DE1797575C3 - Focusing device. Eliminated from: 1422563 - Google Patents

Description

The invention relates to a focusing device for optical instruments having an in 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 known, the photocurrent in the photoelectric converter increases with increasing illuminance /, which is equivalent to a corresponding decrease in the internal resistance R of the converter. This relationship can be expressed by the general formula

Here, k and e are Maierial 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 aid of an imaging lens system, then has the designed image in the in-focus case has the smallest dimensions, and they are essential here Differences in illuminance and brightness distribution compared to the blurred image of the same object present. One should therefore expect this fact to turn into a somehow kind of change in the internal resistance of the converter.

In a focusing device known from German patent specification 709 954 as described at the beginning For this purpose, the image generated by the optical system is thrown onto a photocell, which have a non-linear relationship between the photocurrent and the illuminance, thus shows a curved characteristic curve (<? not equal to 1), whereby from the size of the photo current then inferred the degree of sharpness of the image and thus the setting of a system can be recognized without looking at the image. As eligible for this Photocell types with a curved characteristic curve 0 smaller than J) become barrier photocells, gas-filled and called high vacuum cells. The former show curved characteristics when they are

resistors are connected that are roughly in the order of magnitude of the internal resistance of the cell or above lie. The latter cell types also show saturation of the photocurrent at higher illuminance levels and thus a curvature of the characteristic.

ίο Here it was recognized that the greatest change in current or change in internal resistance occurs when the Ul-ergang from unsharp to sharp setting occurs when the photocell characteristic curve for the mean illuminance of the image has the greatest curvature, while on the other hand in the linear characteristic range (e = 1) at all no such change in current or internal resistance occurs. However, since the average brightness of the individual images can be very different, it is to be expected that in many cases the working point on the photocell line will fall within the linear range of the same, so special circuit measures are necessary 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 one :; curved course with the help of suitable distortion circuits, which are connected between the photocell and the display element, and (2) provision of an automatic control in the sense that the operating point on the characteristic is always relocated anew in an area of the greatest curvature of the characteristic.

These measures are obviously cumbersome and, moreover, by no means always lead to a large, detectable measuring range in the sense of a strongly fluctuating, average brightness of the image to picture. For example, if one considers a characteristic curve that is uniformly curved from the point of origin, the finally turns into a horizontally running straight line (saturation area), so images above a corresponding average brightness can no longer be detected. You want these cases anyway still grasp, an appropriate art circuit would have to ensure that the horizontal running characteristic curve branch comes to lie higher. However, this automatically reduces the curvature in the usable range of the characteristic curve.

which in turn comes at the expense of sensitivity.

In the case of barrier photocells (copper-copper oxide cells and selenium cells) the electrodes are generally flat and close like a capacitor a dielectric, between them the copper

s.s oxydul or selenium, one of the two electrodes is designed to be translucent. The light incident on this electrode sets in the one below Layer at a point under consideration releases electrons due to the internal photo effect

(> o the number of which depends on the illuminance prevailing at the point of view. With this type of elec Electrode arrangement, therefore, surface elements lying next to one another are always seen from an electrical point of view behave as if their associated resistance

f'5 elements are connected in parallel. Appropriate: This also applies to a vacuum or gas-filled photo cell where, due to the external photo effect, ai illuminated parts of the photocathode surface Elek

₁₀

15th

20th

Irene emerge and are transferred to the anode because of the pending suction. Here too if the partial currents exiting from each considered partial gap on the cathode surface add up, see above that in effect also a parallel connection of the individual surface elements is present.

On the other hand, the conditions are completely different for those known as semiconductor photoresistors pptoconductive cells, which are attached to two opposite Front edges of a photo semiconductor layer are provided with the necessary connection electrodes. For the sake of simplicity, it should be assumed here that these electrodes are along the entire extension these front edges run. If, for example, one half of the photo resistor is now illuminated, then so one obtains (regardless of whether the cut-off line is sharp or fuzzy) an electrical one Parallel connection of the partial resistances assigned to the dark or the light half, if the cut-off line runs perpendicular to the electrodes, but a series connection of the same if the cut-off line runs parallel to the electrodes. In those cases in which the cut-off line is inclined runs, the effect is a combination of partial resistances connected in parallel and in series. Attempts have been made to replace the photocells with more modern semiconductor photoresistors. Ais beneficial It turned out that semiconductor photoresistors inherently have a curved resistance-lighting characteristic exhibit. So there are no circuit tools or saturation mode required, such as it is the case with the above-mentioned German patent specification.

However, it has been found that the semiconductor photoresistor has two opposite end edges arranged connection electrodes does not allow any measurement definable for focusing. This is because the resistance that can be measured when the focus is set depends on whether the cut-off line of the object to be measured is perpendicular, parallel or inclined to the electrodes. That means z. B. that when panning the camera around the lens axis for a stationary object, the resistance value changes when the focus is adjusted while panning or that there is no defined one for moving objects Measurement is possible.

