DE102005047595A1 - Method for determining the sensitivity characteristic of a linear array of optoelectronic sensor elements involves repeatedly exposing the array with a light field, moving the array between two exposures and further processing - Google Patents

Abstract

Method for determining the sensitivity characteristic of a linear array of optoelectronic sensor elements involves repeatedly exposing the array with a light field which has an inhomogeneous distribution of radiation intensity over its surface, moving the array between two exposures in directions X or Y so that each sensor element is subjected to different radiation intensities, determining the measured values of the individual sensor elements for each exposure and determining a sensitivity characteristic for the whole array from the measured results. Preferred Features: The sensitivity characteristic is determined as a relative value of the sensitivity of the sensor elements to each other.

Description

The The invention relates to a method for determining the sensitivity characteristic a linear, in a direction X extended, or a flat, a plane X, Y spanning arrays of optoelectronic sensor elements.

at The optoelectronic image capture often CCD or CMOS cameras are used, the equipped with arrays of sensor elements, usually referred to as pixels are. Due to inaccuracies, these sensor arrays which have their causes in the manufacturing process, do not always have an exact uniformity, i. each sensor element has its own background noise and has a specific sensitivity curve.

at Lighting, in particular with UV radiation, results due to these deviations from sensor element to sensor element within an image taken with such an array in unwanted Way brighter or darker points of light, so-called "hot spots", which distort the image.

adversely affects the adulteration especially with regard to the further processing of the array obtained with the array Image information, especially if the intensity of each individual sensor element in the calculation of measured values received.

Around To remedy this it is known, a so-called "clear" picture from the measurement image subtract or the measurement image to divide by the "clear" image. To get such a "clear" picture, the respective array is illuminated with a light field, one over its area has homogeneous distribution of the radiation intensity.

however This method is also flawed, as an influence done by the illumination brightness. In other words, "hot spots" will not be correct recalculated, if during a measuring process the exposure at the location of these "hot spots" from exposure during shooting the "clear" picture on the same Body deviates.

Besides that is technically difficult, a light field with a homogeneous intensity distribution to create. Any existing inhomogeneities falsify the "clear" image, so that this correction method the danger of inaccuracies can only be remedied insufficiently.

From Based on this prior art, the invention has the object underlying, sensitivity characteristics for arrays of the aforementioned Identify type as the basis for better compensation Uniformity deviations serve and thus more accurate measurement results enable.

• – die Sensorelemente P_i,j des Arrays wiederholt mit einem Lichtfeld belichtet werden, das eine inhomogene Vertei lung der Strahlungsintensität über seine Fläche aufweist,
• – die Verteilung der Strahlungsintensität von Belichtung zu Belichtung konstant ist,
• – das Array zwischen zwei Belichtungen in Richtung X oder Y verschoben so wird, so daß jedes Sensorelement P_i,j von Belichtung zu Belichtung unterschiedlichen Strahlungsintensitäten ausgesetzt ist,
• – bei jeder Belichtung die Meßwerte der einzelnen Sensorelemente P_i,j erfaßt werden, und
• – aus den Meßergebnissen eine auf das gesamte Array bezogene Empfindlichkeitscharakteristik bestimmt wird.

The object of the invention is achieved with a method for determining the sensitivity characteristic for linear, in a direction X extended, or for planar, a plane X, Y spanning arrays of optoelectronic sensor elements P _{i, j} , in which

• The sensor elements P _{i, j of} the array are repeatedly exposed to a light field having an inhomogeneous distribution of the radiation intensity over its surface,
• The radiation intensity distribution is constant from exposure to exposure,
• The array is shifted between two exposures in the direction X or Y so that each sensor element P _{i, j} is exposed from exposure to exposure to different radiation intensities,
• - With each exposure, the measured values of the individual sensor elements P _{i, j are} detected, and
• - From the measurement results a relative to the entire array sensitivity characteristic is determined.

In a first embodiment of the invention, the sensitivity characteristic can be determined as a relative value of the sensitivity of the sensor elements P _{i, j} to each other.

