DE1422563C - Focusing device - Google Patents

Description

(R = resistance,

I = light intensity,
k and e = material constants),

one with the greatest possible e-value, expediently greater than 0.8, is provided.

5. Device according to one of claims 1 to 4, characterized in that the optical system of the optical instrument is provided at the same time as the focusing lens system.

6. Device according to one of claims 1 to 5, characterized in that for automatically focussing takes place in drive connection with the focusing lens system, from the semiconductor photoresistor controlled adjustment device is provided.

7. Device according to claim 6, characterized in that the adjusting device has a has servomotor (8) controlled by the photoresistor.

is. This relationship can be expressed by the very common formula

k_ I ^e

The invention relates to a focusing device for optical instruments, with one in the image space a photoelectric converter arranged in a focusing lens system, its resistance when focusing of the image on the transducer surface has an extreme value.

As is well known, the photocurrent of a photoelectric converter increases with increasing illuminance /, which is synonymous with a corresponding decrease in the internal resistance R of the converter. Here, k and e are material constants of the respective converter.

This general relationship is not only obeyed by vacuum photocells, gas-filled photocells, barrier photocells, but also the photoconductive cells known as semiconductor photoresistors, e.g. B. CdS cells.

If an object is imaged on a certain plane with the help of an imaging lens system, so the designed image has the smallest dimensions in the case of focus, and it is here significant differences in illuminance and brightness distribution compared to the blurred image of the same object present. One should therefore expect that this circumstance will result in a expresses any kind of change in the internal resistance of the converter.

In a known focusing device of the type described above, the image generated by the optical system is projected onto a photocell for this purpose, which shows a non-linear relationship between the photocurrent and the illuminance, i.e. a curved characteristic curve (e not equal to 1), which means that the size of the photocurrent then inferred the degree of sharpness of the image and thus the setting of a system can be recognized without looking at the image. The photocell types with curved characteristics (e less than 1) that can be used for this purpose are barrier-layer photocells, gas-filled and high-vacuum cells. The former show curved characteristics when they are connected to external resistances that are roughly in the order of magnitude of the internal resistance of the cell or above. The latter cell types also show saturation of the photocurrent at higher illuminance levels and thus a curvature of the characteristic curve. It was recognized here that the greatest change in current or change in internal resistance occurs when the setting changes from a blurred to a sharp setting when the photocell characteristic curve for the average illuminance of the image has the greatest curvature, while on the other hand no such current or at all in the linear characteristic curve range (e = 1) Internal resistance change occurs. However, since the average brightness of the individual images can be very different, it is to be expected that in many cases the operating point on the photocell characteristic will fall within the linear range of the same, so special circuit measures are necessary in order to achieve a desired change in the photocurrent transition from fuzzy to sharp setting. For this purpose, two ways are basically provided in the known arrangement, namely 1. artificial distortion of the entire characteristic curve to obtain a curved course with the help of suitable distortion circuits that are connected between the photocell and the display element, and 2. provision of an automatic control in the sense, that the working point on the characteristic is always relocated to an area of the greatest curvature of the characteristic.

These measures are obviously cumbersome and, moreover, by no means always lead to one

large, "detectable measuring range in the sense of a strongly fluctuating, average brightness from picture to picture. If, for example, one looks at a characteristic curve that is uniformly curved from the zero point, the finally turns into a horizontal straight line (saturation area), images above a corresponding medium brightness can no longer be detected. If you still want to capture these cases, so it would have to be ensured by an appropriate art circuit that the horizontally running Branch of the characteristic curve comes to lie higher. This automatically reduces the curvature in the usable range of characteristics, which in turn comes at the expense of sensitivity.

In contrast, it is the object of the invention to provide a focusing device of the type in question provide, in which one by no means on a curved characteristic shape of the photoelectric converter is instructed, in particular, the linear part of the characteristic curve can also be used, so that the detectable range of the mean image brightness can be enlarged considerably. This task should 'with the simplest means, i. H. without use any additional art circuits between photoelectric converter and photo current display element, be solved.

The invention is based on the following considerations: In the case of barrier photocells (copper-copper oxide cells and selenium cells), the electrodes are generally used They are flat and, like a capacitor, close a dielectric, the copper oxide between them or selenium, one of the two electrodes being transparent. That on this Light incident on the electrode sets in the layer underneath at an observed point Due to the internal photo effect, electrons are released, the number of which depends on the illuminance at the point of view depends. With this type of electrode arrangement, therefore, they are always adjacent to one another From an electrical point of view, surface elements behave as if their associated resistance elements are connected in parallel. The same applies to a vacuum or gas-filled photocell, in the case of electrons due to the external photo effect on illuminated parts of the photocathode surface exit and are transferred to the anode because of the suction voltage that is present. Here too if the partial currents exiting from each considered section of the cathode surface add up, see above that in effect there is also a parallel connection of the individual surface elements.

