The spectrophotometer should normally be provided with two or more standard lamps, A, B,
C, etc., that are used periodically in checking the state of calibration of the instrument. If lamp A or
lamp B should burn out or be accidentally broken, a new replacement lamp should be wired for use.
It is important that the new lamp be correctly positioned over the spectrophotometer inlet window
before use, particularly if the lamp is of the ultraviolet glass envelope variety that has a C-shaped
tungsten filament. (These lamps are now very rare.) Correct positioning of the lamp is effected by
impressing a small voltage across the lamp so that the filament becomes visible, and adjusting the
lamp in its holder so that the C-shaped filament is situated symmetrically over the
spectrophotometer inlet window with the back of the C pointing downward. Mark this position of
the lamp by drilling a small dimple in the lamp base in line with the hole present in the lamp holder.
All future tests with the new lamp must be performed with the lamp correctly positioned; otherwise,
erroneous data will result.