• a) Belichten des Arrays während einer Zeitspanne Δt mit einer inhomogen über das Array verteilten Strahlungsintensität I(x,y) aus einer im wesentlichen senkrecht auf das Array treffenden Beleuchtungsrichtung,
• b) Erfassen und Speichern von Meßwerten m_i,j in Zuordnung zu jedem einzelnen Sensorelement P_i,j als Äquivalent für die Anzahl Photonen, die während der Zeitspanne Δt von dem jeweiligen Sensorelement P_i,j registriert werden,
• c) Lageveränderung des Arrays durch Verschieben senkrecht zur Beleuchtungsrichtung um einen Betrag, der dem ein- oder mehrfachen des Abstandes zweier benachbarter Sensorelemente P_i,j entspricht,
• d) erneutes Belichten des Arrays während derselben Zeitspanne Δt mit derselben inhomogen über das Array verteilten Strahlungsintensität I(x,y), wobei nun jedes Sensorelement P_i,j einer Strahlungsintensität I_i,j ausgesetzt ist, die von der im Verfahrensschritt a) auftreffenden Strahlungsintensität I(x,y) verschieden ist,
• e) Erfassen und Speichern von weiteren Meßwerten m_i,j analog zu Verfahrensschritt b),
• f) n-faches Wiederholen der Verfahrensschritte a) bis e), dabei jeweiliges Erfassen und Speichern der Meßwerte m_i,j analog zu Verfahrensschritt b) zusätzlich zu den bereits in Zuordnung zu den Sensorelementen P_i,j gespeicherten Meßwerten m_i,j,
• g) Ermitteln der über das Array verteilten Strahlungsintensität I(x,y) beginnend bei (0,0) ins Positive unter der Annahme einer auf das gesamte Array bezogenen mittleren Empfindlichkeit von E = 1 nach der Funktion
  
  – I(x,y) der Intensitätsverteilung in der von dem Array aufgespannten Ebene x,y, wobei der Ursprung (0,0) in einer der äußersten erfaßten Ecken des Arrays liegt, – der mit 1 beginnenden laufenden Nummer k der insgesamt n vorgenommenen Belichtungen, – m^k der Matrix von Meßwerten m^k _i,j = m^k(i,j), welche während der k-ten Belichtung erfaßt und gespeichert wurden, wobei m^k alle Meßwerte der Sensorelemente P_i,j von der k. Belichtung enthält, – p_k und q_k den Differenzen an Sensorelementabständen, um die das Array bei der k-ten Belichtung relativ zur ersten Belichtung in x- oder y-Richtung verschoben wird, – Δx und Δy den Abständen der regelmäßig im Array angeordneten Sensorelemente in x- und y-Richtung, und
• h) Ermitteln der Empfindlichkeitscharakteristik des Arrays in Form von Parameter a₁...a_n einer Kurve m über I durch Regression eines zugrunde gelegten Modells mit n Parametern für die Abhängigkeit m(I) an die Meßwerte jedes Sensorelementes P_i,j, wobei m der zu erwartende Meßwert bei einer Beleuchtung des betreffenden Sensorelements P_i,j mit der lokalen Intensität I ist.

For this purpose, the method according to the invention comprises the following method steps:

• a) exposing the array during a period of time Δt with a radiation intensity I (x, y) distributed in an inhomogeneous manner over the array from a direction of illumination which strikes the array substantially perpendicularly,
• b) acquiring and storing measured values m _{i, j} in association with each individual sensor element P _{i, j} as the equivalent of the number of photons registered by the respective sensor element P _{i, j} during the period Δt,
• c) change in position of the array by shifting perpendicular to the illumination direction by an amount which corresponds to one or more times the distance between two adjacent sensor elements P _{i, j} ,
• d) re-exposing the array during the same period of time Δt with the same inhomogeneously distributed radiation intensity I (x, y) across the array, each sensor element P _{i, j} now being exposed to a radiation intensity I _{i, j which} is incident from the one impinging in method step a) Radiation intensity I (x, y) is different,
• e) acquiring and storing further measured values m _{i, j in} analogy to method step b),
• f) n-times repetition of the method steps a) to e), thereby respective detecting and storing of the measured values m _{i, j} analogous to process step b) in addition to those already in association with the sensor elements P _{i, j} stored measured values m _{i, j,}
• g) determining the radiation intensity I (x, y) distributed over the array starting at (0,0) positive assuming an array-wide mean sensitivity of E = 1 after function
  
  - I (x, y) of the intensity distribution in the plane x, y spanned by the array, the origin (0,0) lying in one of the outermost detected corners of the array, - the starting number k of the total n starting with 1 Exposures, - m ^{k of} the matrix of measured values m ^k _{i, j} = m ^k (i, j), which were acquired and stored during the k th exposure, where m ^{k is} all measured values of the sensor elements P _{i, j} from the k. Exposure contains - p _k and q _k the differences in sensor element distances by which the array is shifted in the x-direction or y-direction relative to the first exposure at the k-th exposure, Δx and Δy the distances of the sensor elements arranged regularly in the array in the x and y directions, and
• h) Determining the sensitivity characteristic of the array in the form of parameters a ₁ ... a _{n of} a curve m over I by regression of an underlying model with n parameters for the dependence m (I) on the measured values of each sensor element P _{i, j} m is the expected measured value at a lighting of the relevant sensor element P _{i, j} with the local intensity I.

