A carto eter is a device to enable three-dimensional measurements to be made of objects viewed with a high-power light microscope. It attaches to the microscope optically via a camera 1ucida and mechanically via a transducer linked to the focusing mechanism. In use the operator looks down the microscope in the usual way where he sees a cursor superimposed over the microscope image. The operator may move the cursor to align it with some point of interest, the cartesian coordinate of which is available from the cartometer.

The cartesian coordinates may be used for a number of applications directly within the cartometer, for example, the measurement of the linear distance between two points or summated linear distance between a series of points, the surface area of a solid, the volume of a solid, or the area of a section through a solid.

The cartesian coordinates are also available through a standard computer interface (RS 232 or similar) from which computer graphics may be employed to generate solid or wire-frame models of the object in the microscope field. This graphic model may be manipulated on the computer screen while the original is still available in the microscope.

A camera 1ucida is a device which allows an image seen in a microscope or other optical instrument to be drawn. In its simplest form, it consists of a thin plate of unsilvered glass, placed above the eyepiece at an angle of 45° with the optical axis of the instrument, so as to reflect into the eye of the observer an image of the drawing surface, which is seen simultaneously with the microscope image. Most microscope manufacturers now offer such a device which sits in the optical path of the microscope before the eyepieces for greater operator comfort. It is usual that they contain a pair of parallel

mirrors arranged in the form of a periscope, with a lens interposed between the mirrors to allow the drawing surface to be focused.

According to the present invention there is provided an optical microscope including partially reflecting mirror means positioned in the optical path from an object plane to an eyepiece to reflect radiation from computer controlled display means to form a first image in an image plane, said first image being viewable by an eyepiece adjusted to view a second image in said image plane of an object in said object plane.

A microscope with a cartometer attachment in accordance with the invention may utilise CMOS RAM (random access memory) with battery backup so that measurements can be off-loaded to a computer located at a different site. An embodiment of the invention will now be described by way of example with reference to the accompanying drawing which shows a microscope with a cartometer attachment in diagrammatic form.

Referring now to the drawing, a microscope comprises an objective 1 and eyepiece 2 adapted to view a specimen in an object plane 3. A semi-reflecting mirror 4 is positioned in the optical path between the eyepiece and the objective. The mirror reflects an image displayed on a cathode ray tube 5 to form a real image in the object plane. The display on the cathode ray tube is controlled by a processor unit 6. A potentiometer 7 is linked to a fine focus control 8 for the microscope which provides depth information to a position control unit 9.

Using this arrangement, a cursor is projected onto the display device. The cursor can be moved by X and Y controls in the cartometer. The cursor position shown on the display unit is controlled from the cartometer by a pair of potentiometers located for example in a track-ball, joy-stick, mouse or similar. Positional information is scaled and amplified to move the cursor across the display device in X and Y directions respectively. To avoid tube burn two cursor patterns are available, a cross

generated from the mains AC supply, centered at the deflection dictated by the X & Y potentiometers, and a spot which is available for a short time after a button on the cartometer has been pressed. The spot is to allow accurate final alignment before making a measurement.

The high-power compound light microscope has a single optical pathway and as such, no true stereo image can be formed. The property utilised by the cartometer to measure the third

(vertical) dimension is depth-of-field which is very shallow in high power lenses of large numerical aperture.

The vertical movement between the lens and specimen is monitored by the cartometer to provide the Z measurement of the cartesian coordinate. This movement has been measured by attaching a transducer to the mechanically amplified movement of the fine focus control of the microscope, but could equally well by taken directly from the relative movement of the stage and nosepiece.

Pressing a button on the cartometer triggers the system to make a measurement. The system first normally reads the voltage available at the reading button. When this changes state the button has been pressed and three separate readings are made, one for each of the axes. In practice the second of two readings are taken since this has proved to be more reliable. The system saves the three values and finally checks the button again to see that it has changed state back to its original, i.e. that the button has been released.

The readings are made by an analogue-to-digital converter which operates at 12-bit resolution. Each result is stored as two 8-bit bytes, the high nibbles being set to null by the software. The six bytes are put into a temporary store prior to being corrected for scale and linearity.

Non-linearity within the electronics and the display device are calculated for each set of hardware by digitising a square grid pattern. Polyno ic expressions are used to describe deviations from linearity. The values of the polynomial

parameters become part of the system definition.

Calibration is carried out using optical standards. Calibration constants for each axis become part of the system definition. The output from the cartometer is then in standard units of length.

The cartometer allows for display, periodic checking and re-setting of this system definition. Definitions for different hardware assemblies (different objective lenses for example) are stored and can be recalled from within the cartometer memory. The cartesian coordinates measured by the cartometer are made available via a computer interface port, RS232 for example. Since the readings are taken manually, the speed of the interface is not a prime consideration. The greater standardisation available with RS232 is deemed advantageous. The interface port will be "intelligent" insofar as it will recognise the usual protocol signals, such as CTS/RTS, ACK/NAK etc..

In addition, the cartometer front panel will display the current cursor position, the last measured position and the linear distance between them. Other status information as may be necessary will also be displayed, such as prompts for calibration, the name of the system definition in use, etc..