The final signal-to-noise ratio and hence speed and accuracy of measurement is dominated by the level of light reaching the detector. In all real systems this light level is limited by the efficiency and light throughput of the monochromator.

When comparing various commercially available monochromators you find that most have an f number in the region of 4. It is a common misconception to assume that light throughput can be assessed by comparing f number alone. This is not the case. For a given spectral bandwidth, a large f/4 monochromator (long focal length, large mirrors and gratings) will operate with wider slits than a small f/4 unit. This wider slit allows more power through and into the fibre.

The Bentham system uses a 300mm focal length, f/4 monochromator to launch 5nW or -53dBm (cw) into a single mode fibre (1300nm, 10nm bandwidth).

Stability

Due to the sequential nature of the measurement, both optical and mechanical stability over the period of the two spectral runs is of critical importance.

For the Bentham range, optical stability is ensured by the IL1 light source, which uses a 100W QH lamp controlled by a highly stabilised constant current power supply. This power supply is at least 10 times more stable than a normal laboratory stabilised supply.

Mechanical instability in such systems results in short or long term wander of the focussed spot over the end of the fibre at the launch end. In the Bentham system the light source and launch optics are bolted firmly to the monochromator, which is constructed from a single rigid casting.