On the other hand, the conditions are completely different for those photoconductive ones known as semiconductor photoresistors Cells, which on two opposite front edges of a photo semiconductor layer with the required Connection electrodes are provided. For the sake of simplicity, it should be assumed here that these electrodes run along the entire extent of these front edges. For example, if the one half of the photo resistor is illuminated, one obtains (irrespective of whether the cut-off line is is sharp or out of focus) an electrical parallel connection of the dark and light halves assigned Partial resistances if the cut-off line is perpendicular to the electrodes, but one Series connection of the same if the cut-off line runs parallel to the electrodes. In the In cases in which the cut-off line runs obliquely, the effect is a combination of Partial resistors connected in parallel and in series.

As has now been found and in detail

will be explained, is obtained whenever such an electrical series circuit of different illuminated surface or resistance elements of a transducer in addition to or instead of a pure electrical parallel connection of such surface elements occurs, both in linear and in

ίο curved characteristic area when transitioning from fuzzy to sharp setting a change in the total resistance, therefore a corresponding one Change in current, which is then used to assess or control the degree of focus.

Accordingly, the invention is characterized in that a semiconductor photoresistor is provided as the converter is, whose photoconductor layer is divided between the connection electrodes in a strip shape in such a way that the differently illuminated sections lying along such a strip are connected in series Form resistance.

From equation (1) it can be seen that the resulting change in resistance for a given change in illuminance, i.e. the sensitivity, will be greater the greater the value e . As mentioned, in the known focusing device, the presence of a curved characteristic curve shape corresponding to e values less than 1 is an imperative. On the other hand, according to the invention, the linear range of characteristics for which e − 1 can now also be used. It is therefore not only possible to enlarge the detectable measuring range, but also to achieve a higher sensitivity of the arrangement according to the invention.

In the following the invention is illustrated by means of exemplary embodiments in the drawing; it indicates

F i g. 1 a schematic representation of a focusing device,

F i g. 2 and 3 illuminance distribution curves in the image plane of the arrangement according to FIG. 1 at sharp or blurred image,

F i g. 4 and 5 are schematic representations of display circuits with semiconductor photo resistors,

F i g. 6 and 7 different embodiments for the photoresistors used and

Fig. 8 is a schematic representation of an automatic Focusing device with semiconductor photoresistor.

According to FIG. 1, an objective lens 1 is displaceable in both directions along the optical axis arranged. A photoresistor 3 is located in its focal plane 2. There is also an electrical power source 4 and a galvanometer 6 located in the circuit 5 is provided.

For the explanation it is assumed that the object P to be imaged consists of a light and a dark half (shown hatched in FIG. 1) and that the light-dark boundary ρ intersects the optical axis. The object P with its light-dark boundary ρ is imaged on the photoresistor 3 as an image P ' with a corresponding light-dark boundary p'. The distribution of the illuminance in the image plane will therefore have a pronounced step when the image is in focus (curve C in FIG. 2), while there will be a gradual transition when the image is out of focus (curve C in FIG. 3).

In the diagrams according to FIG. 2 and 3 is the

Illuminance distribution C or C, which results in the case of a sharp or unsharp image in the image plane for the example under consideration, with the illuminance / plotted as the ordinate. For reasons of symmetry, the ordinate (x = 0) of the coordinate system used is relocated to the light-dark boundary p '. Let the illuminance of the dark part of the picture surface be denoted by I _d , that of the bright part by I _b and the ratio of this to that by m; so it applies

L = mL, with m> 1.

(2)

Hx) = / on ,.■■ ■ ./(-χ) = h

and for F i g. 3

7 (x) = I _b -hl _d
or because of (2)

(3)

(4)

(-x) = I _d -hI _d =

with the one from FIG. 3 apparent meaning for h.

In Fig. 4 shows the case in which the photoresistor has the shape of a narrow, elongated semiconductor body with electrodes t on the end faces and the light-dark boundary p ' is oriented perpendicular to the direction of the electrical current in the semiconductor body (s). The total resistance of this semiconductor body can then be understood as a series circuit of successive small resistance elements assumed within the semiconductor body.

