Explain the types of double beam systems?

Following are types of double beam systems:

a) Dual beam in space type

b) Dual beam in time type

In type (a) separate detectors monitor the light intensity from the sample cuvettes and reference cuvettes. Whereas in the type (b) the light from the monochromator is directed to half silvered mirror. Half of the light is reflected from the reference cuvettes and half from the sample cuvette. A half mirror rotates to direct light alternately from sample and reference cell to single detector. In both the types, the ratio of intensity of sample light to the reference cuvettes is measured.