CL-Hg Low Pressure Mercury Lamp

The Bentham CL-Hg is a low pressure mercury lamp which can be operated directly from the mains supply.

The wavelengths of the spectral lines emitted by the lamp are dependant only on the energy levels occupied by the electrons in the mercury atom and are thus constant for any source in which mercury atoms are caused to emit light. No measurements have been made on the lamp supplied nor does it need calibration in any way. The fact that the lamp contains mercury is enough to guarantee that the spectral output characteristic of mercury will occur.

The list below shows only the more prominent lines which have been found useful for calibrating spectral measuring equipment. The wavelengths given are taken from a NIST (NBS) document titled "Wavelengths and Transition Probabilities for Atoms and Atomic Ions. Part 1. Wavelengths" . Reference NSRDS-NBS 68

253.65nm 435.83nm
296.73nm 546.08nm
365.02nm 576.96nm
404.66nm 579.07nm

The NBS document actually lists the wavelengths for approximately 70 spectral lines. The limited selection above have been chosen because in the CL-Hg they are sufficiently intense to allow monochromator calibration and because there are no intense lines in the same wavelength region. This last point is especially important if the calibration of the monochromator is checked by measuring the half power points with the normal operating bandwidth e.g. 5nm (576.96nm and 579.07nm would not be suitable in this case).

Using Higher Orders

Unfortunately, the mercury spectrum contains no useful lines above 579.07nm but the shorter wavelength lines can be used to calibrate monochromators by exploiting higher order transmission of the grating. N.B. To allow higher order transmission the order sorting filters must be removed or the order sporting filter wheel be set to its open position.

As an example of higher order transmission, the green 546.08nm line can be used to calibrate monochromators in second order at 1092.16nm or in third order at 1638.23nm.