It is therefore the object of the invention to provide a focusing device of the type described at the outset to make, which allows a defined measurement of the focus condition at any time.

According to the invention, this object is achieved in that a semiconductor photoresistor is used as the converter is provided, the photoconductor layer divided by partitions into a plurality of sections is and that the partitions are designed in a known manner as electrodes, wherein the Finding the focus condition from the entirety of the section output signals.

An advantage of the invention is that ourch the Electrode arrangement a plurality of sections of the photoconductor layer is connected in parallel, so that the entire semiconductor component has a low internal resistance. This results in the Possibility of using the one for the conveyor Semiconductor component used to carry out an exposure measurement at the same time.

From equation (1) it can be seen that the resulting change in resistance for a given change in the light intensity, i.e. the sensitivity, will be greater the greater the value e . As mentioned, in the known focusing arrangement, the presence of a curved characteristic curve shape corresponding to e-value is less than the mandatory requirement. According to the invention, the characteristic limit range for which e> 1 can also be used. It is therefore not only possible to enlarge the detectable measuring range, but also to achieve a higher sensitivity with the arrangement according to the invention.

F i g. 1 shows a scheraatic representation of a Focusing device, the HeU-dark border of the object running vertically,

FIG. 2 shows the focusing device according to FIG. 1, the light-dark boundary of the object, however, running horizontally.

F i g. 3 and 4 an embodiment according to the invention a photo resistor with parallel or perpendicular to the partition walls designed as electrodes running light-dark borders,

F i g. 5 and 6 lighting intensity distribution curves in the image plane of the arrangement according to FIG sharp or unsharp image and is F i g. 7 is a schematic representation of a display circuit with a semiconductor photo resistor.

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 purpose of explanation, it is assumed that the object P to be imaged is composed of a light and a dark FIG. 1 shown hatched) 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 hay-dark boundary p'. The distribution of the illuminance in the image plane will therefore have a pronounced step in the case of sharpness (curve C in FIG. 5). while in the case of a blurred image there will be a gradual transition (curve C in FIG. 6). 4s In Fig. 2 is the same arrangement as in Fig. 1 shown. While in FIG. 1 the light-dark boundary runs vertically and thus parallel to the electrodes of the photoresistor 3, in FIG. 2 it is horizontal and thus perpendicular to the electrodes of the photoresistor. In general, the partial resistance in the dark area of the photo resistor is a multiple of the partial resistance in the light area of the photo resistor. In the case of FIG. 1 the high dark-light resistance and the low light-part resistance are arranged one behind the other, so that the total resistance of the photoresistor is essentially formed by the dark resistance. Since in the case of Fig. 2 dark and helicopter partial resistance are connected in parallel to each other, the Gcsamti.o resistance of the photoresistor is essentially formed by the low light partial resistance. If the difference in brightness between the dark and the healthy area remains the same, the total resistance of the photoresistor * 3 and thus the measured value on the measuring device 6, depending on whether the light-dark border runs horizontally or vertically, is completely different. If the camera is rotated around its lens axis while the object is stationary, that for the changes

Distance setting measured value to be evaluated. The same goes for a moving one Object. It is therefore not possible to make a clear measurement for the purpose of setting the distance.

The present invention achieves that. regardless of whether the cut-off line is horizontal or runs vertically, always a parallel connection of dark and light partial resistances occurs. On the basis of Fi g. 3 and 4 can be demonstrated that a photoresistor divided into a plurality of sections by partitions shows a measured value that is practically independent of the position of the cut-off line.

Since the object to be photographed is generally placed in the center of the image, it is reasonable for the Calculation assume that the cut-off line is practically in the middle of the photo resistor located. In Fig. 3 it is assumed that this boundary is parallel to the partition walls designed as electrodes runs, while in F i g. 4 is perpendicular to the partitions. Inclined helical-dark borders can be seen as a combination of these two cases.

First, let the case according to FIG. 3 considered. The resistance layer between two partition walls can alternatively be represented as a resistance which, depending on whether it is in the dark or in the healing zone, should have the resistance value D or H. Assuming η resistance paths between each pair of electrodes. so applies

DH
- parallel

n_- 1

parallel - ^ D + -i H. (1)

from which the following total resistance results:

_{R =} 2- DH [D + H)

^s " (1 + n) - (D ² + H ² + D- H) · ⁽

2D

parallel

2H

(III)

from which the following total resistance results:

2HD
π (H + D) '

(IV)

then, if one assumes a dark resistance, which is only 10 times as great as the light resistance. this difference is only a maximum of 7%. However, these measured value deviations are within the ι Reading accuracy with the galvanometers commonly used for cameras. Also this will Measured value deviation only cause such a misalignment of the lens that the object to be imaged is still in the depth of field. Is the

ίο The depth of field of the lens used is very small, the measured value deviation can be reduced by increasing η. For example, with 50 resistance paths between 2 electrodes each with a dark resistance, the result is 100 times

is greater than the light resistance, a measured value deviation less than 2%.