Ik i,j = I{{i – pk}·Δx, {j – qk}·Δy) und mk i,j = mk(i,j) In the simplest case, in which m (I) is assumed to be a linear m = E * I + m ₀ with an increase corresponding to the sensitivity E, a matrix of sensitivities E _{i, j is determined} for each sensor element P _{i, j} function

I k i, j = I {{i - p k } · Δx, {j - q k } · Δy) and m k i, j = m k (I, j)

Prefers The array should be between two or more consecutive Exposures are shifted in each case in the X or Y direction. It can the shift can be made alternately in the X and Y directions. The amounts, by which the shift takes place in each case can be constant.

As an alternative to the first embodiment, in a second variant, the sensitivity characteristic can be determined as an absolute value by first measuring an absolute value for the sensitivity of one of the sensor elements P _{i, j} and then relating the sensitivity of the remaining sensor elements P _{i, j} thereto.

As well Is it possible, the sensitivity with a uniform lighting integral over the Array to determine and thus the value of the averaged sensitivity E replaced by the measured mean.

By means of the thus obtained sensitivity characteristics associated with each array compensate for uniformity deviations, which is particularly important in the optoelectronic image acquisition by means of CCD or CMOS cameras, for example, in image measuring devices with high resolution for the UV range.

The inventive method will be explained below with reference to a linear, in one direction X extended array of optoelectronic sensor elements P _{i, j} , which consists for the sake of simplicity of only five sensor elements P _i (i = 1 ... 5). In the accompanying drawings show:

1 the simplified representation of an assumed inhomogeneous distribution of the radiation intensity in a light field striking an array of sensor elements,

2 the simplified representation of an assumed characteristic of the sensitivities of the sensor elements of an array X extended in one direction X,

3 an example of a series of measurements from four measurements, wherein the measured values at a first exposure to the light field from 1 were obtained to determine the sensitivity of the sensor elements 2 .

4 the simplified representation of the sensor elements taking into account their sensitivities 2 measured intensity values 1 and the intensity averaged for each position from the measured values,

5 the basis of the on the sensor elements 3 related sensitivity measured sensitivity of the array.

It is assumed that a light field is available for the exposure of the array, which has a light as in 1 shown, extending in the direction X has inhomogeneous distribution of the radiation intensity and with this distribution strikes the arrayed in the direction X sensor elements P _i (i = 1 ... 5).

Furthermore, assume a relative sensitivity for the sensor elements P _i (i = 1 ... 5) as in 2 shown, wherein the sensor element P _{1 should have} the relative sensitivity E (i) = 1. Based on this, the relative sensitivity E (i) = 0.9 applies to the sensor element P ₂ ; for the sensor element P _3, the relative sensitivity E (i) = 0.9; for the sensor element P _4, the relative sensitivity E (i) = 1.1; for the sensor element P _5, the relative sensitivity E (i) = 1.1.

At a first exposure during a period of time Δt with the light field corresponding to the in 1 shown intensity distribution, are at the sensor elements P _i (i = 1 ... 5) depending on their respective sensitivity (see. 2 ) Measured values can be tapped as in 3 , Picture a).

If the sensor elements P _i (i = 1... 5) have spacings of respectively .DELTA.x = 0.3 relative to one another, the array is shifted, for example, by the amount .DELTA.x = 0.3 before a second exposure, so that during the second exposure each of the sensor elements P _i (i = 1 ... 5) is exposed to a radiation intensity that is different from the radiation intensity incident on the first exposure.

In this way one obtains with a second, third and fourth exposure, wherein the exposures in each case during the same period of time .DELTA.t and after each shift of the array by the amount .DELTA.x = 0.3 is made, in measurement series m ¹ to m ⁴ in 3 Image b), c) and d), related to the individual sensor elements P _i (i = 1 ... 5) measured values.

In 3 p ₁ = 0 denote the position of the array in the first exposure or first measurement m ¹ , p ₂ = 0.6, the position of the array in the second exposure or second measurement m ² , etc.

The Exposures and the positions are shown in the pictures a), b), c) and d) each associated with the measurement results.

In the case of concrete, practical method application, the measurement results shown in pictures are superimposed on one another for the purpose of evaluation, so that physically identical locations in the arrays lie one above the other. The magnitude of the displacement can be determined physically by measurements of the displacement of the Sensors are detected in the measured value recording or by correlation algorithms from the measured values, if the sensor element fluctuations are not too large.

The measured values, who are in the same place are averaged. This mean corresponds to the mean intensity of the light field at this Digit multiplied by the mean sensitivity factor E. You get from it an intensity distribution, which of true intensity comes very close.

This is exemplary for the above-described series of measurements m ¹ to m ⁴ in 4 shown.

For each sensor element P _i in linear arrays or P _{i, j} in flat arrays, a count value and the associated value from the intensity distribution determined in a preceding step can be assigned from the individual measured values of the images m ^k . This assignment gives a series of pairs of count intensity values for each sensor element.

