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