FR2719916A1 - Brightness measurement device for a camera and associated control method. - Google Patents

Abstract

An exposure value determination system is implemented which measures light reflected from an object whose image is to be recorded when a first trigger (S1) is actuated. The image of the object is recorded at a calculated exposure value when a second trigger (S2) is actuated. If the time between activation of the first step trigger and the second trigger exceeds a predetermined time limit, an exposure value determination circuit (40) measures the second light time and calculates a new exposure value. . If the predetermined time is exceeded again before the second trigger is actuated, the exposure value determining circuit will again measure the light and calculate a new exposure value.

Description

i Brightness measurement device for a camera and

  The present invention relates to an exposure control system for taking pictures. More particularly, the present invention relates to the determination of an exposure value to form an image of animated objects or objects whose

lighting conditions vary.

  When shooting, the exposure value is related to the amount of light reaching the imaging medium, such as film. A brightness measurement circuit measures the light reflected from an object to

determine the exposure value.

  In a conventional still image camera, when a user presses the shutter button to take a photograph, a

  first and second triggers are actuated.

  When the first trigger is actuated, an automatic distance measuring circuit measures the distance to the object, and the brightness measuring circuit measures the ambient brightness around the object. A microcomputer calculates an appropriate exposure value in accordance with the brightness value measured by the brightness measurement circuit. When the second trigger is actuated, the microcomputer controls the opening of the shutter and fixes any variable opening in accordance with the exposure value for

expose the film.

  A conventional brightness meter only measures light once per exposure, and the shutter speed is determined based on this measurement. Conditions may, however, vary between

  actuation of the first and second triggers.

  Object can move, camera

  may move, or the light may vary.

  The user cannot take a photograph with the correct exposure when the object is moving during the time interval before the operation of the second trigger because the exposure value has been determined in accordance with the measured brightness value before moving

of the object.

  Therefore, with a conventional brightness meter, the user cannot obtain a photograph having a correct exposure because the shutter is operated in accordance with the exposure value calculated using the previously measured brightness value and the fact that the ambient light around the object may vary before the second

trigger is activated.

  One of the aims of the present invention is to propose a device for measuring brightness for a camera and an associated control method which solves the problems and the drawbacks.

conventional devices.

  The present invention relates to a brightness measurement device for an image recorder such as a camera. A user operates first and second triggers. Ambient brightness around an object is measured when the first shutter button is pressed, then. the ambient brightness is successively measured around the object during each predetermined time interval when the second trigger is not actuated for a predetermined time after the actuation of the first trigger. An exposure value is then calculated according to the degree of brightness finally measured when the

second trigger is activated.

  The method according to the invention for measuring an ambient brightness for a camera comprises the steps consisting in measuring the ambient brightness around an object when the first trigger is actuated; determining whether a predetermined time has elapsed after the first trigger has been actuated, while the second trigger has not elapsed successively measuring the ambient brightness around the object during each predetermined time interval depending on the result determination: and to calculate an exposure value of the camera in accordance with the degree of brightness finally measured before actuation of the

second trigger.

  It will be understood that both the description

  general above and detailed description which

  will follow are given by way of example and explanation only and are not limiting

  of the invention, as claimed.

  The accompanying drawings which are incorporated into and

  constitute a part of this description,

  represent an embodiment of the invention and

  together with the description, serve to

  explain the principles of the invention. In the Figures: Figure 1 is a block diagram showing a brightness measuring device according to a preferred embodiment of the present invention; Fig. 2 is a detailed circuit diagram showing a brightness measuring circuit of the brightness measuring device in accordance with the preferred embodiment of the present invention; o Figure 3 is a graph showing the charging time of a capacitor of the brightness measurement circuit according to the preferred embodiment of the present invention; Figure 4 is a flow diagram of the steps of a method of measuring brightness in accordance with the preferred embodiment of the present invention; and Figure 5 is a time diagram showing the operation of the brightness measurement device according to the mode of

  realization of the present invention.