If one considers the resistance elements r at the points χ and - χ independently of the focus state as a unit, i.e. as a series connection, and denotes the total resistance of the same with sharp setting with r {- x, x) and that with unsharp setting with r '(- x, x), then for these quantities result from equations (2) to (4) in combination with the equation

R = ψ (U

after some intermediate calculation the expressions

^(5b)

A zzz ι * ' ( γ yI r ( γ Y ^

₌ 1 [YJ

■ / 5 LMi + A) "

One can show that the sign of Δ is given by the magnitude

If one takes on the basis of the F i g. 2 and 3 indicate that the illuminance / on outside of the origin of coordinates at χ and - χ is / (x) and / (- x), results for the case the F i g. 2

15th eh

(10)

30th

35

40

45 is determined. Since e is greater than zero in all cases and, by definition, m is always greater than 1, J is always negative, so the relationship applies

r '(- x, x) < r (-x, x) (8)

always.

In words, this means that the resistance when the setting is unsharp is always smaller than the resistance when the setting is sharp, for all values of e> O.

Currently available materials exhibit a β value of about 0.6 to 0.8, but materials with e values above 0.8 are also desirable.

In the example under consideration according to FIG. 4, in which a series connection of all resistance elements is assumed was, therefore, results for the total resistance between the two electrodes in the case of focus

R = Σ> (- χ, χ) (9)

X '

and with a fuzzy setting

So it always applies
R '<R.

The electric current flowing through the photoconductor for all e > 0, also for e = 1 (linear characteristic), is therefore always smaller in the case of focus than in the case of an unsharp setting (current minimum with focus).

Next is the one shown in FIG. 5 is considered, in which the light-dark boundary p 'of the image on the photoconductive cell lies parallel to the direction of the current flowing in the semiconductor body s. The total resistance of the photoresistor is here a parallel connection of individual resistances of many successive small strip elements, which are assumed to be between the two electrodes.

If one again considers the resistance elements r at the points χ and - χ as a unit, i. H. as a parallel connection and the total resistance of the same is called again with a sharp setting r (—x, x) and those with a fuzzy setting with r '(- "x, x), then result for these values from the Equations (1) to (5) after some intermediate calculation the expressions

55

r (-x, x)

^{1 1} ₌ ! L (i _{+ nf) t} (12a)

For the resistance difference Δ between the sharp and the unsharp setting, the equations (5a) and (5b) must be subtracted from one another, so the equation is obtained

-hi _d ) ') \ (i _d -hi _d r)

(12b)

65 To calculate the change in resistance / I in the event of a transition from fuzzy to sharp

Position, in turn, equations (12a) and (12b) are to be subtracted:

A =

r '(- x, x) r (-x, x)

(13)

or

Δ = il [(I + h) ^e + (m - h) ^e - (1 + m ^e )]. (14)

One can show that the sign of Δ is given by the magnitude

(1 - rrf- ") (15)

is determined.

In the example under consideration according to FIG. 5 therefore results for the total resistance in the case of focusing

(16)

Λ χ "K Λ, Χ)

and for fuzzy setting

R '

r '(- x, x)

From equations (16) and (17) it again follows that the sign of the quantity

is determined by the sign of A. However, the sign of Δ depends on the quantity e , as determined by formula (15). So we have to distinguish three cases:

a) For e> 1, Δ < 0; so becomes - <-. (19)

K K

b) For e = 1, A = O; so ^ 7 = 4 "■ ( ²⁰ )

K K

c) For e < 1, A> 0; so becomes - ^> -. (21)

It therefore follows that the electric current in the photoconductive cell is a maximum in the case of focus when e> 1, that it is independent of the degree of focus when e = 1 and that it is a minimum when e < 1 in the case of focus is.

In the case of conventional photoresistors, e lies between 0 and 1. On the other hand, it should be pointed out that cells with e> 1 are highly desirable in view of the sensitivity increasing with e. Cells with e = 1 cannot be used at all if their resistance elements are connected in parallel. Therefore, in this case, the size e should be made significantly different from 1.

The foregoing are considerations regarding the simplest, shown in Figs. 4 and 5 illustrated borderline cases where the cut-off line is either perpendicular or parallel to the direction of the electric current in the photoresistor runs, so a pure series connection or pure parallel connection of the individually illuminated differently Resistance elements are present:

Similar considerations can be made, for example, with regard to an image in which a Cut-off line at an angle to the electrical direction Current in the photo resistor runs, because in this case the cut-off line is made up of elements Can think of composite, alternating at right angles and alternating parallel are oriented towards the direction of the electrical current, thus a combination of series and parallel connections differently illuminated resistance elements, d. H. a superposition of the two borderline cases, is available. The same considerations apply to the general cases; which are, for example around a pattern of black stripes on a white surface or around a pattern of any kind other training acts. Here, too, the effect is always a corresponding combination of series and parallel connections of differently illuminated resistance elements, d. H. a corresponding overlay of the two borderline cases.