These Value pairs are described by a custom function. in the The simplest case is a linear function with 2 parameters. For complicated Sensor behavior is also to be considered nonlinearities or saturations. The overall result is parameter vectors or matrices with the Size of the array.

In In the application these parameter matrices are used to calculate the readings correct according to the attached model. Im simple In the case of the linear approach, the correction consists of the subtraction of the zero value and division of the measured value by the value of the sensor sensitivity. That it is only one subtraction and one division per sensor element required.

How to get an in 5 shown, on the sensor elements 3 related sensitivity characteristic of the array in the form of a sensitivity curve.

Claims (9)

Method for determining the sensitivity characteristic of a linear, in a direction X extended, or a planar, a plane X, Y spanning array of optoelectronic sensor elements P _{i, j} , in which - the sensor elements P _{i, j of} the array are repeatedly exposed to a light field having an inhomogeneous distribution of the radiation intensity over its surface, - the radiation intensity distribution is constant from exposure to exposure, - the array is shifted between two exposures in the X or Y direction so that each sensor element P _{i, j increases} from exposure to Exposure is exposed to different radiation intensities, - the measured values of the individual sensor elements P _{i, j are} detected at each exposure, and - a sensitivity characteristic based on the entire array is determined from the measurement results. Method according to Claim 1, in which the sensitivity characteristic is determined as the relative value of the sensitivity of the sensor elements P _{i, j} to one another. Method according to claim 1 or 2, comprising the following method steps: a) exposure of the array during a period Δt with a radiation intensity I (xy) distributed inhomogeneously over the array from an illumination direction substantially perpendicular to the array, b) acquisition and storage of measured values m _{i, j} in association with each individual sensor element P _{i, j} as the equivalent of the number of photons registered during the period Δt of the respective sensor element P _{i, j} , c) positional change of the array by shifting perpendicular to the illumination direction by an amount corresponding to the one or more times the distance of two adjacent sensor elements P _{i, j} corresponds to d) Re-exposure of the array during the same period of time .DELTA.t with the same inhomogeneous across the array distributed radiation intensity I (x, y), where now, each sensor element P _{i, j} is exposed to a radiation intensity I ^{k + 1} (i, j) which impinge on that in method step a) the radiation intensity I ^k (i, j) is different, e) capturing and storing other measured values m _{i, j} analogous to process step b), f) n-times repetition of the method steps a) to e), thereby respective detecting and storing the Measured values m _{i, j in} analogy to method step b), in addition to the measured values m _{i, j} , g already stored in association with the sensor elements P _{i, j} , determining the radiation intensity I (x, y) distributed over the array starting at (0, 0) positive under the Assuming an array-wide average sensitivity of E = 1 after function

- I (x, y) of the intensity distribution in the plane x, y spanned by the array, the origin (0,0) lying in one of the outermost detected corners of the array, - the starting number k of the total n starting with 1 Exposures, - m ^{k of} the matrix of measured values m ^k _{i, j} = m ^k (i, j), which were acquired and stored during the k th exposure, where m ^{k is} all measured values of the sensor elements P _{i, j} from the k. Exposure contains - p _k and q _k the differences in sensor element distances by which the array is shifted in the x-direction or y-direction relative to the first exposure at the k-th exposure, Δx and Δy the distances of the sensor elements arranged regularly in the array in x- and y-direction, and h) determining the sensitivity characteristic of the array in the form of parameters a ₁ ..a _{n of} a curve m over I by regression of an underlying model with n parameters for the dependence m (I) on the measured values each sensor element P _{i, j} , where m is the expected measured value at a lighting of the relevant sensor element P _{i, j} with the local intensity I. Method according to claim 3, wherein - m (I) is a linear m = E · I + m ₀ with a slope corresponding to the sensitivity E, and - a matrix of sensitivities E _{i, j is determined} for each sensor element P _{i, j} after the function

I k i, j = I ({i - p k } · Δx, {j - q k } · Δy) and m k i, j = m k (I, j). Method according to one of the preceding claims, in the array between successive exposures is moved in the X or Y direction. The method of claim 5, wherein the array is alternating in the X and Y direction, preferably by constant amounts, shifted becomes. Method according to Claim 1, in which the sensitivity characteristic is determined as an absolute value by first measuring an absolute value for the sensitivity E _{i, j of} one of the sensor elements P _{i, j,} and then relating the sensitivity E _{i, j of} the remaining sensor elements P _{i, j to} this becomes. Method according to Claim 7, in which first the average sensitivity E of the entire array is determined in a separate measurement, and the relative sensitivities E _{i, j of} all sensor elements P _{i, j} , the average of which 1 is multiplied by the mean sensitivity E. Use of the method steps according to the aforementioned claims determined sensitivity characteristics for the correction of uniformity deviations the use of arrays of optoelectronic sensor elements for image capture.