In the diagrams according to FIG. 5 and 6, 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, is plotted with the illuminance / as the ordinate. For reasons of symmetry, the ordinate (x = O) of the coordinate system used is relocated to the light-dark boundary p '. The illuminance of the dark part

2s of the image area is denoted by / _d , that of the bright part is denoted by l _b and the ratio of this to that is denoted by m; so it applies

In the case of FIG. 4. that the cut-off line is vertical and in the center of the photoresist surface runs, there is a parallel connection of a dark and of a healing resistance. Now applies here

For the following calculation example, η is assumed to be 19, ie 19 resistance paths between each pair of electrodes. If one assumes that the dark resistance D is 100 times as great as the light resistance H, then in the case of FIG. 3 a total resistance of 0.099 H, in the case of FIG. 4 a total resistance of 0.104 H. Depending on whether the cut-off line runs parallel or perpendicular to the partition walls, the difference in the total resistance is a maximum of 4.8%. Even l _h = ml .: with m> 1

If one takes on the basis of FIG. 5 and 6 indicate that the illuminance / on is outside the Origin of coordinates at χ and - χ points / (x) and / (- x) results for the case of FIG. 3

Kx) = U

and for Fig. 6th

I (x) = l _h - hl _d or because of (2) = / _d (m - / i)

with the one from FIG. 6 apparent meaning for Jj.

Let it be in FIG. 7 is considered, in which the light-dark boundary p 'of the image on Jer photoconductive cell lies parallel to the direction of the current flowing in the semiconductor body s. The total resistance of the photoresistor is thus a parallel connection of individual resistors r many successive small tire elements. which are accepted lying between the two electrodes.

If one considers the resistance elements r at the points χ and - χ as a unit, i.e. as a parallel connection! and if the total resistance is called the same! with sharp focus with r (—x, x) and the b < fuzzy setting with r '(-x, x), they result

f> o for these quantities from equations (1) to (^ after some intermediate calculation the expressions

r (-x, x)

(i)

(1 + m ^e )

(5 a)

I] I and for fuzzy setting

r '(- xx) / k \ (k

((/ ,, - / ι / J ¹ j KiI ₁ , - hl J ')

R '~ χ ? (- x. Ν)

- '''ΠΙ + / if + (' »- Ί) 'Ί (5b) From ^c ' ^cn equations (9) and (K)) it follows that the

ic sign of magnitude

IO

To calculate the change in resistance I.
a transition from a blurred to a sharp setting
the equations (5 a) and (5 b) are to be subtracted:

is determined by the sign of I. The sign of I depends on the quantity e . how

I _.! J (61 this is determined by the formula (8). So one has

r '(- x. x) r (~ x. .ν) differentiate between three cases:

_{0 (} j _er a) For e> 1 v becomes I <O: thus becomes - ^ r <- ^ -. (12)

1* ¹⁰ 1 1

\ - JJL [_ {\ + /,) <· + (»ι - / i) ¹ '- (1 + m'')]. (7) b) For e = 1 we get I = 0: so we get - ^ r = - ^ -. (13)

,. c) For <1 becomes I> 0; so becomes -57> - ^ - · (14) One can show that the sign of I is given by ««

the size 25 It therefore follows that the electric current in the

photoconductive cell in the focus trap

(1 - m ¹ "" ¹ ) (8) is maximum. if e> 1, that it is independent of the degree of focus if e = 1. and is determined. d ^a ß ^{he 'm-focus} case, a minimum is

In the example under consideration according to FIG. 7 results in 30 if e < 1.

therefore in sharp focus for the entire contradiction- With common photo resistors, e lies between (

Positional ^{trap ur>} d 1 - On the other hand, it must be pointed out that Zeller

with e> 1 with respect to the increasing with e i

J, m sensitivity are highly desirable. The size

r (—x. x) 35 ße e should be made significantly different from 1.

1 sheet of drawings

Claims (1)

1 797S75 Claim: Focusing device for optical instruments, with a photoelectric converter arranged in the image space of a focusing lens system, the resistance of which has an extreme value when the image is focused on the converter surface, characterized in that a semiconductor photoresistor is provided as the converter, which has a slope of e Φ 1 and its photoconductor layer is divided by partition walls into a plurality of sections, and that the partition walls are formed in a manner known per se as electrodes, the determination of the focus condition being obtained from the entirety of the section output signals.