  An exposure value determination system measures the light reflected from an object whose image is to be recorded when a first shutter button is pressed. The object image is recorded at the calculated exposure value when a second shutter button is pressed. If the time between actuation of the first trigger and the second trigger exceeds a predetermined time limit, the exposure value determining circuit measures the light a second time and calculates a new exposure value. If the predetermined time is again exceeded before the second trigger is actuated, the exposure value determining circuit will measure the light again and calculate a second and new exposure value. Thus, if the object whose image is to be recorded moves between the actuation of the first and second 2ut7eLu cheurs, a new exposure value will be used to record the image. In addition, if the light conditions vary between the actuation of the first and second triggers, the exposure value determination circuit will determine a new exposure value based on the news.

light conditions.

  As shown in Figure 1, the brightness measurement device according to an embodiment of the present invention is controlled by a microcomputer 40. The microcomputer 40 is connected to a first trigger S1, to a second trigger S2, to a circuit automatic distance measurement 10, to a brightness measurement circuit, to a film sensitivity sensor 30, and

  across the triggers S1 and S2.

  The automatic distance measuring circuit 10 measures the distance between the camera and an object to be photographed. The brightness measurement circuit 20 measures the ambient brightness around the object. The film sensitivity sensor 30 measures the sensitivity of the film. The microcomputer also controls a shutter 50, a motor actuator, and an information display 70. The circuit

  Brightness measurement can take many forms.

  For example, an incident light measurement device or a reflected light measurement device, such as a point, average or matrix measurement device, can be used. Figure 2 shows a diagram of an embodiment of a brightness measurement circuit which can be used in accordance with the present invention. The brightness measurement circuit includes a photoresist light receiving element Cds. The ohmic value of the light receiving element Cds varies in accordance with the amount of light striking the light receiving element Cds. The microcomputer sends signals to the brightness measurement circuit on a terminal referenced OUT and receives signals on a terminal

referenced IN.

  The brightness measurement circuit includes a comparator JC1 and three bipolar transistors Tl, T2 and T3. Each transistor T1, T2 and

  T3 has a base, a collector and a transmitter.

  A voltage source Vcc and a capacitor Cl are connected in parallel between a ground voltage and the emitter of transistor Tl. The emitter of transistor T2 is connected to the base of transistor T1, and the emitter of transistor T2 is connected to ground voltage. The base of transistor T2 receives a pulse

  of the microcomputer 40 on the OUT terminal.

  The collector of transistor Tl is connected to a resistor R1, to the light receiving element Cds, to a variable resistor VR1, and to a resistor R4. The resistors Ri and R2 constitute a voltage divider between the collector of the transistor T1 and the ground voltage. The output of the voltage divider is connected to the inverting input terminal of the comparator IC1. The light receiving element Cds and the variable resistor VR1 are connected in parallel between the collector of the transistor T1 and a resistor R3. The resistor R3 connects the light receiving element Cds to the non-inverting terminal of the comparator IC1 and to the ground voltage by

  through a capacitor C2.

  The collector of the bipolar transistor T3 is connected to the non-inverting terminal of the comparator IC1, and the emitter of the transistor T3 is connected to ground. The base of transistor T3 is connected to the base

  of transistor T1 and to the collector of transistor T2.

  The output of comparator IC1 is connected to resistor R4 and to microprocessor 40 by

through the IN terminal.

  The operation of the camera according to an embodiment of the present invention will be explained with reference to

attached drawings.

  As shown in Figure 4, the power supply is applied at the start (STEP 100). The microcomputer 40 initializes all the operating circuits, variable, and an interior timer (STEP 110) and then determines whether the first trigger S1 is actuated (STEP). When a user presses the shutter button to take a photograph of an object, the first trigger S1 is actuated and a corresponding electrical signal is applied to the

microcomputer 40.

  The microcomputer 40 checks the voltage condition and the charge condition of a flash, not shown (STEP 130) and measures the sensitivity of the film loaded in the camera. The film sensitivity sensor 30 detects the sensitivity of the film and sends an electrical signal corresponding to the sensitivity of the film to the microcomputer 40

(STEP 140).

