Patent GB-1385679-A (GB)
Title: Density measurement by image analysis
IPC: G01N21/59
Filed: 1971-04-17
Abstract: 1385679 Photo-electric analysis of images IMAGE ANALYSING COMPUTERS Ltd 27 March 1972 [17 April 1971(3)] 9737/71 9739/71 and 9740/71 Heading G1A In the determination of dimensional parameters of a feature in a field, in which the feature is distinguishable by its having a greater optical density than the rest of the field, the field is illuminated and light transmitted therethrough focused on a TV camera tube, the video output of which is passed on a logarithmic amplifier to an integrator. The invention is applicable to the determination of the area, volume or mean optical density (obtained by dividing a volume representative signal by an area representative signal) of (e.g. stained) cells in animal or vegetable tissue in a microscope slide. One or more associated parameter computers are used with an anti-coincidence detector in order to separate the information relating to respective cells if more than one are present in the field, as described with reference to Fig. 3. The apparatus comprises a T.V. camera 10, in which a line scan on path 26, Fig. 2, passing through cell 18 (and nucleus 22), would produce a video signal at A of the form shown in (a), Fig. 2. In order to reduce the area of the frame considered, a blank frame 24 facility is provided, by which video signals for e.g. a particular single cell on the field may be analysed. Each field may be identified in turn automatically, or a light pen used. In either case only video signals within blank frame 24, are gated through gate 14 to  inverting amplifier 12 whose output is shown at (b), Fig. 2. This is followed by logarithmic amplifier 36 (output at Fig. 2 (c) ) and linear amplifier 38 which multiplies by a constant K to convert the signal to one directly related to optical density and is dependent on illumination intensity, microscope transmission characteristics &c. The integrator is shown schematically at 40 and typically integrates over a single frame scan. Since the background signal level (i.e. in the area outside the feature area) may produce a cumulative effect e.g. due to the presence of dirt particles and foreign matter, the circuit between the camera and gate 14 is modified as in Fig. 3. Only video signals above a certain level are allowed to reach gate 14, by the use of a differential amplifier 44, having one input as this reference level, followed by a bi-stable circuit 48 and a gate 50. The video signal passes via a delay device 56 which is to accommodate any delay introduced by the gating circuit. The effect is to reduce the video signal to be analysed to the area within lines 52, 54 in Fig. 2, i.e. to the feature (or cell) area itself. This gating circuit may also include pulse stretching to allow for diffuse boundaries. The feature gating pulses may be chopped at high frequency to produce pulses for counting to provide an area representative signal. The arrangement of Fig. 4 (not shown) for analysing each of two or more features individually, includes two associated parameter computers one connected at X in Fig. 3, obtaining an area and the other, connected at 39 in Fig. 1, for obtaining a volume representative signal which are released by an anti-coincidence detector after the last line scan intersection with each feature, to a divider and printer. Digital or analog or a combination of both techniques may be used.