OPTICAL MEASUREMENT OF MARINE CONDITIONS

: The apparatus provides a method of observing a water sample that is caused to flow rapidly past a beam of light of rectangular cross section. Attenuation of the beam by the sample is measured in a first detector that generates an output signal that has a basic, relatively slowly changing (e.g. half a second) component representative of the overall turbidity of the sample, and a series of short (e.g. 6 millisecond) pulses caused by individual, larger particles in the sample. The basic component is fed to a feedback circuit which so controls the light source as to maintain such component substantially constant. The intensity of the light source then provides a measure of the turbidity. A second detector counts the number of pulses above a certain threshold magnitude. In addition, this second detector can momentarily (during the occurrence of such a pulse) open a switch to prevent the feedback circuit from receiving the output signal of the first detector and hence avoid its reacting to the pulse. During this switched off time the output of the feedback circuit is held substantially constant by a capacitor. In what is expected to be its most important application, the apparatus will be used with sea water samples to obtain simultaneous measurements of the concentration of phytoplankton (typically 1 to 10 .mu.m in diameter), as reflected in the turbidity of the sample, and counts of zooplankton (typically 0.4 to 20 mm diameter) which can be individually detected as pulses and counted by the second detector. However, the apparatus can also be used for measuring other factors causing turbidity in water samples, e.g. the presence, of small non-organic particles, while simultaneously counting larger particles, which can include small fish as well as zooplankton.