  The microcomputer 40 (STEP 140) carries the control signal OUT from the high level to the low level and controls the brightness measurement circuit 20 to measure the ambient brightness around the object after the end of the above step (STEP 150). Then, the microcomputer 40 counts the time until the high level signal is delivered by the brightness measurement circuit 20 in

  activating the interior timer.

  The high level control signal OUT delivered by the microcomputer 40 is applied to the base terminal of the transistor T2 of the measurement circuit

  of brightness 20, and the transistor T2 conducts.

  When the transistor T2 is conductive, the electrical potential on the collector of the transistor T2 is lowered, and the low signal is applied to the bases of the transistors Tl and T3. The transistor Tl becomes

  conductor, and transistor T3 is blocked.

  The brightness measurement circuit 20 measures the ambient brightness around the object when the transistor T1 conducts and when the power is applied to the light receiving element Cds. Resistors Rl and R2 divide the voltage applied to the brightness measurement circuit and thus determine a reference voltage

  applied to the inverting terminal of comparator IC1.

  The voltage applied via the transistor T1 is charged by the capacitor C2 in accordance with the ohmic value of the light receiving element Cds. The ohmic value of the Cds light-receiving element varies according to the ambient light around the object. The charging speed of the capacitor C2 varies according to the ohmic value of the element of

Cds light reception.

  When the voltage applied to the non-inverting terminal of comparator IC1 is greater than the voltage applied to the inverting terminal of comparator IC1, due to an increase in the charging voltage of capacitor C2, the output of comparator IC1 goes from one low level to high level. This output signal is then sent to

microcomputer 40 (STEP 160).

  The microcomputer 40 reads the counting of the interior timer and determines the brightness condition of the object when the high level signal appears on the IN terminal of the brightness measurement circuit 20. The microcomputer 40 determines that the brightness of the object is too high when the read account is below a first predetermined value and is too low when the read account is greater than a second predetermined value. The microcomputer 40 determines that the brightness is high when the count is less than 8 and low when the count is more than 80. Figure 3 shows the charge speed of the capacitor C2 in different light conditions A, B and C. The curve A indicates high luminosity, while curve C indicates low luminosity in accordance with the charging speed of the capacitor as modified by the ambient luminosity around the object. Curve B designates a brightness between λ and C. If the brightness is too low (STEP 170), the microcomputer 40 alerts the user by controlling the display of information 70 (STEP to STEP 190). The microcomputer 40 also sets the shutter speed to the minimum or the lowest speed. If the brightness is too high (STEP 200), the microcomputer 40 fixes the shutter speed at maximum or at the speed

faster (STEP 210).

  The microcomputer 40 determines the shutter speed (STEP 220) by controlling the automatic exposure program in accordance with the metering count when the metering count is between the first and second predetermined values, and therefore, the brightness is neither too low

neither too high.

  The microcomputer 40 measures the distance between the camera and the object by controlling the automatic distance measuring circuit 10 after determining the speed

  shutter depending on the brightness of the object.

  The microcomputer 40 then performs an automatic distance calculation (STEP 230 to STEP 240)

  according to the distance measured.

  o10 After completion of the automatic distance calculation, the microcomputer 40 determines (STEP 250) if the second trigger S2 is actuated and determines (STEP 260) when a predetermined time elapses during which the second trigger S2 is not actuated . If the predetermined time has elapsed, the microcomputer 40 returns to STEP 150. During STEP 150, the microcomputer 40 controls the measurement circuit of

  brightness 20 to measure the brightness.

  Thus, if the object changes position or if the camera changes position, the microcomputer 40 calculates a new exposure value. If the lighting varies between the first and second trips, a new exposure value is also calculated. The microcomputer 40 calculates exposure values as a function of the brightness value measured just before the actuation of the second trigger S2 after successive measurement of the ambient brightness around the object when the second trigger S2 is not

  operated for the predetermined time.

  The microcomputer 40 takes the photograph of the object when the second step trigger is actuated (STEP 270). The microcomputer controls the shutter 50 in accordance with the value

  exposure determined during STEP 220.

  After determined exposure during STEP 220.

  After exposure of a section of film, the microcomputer 40 advances the film by actuating the actuating member of the motor 60 and

  ends all steps (STEP 280 to STEP 290).

