GB2062894A

GB2062894A - Optical density measuring device

Info

Publication number: GB2062894A
Application number: GB8020523A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-06-22
Filing date: 1980-06-23
Publication date: 1981-05-28
Also published as: CH647866A5; GB2062894B; DE2943474A1; DK259480A; DE2943474C2

Abstract

A measuring probe for measuring the density of originals when copying the same in copying cameras comprises a housing 1 having a measuring opening 2, and a photoelectrical transducer 3' disposed behind an optical system 3. For imaging a narrow range (measuring spot) of the inaccessible area of the focusing screen 16 on which an image of the original is formed, the optical system 3 has a larger opening angle than the largest field angle of the copying camera. The image of the spot is focussed on the transducer 3' which converts the reading into density values. <IMAGE>

Description

SPECIFICATION Optical density measuring probe The present invention is concerned with a probe for measuring the density of an original for copying it by means of a copying camera, comprising a housing including a measuring opening and a photoelectrical tran,sducer disposed in the housing behind an optical system.

Measuring probes of this type are known for example from DE-OS2202635 and DE PS 1111 004.

Density measurements in the inaccessible area of the focusing screen plane of copying cameras, which cannot be performed by way of the known measuring systems, are a prerequisite for a controlled performance of predetermined copying operations. Essentially, it is a question of measuring the illumination strength of the lightest and the darkest image points, with the density being a logarithmic measurement of the relative illumination strengths.

The measuring spot has to be as small as possible, about 2 to 5 mm in diameter, and the sensitivity of the measuring element, as a rule, will have to be very high. For this reason, in respect of similar measuring operations, preferably photomultipliers are used as photoelectric receivers. However, up to now precise density measurements in the focusing screen plane have failed in that it has not been possible to measure, through focusing screens of different thicknesses, a correspondingly small point in the inaccessible area.

The second difficulty encountered with such measurements resides in that the light incident on the focusing screen is scattered only in an angular range which encloses the direction of incidence more or less closely.

Accordingly, it is the object of the present invention to provide a measuring probe permitting density measurement in the inaccessible measuring plane of the focusing screen, i.e. at the point where later the material to be exposed is positioned.

This problem is solved in accordance with the invention with a measuring probe of the aforementioned type wherein, for imaging a narrow range of the inaccessible face of the focusing screen of a copying camera, an optical system with a larger opening angle than the largest field angle of the copying camera is arranged in a housing of the measuring probe directly behind the measuring opening thereof but ahead of transducer.

In this connection, the "field angle" is the angle under which the image diagonal appears from the lens.

Thus, in a measuring probe according to the invention, an optical system is provided ahead of the transducer. It is the function of the optical system to detect a measuring point in the focusing screen plane through a focusing screen of any desired thickness and to reproduce the same on the light sensitive face of the receiver. The features of the optical system reside in that it has a sufficiently large opening angle to detect the focusing screen light scattered in a close angular range around the direction of incidence, particularly in the image corners (largest angle of incidence). As a guideline, the sum of the largest field angle of the camera and the largest opening angle of the camera lens can be used as the minimum required opening angle of the optical system.

The size of the measuring spot detected is determined by the size of the aperture between the optical system and the transducer, if a so-called photomultiplier is used as the transducer. When using a correspondingly small photocell as the transducer an aperture will not be absolutely necessary.

For eliminating scattered light and errors of imaging, the aperture size is selected to be smaller than the nominal measuring face.

What is essential is sharp focusing of the focusing screen plane on the aperture plane, with the scale of focusing being selected to be, for example, 1:1.

The scattered light has a particularly strong effect with very small dark measuring faces in the light surround field. In a particularly advantageous construction, in which the aperture is exchangeably disposed against laterally staggered aperture openings for detecting the scattered light, the influence of the scattered light can be detected and used for numerically correcting the measured value if, in a separate measuring operation, aperture openings are used that are disposed, for example, circularly around the actual aperture. However, this problem can be solved even more elegantly in that the optical system and the light-electric transducer may be adjustably disposed in the housing, which will be explained in further detail.

If a large image field is of a uniform density the measured value can nevertheless show an angular dependence caused by the geometry of the optical system and of the receiver. This can be so compensated by a corresponding construction that the optical system is provided with correcting means, for example, a neutral filter with a radial density gradient, to compensate the angular dependence of the measured value.

Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 in part is a sectional view of a measuring probe; Figure 2 is a side view of the measuring probe in the sighting position; Figure 3 is a view of the measuring probe according to Fig. 2 in sighting direction; Figure 4 in part is a sectional view of another embodiment of measuring probe; Figure 5 is a sectional view of another embodiment of measuring probe including a collimating projector, in sighting direction; Figure 6 is a special embodiment of a measuring probe in section; Figure 7 is a schematic view of the beam path during detection of the light surround field by the measuring probe according to Fig.

