow absorption profile. Pumped with krypton, these lasers can achieve up to twice the output power obtainable from xenon. Spectral line emission is usually chosen when pumping Nd:YAG with krypton, but since all of xenon's spectral lines miss the absorption bands of Nd:YAG, when pumping with xenon the continuum emission is used.