  The second trigger is activated by pressing the shutter button and measuring the ambient light around the object. The ambient brightness around the object is successively measured when the second trigger is not actuated for the predetermined time, thereby obtaining a photograph having an appropriate exposure in accordance with the variation in brightness of the object as a result of a variation. the position of the object or the position of the camera before actuation of the second shutter S2. The present invention has been described in

  About a still image camera.

  However, the present invention can be used in other devices which measure the brightness around an animated object or in varying light conditions. For example, the present invention can be used in an optical measurement device. In addition, the present invention presents a

  low manufacturing cost and high reliability.

  Although only a preferred embodiment and selected variants of the invention have been disclosed and described, it is evident that other embodiments and variants of the invention are possible within

  of the scope of the appended claims.

R E V E N D I CATIONS

  1 - Brightness measurement device for a photographic camera, characterized in that it comprises: first and second triggers (S1, S2) actuated by a user, means (20) for successively measuring an ambient brightness around a object at each predetermined time interval when said second trigger is not actuated for a predetermined time after said first trigger has been actuated, and means (40) for calculating an exposure value in accordance with a finally measured degree of brightness when said second

trigger is activated.

  2 - Device according to claim 1, characterized in that it further comprises: means (10) for measuring a distance between the camera and the object, means (30) for detecting the sensitivity of a film mounted on the camera, means (50) for exposing the film mounted on the camera, means (60) for transferring exposed film, means (70) for notifying the user that said degree of brightness measured is too low when the ambient light around

of the object is weak.

  3 - Device according to claim 2, characterized in that said detection means (30) detect the sensitivity of the film and said measuring means (10) measure the distance when the first trigger (Si) is actuated, that means are provided for fixing a shutter speed at minimum when the ambient brightness around the object is too low and at maximum when the ambient brightness around the object is too high, that said calculation means (40) determine the value d exposure according to a value measured from the object and the degree of brightness measured when the ambient brightness is neither too low nor too high, and means are further provided for photographing the object by actuating a shutter (50) according to a determined exposure value when said second trigger (S2) is actuated. 4 - Device according to claim 3, characterized in that said means (20) for successively measuring an ambient brightness comprise means for measuring an elapsed time and determining whether the ambient brightness around the object is too low or too high depending of

time elapsed.

  5 - Method for measuring a brightness for a camera having first and selond triggers (Si, S2), characterized in that it comprises the steps consisting in: measuring an ambient brightness around an object when the first trigger (S1 ) is actuated, determine if a predetermined time has elapsed after the first trigger (S1) has been actuated, while the second trigger (S2) has not been actuated, successively measure the ambient brightness around the object to be any predetermined time interval as a function of the result of said determination, and calculating an exposure value in accordance with a degree of brightness finally measured before actuation of the second trigger. 6 - Method according to claim 5, characterized in that it comprises the steps of detecting the sensitivity of a film mounted on the camera and measuring a distance from the object when the first trigger is actuated, determining a speed shutter to a minimum when the ambient brightness around the object is too low and to the maximum when the brightness around the object is too high and the exposure value according to the degree of brightness measured when the brightness is not neither too low nor too high, and take a photograph at the value

determined exposure.

  7 - Device for determining the exposure value for an image recorder, comprising means for recording an image of an object, a light measurement device for measuring the light on the object, means for actuating the image recording means, and means for actuating the light measuring device, characterized in that the means for actuating the light measuring device actuate the latter at predetermined time intervals for measuring the light on the object, and the light measuring device can be actuated a plurality of times between the instant when the light measuring device is actuated for the first time to measure the light on the object and when the recording means d are

  operated to save the object image.

  8 - Device according to claim 7, characterized in that the means for recording an image are a light-sensitive support and the means for actuating the recording means

image are a shutter.

  9 - Device according to claim 8, characterized in that the shutter speed is determined in accordance with the last light measurement carried out by the device for measuring

  light before actuation of the shutter.

  10 - Device according to claim 8, characterized in that the shutter speed is determined according to an average of the light measurements made between the instant when the light measurement device is actuated for the first time to 2Q record the image of the object and time o

the shutter is activated.