All ceilometers which are set up for  long range cloud height measurement are “far sighted”,  having a blind region in front of the unit.  This is shown in the diagram below ,  and the height of the blind spot Rio is heavily dependent on the axial separation d ,  the beam divergence and the telescope angle of acceptance.  The signal is maximised at the full overlap distance Rovf as shown below.

Since most ceilometers are designed for the best acheivable  signal to noise ratio,  the telescope angle of acceptance is set to the limit of  focal length,  sensor active area and lens aberration.

The single lens designs,  such as the CL51 and 8200-CHS feature a low  value of d and thus a much reduced overlap height

Single lens overlap geometry

There are a number of different optical arrangements to enable the reduction of d to zero or to a small value to minimise the overlap height.

One form of “single lens” Ceilometer,  using a “split lens ” approach (reference (Vande Hey, J. ;  Coupland, J. ; Richards, J. ; Sandford, A. )

It is worth noting that earlier designs of dual lens ceilometers actually utilised the blind spot to reduce the required dynamic range to prevent overload of the return signal processing channel, and greatly reduce optical  crosstalk in the instrument itself ( known as To crosstalk)

Later ceilometers using the single lens optics,  such as the MTECH SYSTEMS  8200-CHS feature special techniques to minimise optical crosstalk and very high dynamic range analog to digital converters to enable detection of fog close to the ground without saturation of the signal

There are a great many factors effecting the performance of ceilometers,  but the key 2 issues for any given cloud volume backscatter coefficient  are:

1. The eye safe limit of the ceilometer .  Infra Red Ceilometers must operate as Class 1M lasers  which limits the energy density of the beam.
2. The noise level.      The inherent noise level of the ceilometer is the ultimate determinant of the signal to noise ratio which enables the ceilometer to discriminate  cloud boundary.

The 8200-CHS has an extremely low level of inherent noise,   which is tested for each ceilometer and is recorded as per the backscatter profile below.

The external source of noise is the shot noise of the scattered and or direct solar radiation within the spectral acceptance of the sensor .   The laser operates around 910 nm and the filtering can only limit the “out of  band”  component of the solar noise spectrum.  The ceilometer will thus t\detect cloud at higher altitudes at night.