For such an overlapping case, a comparison of equation (11) with equations (19), (20), (21) immediately shows that for e < 1 the parallel and series resistance change components run in the same direction, i.e. add up, hence the The expected resulting overall change in resistance (sensitivity) will be significantly greater than in the case known from gas-filled vacuum photocells or barrier photocells, where there is a pure parallel connection of all differently illuminated surface or resistance elements. Furthermore, it is immediately apparent from equations (11) and (20) that a change in the total resistance can also be expected for e = 1, but in this case this only comes from the series circuit resistance change component. On the other hand, since the overall sensitivity increases with increasing e , the disappearance [s. Equation (20)] of the parallel connection resistance change proportion com-.

pensed-t; Thus, not only the (curved) saturation range of a photoconductor characteristic, in which e < 1, can be used, but also the linear characteristic range, in which e = 1. This means that practically the entire characteristic curve can be used for the purpose in question, if it is only ensured that a series circuit resistance change component always occurs. It remains to discuss the comparison of equations (11) and (21), that is, the case where e> 1. Here the immediate

bar that the parallel and the series circuit resistance change components run in opposite directions to each other, so will subtract. However, the decrease in sensitivity to be expected will not appear, at least not significantly, because, as already mentioned, the sensitivity increases with increasing size of e anyway, so such a characteristic curve shape can also be used successfully.

The focusing device thus has the properties discussed above and enables an objective one Focusing in which the current flowing through the galvanometer 6 in the case of focus assumes an extreme value. This extreme value can depend on the shape of the characteristic curve and the formation of the photo resistor be a maximum or a minimum. The F i g. 6 to 7 show different embodiments for the photoresistors. In the embodiment according to FIGS. 6 and 6 'is an between

309 610.2

Electrodes t arranged semiconductor body s divided in the longitudinal direction into numerous, mutually parallel strips by insulating walls u . In effect, this is an arrangement in which the basic arrangement according to FIG. 4, multiplied accordingly, is connected in parallel to increase the sensitivity. This arrangement obviously has a lower internal resistance than that of FIG. 4, which is advantageous for certain use cases. The shape shown in FIG. 6 ″ is made up of two cells designed in the manner of the cells in FIG The shape shown in FIG. 7 has a semiconductor body s between a center electrode I ₁ and a circumferential electrode t ₂ , which is divided into narrow sectors by radially extending insulators u. in Fig. 7 'is the semiconductor body s u by insulation rings divided into annular, mutually coaxial semiconductor stripes. the embodiments shown in F i g. 7 and T are variations of g in F i. form shown 6, so that blurring in virtually any extending Light -Dark limits (as in the exemplary embodiment according to FIG. 6 ") can be effectively used.

In Fig. 7 ", the semiconductor body has a spiral shape. In effect, there is an arrangement here which the basic arrangement according to F i g. 4, correspondingly multiplied, is present in a pure series circuit, whereby on the one hand the sensitivity and on the other hand also the internal resistance of the photoresistor is increased, which is also desirable in certain applications. Also can also in this embodiment the blurring

ίο effective with practically any running light-dark borders, ie in the sense of a series circuit - resistance change component that is always present.
In Fig. 8 shows an embodiment of a focusing device with a photoresistor in one of the types described. The device is designed for automatic focusing and for this purpose has a drive device 7, which is in drive connection with a servomotor 8, for the displacement of the objective lens 1. A control device 6 ', which (instead of the galvanometer 6 in FIG. 1) is provided for the servomotor 8, is located in the circuit 5 of the photoresistor and ensures that - when the electrical current in the circuit 5 reaches an extreme value - the servomotor8 and thereby stopping the movement of the objective lens 1 to achieve automatic focusing.

1 sheet of drawings.

Claims (4)

Patent claims: 1. Focusing device for optical instruments with one in the image space of a focusing lens system arranged photoelectric converter, its resistance when focusing the image has an extreme value on the transducer surface, characterized in that as Converter a semiconductor photoresistor is provided, the photoconductor layer between the connection electrodes (f) is subdivided in strips in such a way that the lying along such a strip, differently illuminated sections form resistors connected in series. 2. Device according to claim 1, characterized in that that the photoconductor layer consists of a single strip applied in a spiral. 3. Device according to claim 1, characterized in that two parallel-connected, strip-shaped semiconductor photoresistors having divided photoconductor layers so arranged • are that the strips of the two photoresistors cross each other. 4. Device according to one of claims 1 to 3, characterized in that the material for the Photoconductor layer, the shape of which is given by the characteristic curve