6; Figure 8 is a curve demonstration of the measured density value in response to the size of the measuring spot; Figure 9 is a displaceable aperture for the detection of the scattered light; Figure 10 is a schematic view of the measuring process with the measuring probe at a focusing screen of a copying camera; and Figure ii is a view of the arrangement of circular neutral filters on a lens of the optical system.

Referring now to Fig. 1 of the drawings, there is provided in the tubular housing 1 of the measuring probe, directly behind the measuring opening 2, an optical system 3 composed for example of a capacitor-type lens arrangement behind which is provided transducer 3' in the form of a photomultiplier, with a reiaceable aperture 5 interposed therebetween.

The opening angle 4 of the optical system is so dimensioned that it is larger than the largest field angle of the copying camera 33 concerned, of which only the focusing screen 1 6 is shown in closer detail in Fig. 10.

Measuring 16' of focusing screen 1 6 in which, in making the foilowing picture, the material to be exposed will lie.

Preferably the optical system 3 and aperture 5 are axially adjustable in the housing, i.e.

arranged displaceably so that with focusing screens 1 6 of different thicknesses the measuring spot 1 7 is always in sharp focus on the aperture.

The size of measuring spot 1 7 to be detected depends on the size of opening 18 of aperture 5, which is inserted between the optical system and the photo cell 3' and can be relaceably arranged in housing 1.

The aforementioned possibility of arranging a plurality of aperture openings 5' for detecting the scattered light is demonstrated in Fig.

9, with the actual aperture openings 5' (positioned on an imaginary circle) being provided, for example, in a common aperture slide 34 suitably arranged in housing 1 of the measuring probe so as to be displaceable in the transverse direction.

If a large image field is of a uniform density the measured value can nevertheless exhibit an angular dependence which is caused by the geometry of the optical system and the transducer. However, this can be compensated for by a construction according to Fig. 11 in which the optical system is furnished with correcting means, for example a neutral filter 35 with a radial density gradient which compensates for the angular dependence of the measured value.

On account of the combination of the optical system 3 and the size of the measuring opening 2, the bearing face of the measuring probe in which the measuring opening is provided is relatively large, and certain difficulties are involved in so precisely mounting the measuring probe onto the focusing screen 1 6 of the copying camera 33 (schematically demonstrated in Fig. 10 by way of example) that the optical axis of the measuring probe passes precisely through the centre of the measuring spot 1 7, thereby attaining a parallax-free detection of the measuring spot.

For this purpose, in a preferred embodiment the measuring probe and the housing, respectively, is provided with a sighting means 6, preferably formed from a collimator 9 displaceably disposed at the mounting end 1' of housing 1 and focused with an opening 7, with the path of displacement of housing 1 corresponding to the distance between the center of the measuring opening 2 and the collimating line 9'.

The carry out the measuring process, the measuring probe with the opening 1 3 or plate 8 thereof is mounted to the focusing screen 16 of the copying camera 33 (Fig. 10), with housing 1 being in the lower position according to Figs. 2, 3 relative to plate 8. The center of the measuring spot 1 7 is collimated via collimator 9 in which connection plate 8 is displaced until the collimating line 9' coincides with the measuring spot 1 7. Then housing 1, with plate 8 being held stationary, is displaced upwardly until stop 8' is reached so that the optical axis 1 5 of the measuring probe coincides with the preceding collimating line 9'.This will of course require that, as set forth above, the path of displacement of housing 1 correspond to the distance between the center of the measuring opening 2 and the collimating line 9', based on the lowest position (sighting position) of housing 1 at plate 8 which, preferably, is provided with a non-slip coating 8".

The sighting means 6 can, in accordance with Fig. 4, also be formed of two relatively pivotable legs 11 and 1 2 with the leg 1 2 being connected at right angles to the bearing surface end 1' of housing 1, and the other leg 11 being provided with a view opening 13, the centre of which 14 lies in the optical axis 1 5 of the measuring probe when legs 11, 1 2 are in side-by-side relationship.

In this form of embodiment, in which opening 1 3 can be provided with cross hairs-leg 11, with housing 1 turned downwardly (or upwardly) together with leg 12, is simply mounted onto the focusing screen 1 6 and its opening 1 3 is aligned with the center of the measuring face. After that leg 1 2 is swung in the direction of the arrow until the stop is reached and measurement can be effected.

The collimators of Figs. 2 and 3 need not necessarily have the shape as shown; a sighting tube or a notch and bead sight arrangement or the like can be equally well provided.

In another advantageous embodiment the measuring probe according to Fig 5 is provided with a collimating projector imaging a self-illuminating mark 20, such as the luminescent face of a luminescence diode, through optical system 1 9 and mirror 21, exactly to the measuring point 1 7. The viewer's eye 23 can thus see the collimation point free of parallax in the focusing screen plane. If the point of collimation coincides with the desired measuring point the probe is swung about the center of rotation 22 in the measuring position, i.e. from the illustrated approximately horizontal position into the vertical position.

According to Fig. 6 the entire unit 24 formed of the optical system 3 and the photoelectric transducer 3' (composed of photocell 31 and electronic converter 32 coupled thereto) is disposed axially displaceably in housing 1 of the measuring probe, which axial displacement is effected by means of a guide ring 27 seated on the external circumferential face 29 of housing 1 and being displaceable between two circular stops 25, 26 arranged fixedly yet adjustably on housing 1. Driving of unit 24 is effected by a dog 28 interconnecting guide ring 27 and unit 24 and gripping through housing 1 in a correspondingly long elongated slot.

According to Fig. 7 the above described measuring probe, mounted to the focusing screen 16, is used to measure the measuring spot 1 7 (density DS) in face 16' of focusing screen 16 in which later the material to be exposed is provided.

After a corresponding displacement of the unit 24 within the measuring probe remaining in situ the whole of the lighter surround field 1 7' and also the density thereof (Du) are detected.

The different density values of measuring areas of different sizes and the surround fields thereof are graphically demonstrated in Fig. 8, revealing that the density values in decreasing areas become drastically smaller and that the influence of lighter surround fileds becomes stronger while in larger measuring faces the curves approach the measured density value (Dw) true of the exposure adjustment thus being the ideal value.

The actual and true density value Du of the exposure to be adjusted, can be readily computed from the ascertained measured density values Ds and Du, using the approximate formula: Dw = Ds + K(Ds-Du) wherein K represents a factor depending on typical apparatus parameters.

The construction of the measuring probe as described, advantageously, and without the aforementioned aperture manipulation, with the measuring probe remaining at the corresponding point of the focusing screen, permits on the one hand detecting a measuring spot of small area, and on the other hand also the influential light surround field. Both measured density values, after a summarizing calculation, can be converted into the density value Du critical of the exposure.

Claims

1. A probe for measuring the density of an original for copying it by means of a copying camera, comprising a housing including a measuring opening and a photoelectrical transducer disposed in the housing behind an optical system, wherein, for imaging a narrow range of the inaccessible face of the focusing screen of a copying camera, an optical system with a larger opening angle than the largest field angle of the copying camera is arranged in a housing of the measuring probe directly behind the measuring opening thereof but ahead of transducer.

2. A measuring probe according to claim 1, wherein an aperture is provided between the optical system and the transducer.

3. A measuring probe according to claim 2, the said aperture is disposed exchangeably.

4. A measuring probe according to claim 2 or 3, wherein the said optical system and aperture are axially adjustable or displaceably disposed in the housing.

5. A measuring probe according to claim 2, characterised in that the said aperture is so formed that it can be exchanged with an aperture having laterally staggered aperture openings for the detection of scattered light.

6. A measuring probe according to claim 1, wherein the said optical system is adapted to be furnished with correcting means, such as neutral filters with a radial density gradient for compensating the angular dependence of the measured value.

7. A measuring probe according to claim 1, wherein the housing is provided with a sighting device.

8. A measuring probe according to claim 7, wherein the said sighting device comprises a plate arranged displaceably on a mounting end of the probe housing and provided with an opening, with plate having housing stop, and of a collimator focused to the center of said opening, with the path of displacement of the housing corresponding to the distance between the center of the measuring opening and the collimating lens.

9. A measuring probe according to claim 7, wherein the said sighting device comprises two relatively pivotable legs one said leg being connected at right angles to a mounting end of the probe housing and the other leg being provided with a view opening.

10. A measuring probe according to claim 7, wherein the said sighting device is formed as a collimating projector system for generating a beam of light to be directed to the measuring point and for forming a luminescent sighting mark, respectively.

11. A measuring probe according to claim 10 wherein the collimating projection system comprises a self-illuminating sighting mark, an optical system and a mirror and the entire housing of the measuring probe being pivotably disposed about a center of rotation.

1 2. A measuring probe according to claim 1, wherein the optical system and the transducer are formed as a unit axially displaceable in the probe housing.

1 3. A measuring probe according to claim 12, wherein a guide ring displaceable between two stops is disposed on the outer circumference of the probe housing the same being connected to the said unit via a dog extending through the probe housing wall into a longitudinal slot.

14. Measuring probes for optical density measurements, substantially as hereinbefore described with reference to the accompanying drawings.