Abstract: A light measurement device includes a photoelectric converter for receiving light from an object to be measured and converting the light to a measuring electrical energy in accordance with the intensity of the light, an offset amount measurement device for measuring the offset amount of the photoelectric converter, temperature measurement device for measuring a first temperature of the photoelectric converter when measuring the offset amount, and a second temperature of the photoelectric converter when measuring the light from the object, a memory device for storing a characteristic of the photoelectric converter with respect to temperature, correction amount calculation device for calculating a correction amount based on the characteristic, the first temperature, and the second temperature, and a light intensity calculation device for calculating a light intensity based on the measuring electrical energy, the offset amount, and the correction amount.

Abstract: A method of measuring light levels, particularly the measurement of exterior light levels in the zone of approach to a tunnel in which vehicles are moving, and from which exterior light levels the minimum light level to be programed inside the tunnel can be calculated, by means of a photo-sensitive cell installed in a recessed tube in order to sense light levels contained within a cone defined by the center of the cell, which constitutes the apex of the cone, and by at least the open end of the tube. The method includes limiting the quantity of light rays entering into tube via the cone, and a device is provided for implementing the method.

Abstract: A subtractive measurement technique in which a CRT display light intensity output is biased above a noise range of a low cost sensor by turning on one or more electron beam guns to produce light levels above the noise range. A bias value is measured. Electron beam guns for colors other than the color being measured are turned on. The gun for the color to be measured is then incremented through its operating range and measured light intensity values at each step are obtained using the sensor. The bias value is subtracted from the measured values to obtain the actual light intensity values for the color primary control value to luminance transfer curve being characterized.

Abstract: A light receiver converts light to be measured into an electrical signal, which during a measuring interval is integrated into a measuring signal. In a first time segment a first mean value signal is formed, corresponding to the mean value of the measuring signal over the first time segment. In a second time segment of equal duration, a second mean value signal is formed within the same measuring interval, corresponding to the mean value of the measuring signal over the second time segment. Finally a difference signal is formed corresponding to the difference of the two mean value signals and constituting a measure of the light received. The difference signal is passed on for further signal processing.

Abstract: A photometric apparatus includes a charge accumulation type photoelectric transfer unit, a real time monitor unit for monitoring the intensity of incident light in a charge accumulation time of the photoelectric transfer unit, a counting unit for counting a time required for the output of the real time monitor unit to reach a predetermined threshold value, and a memory unit for storing the counted value of the counting unit. A brightness of an object is calculated using the previous counted value stored in the memory unit, the present counted value, the present charge accumulation time, and the A/D converted values of the individual pixel signals obtained on the basis of the present charge accumulation of the photoelectric transfer unit.

Abstract: A laser power measuring jig is adapted for use in measuring a laser power of a laser beam which is emitted from an optical head of an optical disk unit which records and/or reproduces information on and/or from a medium. The laser power measuring jig supplies power information related to the laser power of the laser beam to a power meter which measures the laser power. The laser power measuring jig includes a main jig body which has a size and a shape which are approximately the same as those of the medium, and this main jig body is loaded into the optical disk unit. The laser power measuring jig also includes a mechanism for outputting the power information outside the optical disk unit and supplying the laser power information to the power meter when the main jig body is loaded into the optical disk unit.

Abstract: A twilight sensor for controlling the headlamps of vehicles employs a photodiode having a predetermined spectral response and a diffuser which also has a predetermined spectral response, the spectral responses being combined to achieve an overall spectral response which corresponds to that of the human eye. With this arrangement, the headlamps of a motor vehicle are turned on and off under different ambient light conditions, in a manner which corresponds to the perception of the driver. In addition, the invention employs a control system which turns the lights on rapidly after the vehicle enters a tunnel.

Abstract: A photometer of a camera employs a MOS-type CPU having an open drain type transistor whose drain serving as an output terminal, a CMOS input terminal, and an input gate inverting an input with a value exceeding a predetermined potential. In the photometer, the transistor is set in a high-impedance state, thereby releasing a charge accumulated in a parasitic capacitor of a photodiode for generating a photoelectric current corresponding to an incident light amount. Thus, a voltage at the CPU terminal gradually increases, and, when it exceeds a threshold of a Schmitt inverter, the output of the Schmitt inverter is inverted. The time period from when the transistor is set in the high-impedance state to when the output of the Schmitt inverter is inverted is proportional to the photoelectric current. By measuring the time period, the photoelectric current value or luminance is detected.

Abstract: A portable traffic signal photometer is capable of rapidly and effectively measuring the intensity of light emanating from a traffic signal light. The photometer includes a housing member with a circular front opening for engagement with the lens of a traffic signal light. The photometer is pressed against the lens, and the light emanating from the traffic signal light is directed through an optical filter located inside the housing, which permits light from the traffic signal light emanating at a predetermined downward angle to pass through it. The light from the filter is directed onto a light-sensitive device, which produces an output signal proportional to the intensity of the light emitted by the traffic signal lens.

Abstract: A system and method for monitoring the output of a laser is provided in which the output of a photodiode disposed in the cavity of the laser is used to provide a correlated indication of the laser power. The photodiode is disposed out of the laser beam to view the extraneous light generated in the laser cavity whose intensity has been found to be a direct correlation of the laser beam output power level. Further, the system provides means for monitoring the phase of the laser output beam relative to a modulated control signal through the photodiode monitor.

Abstract: A light beam is chopped and split into two beams. Intensity of a first beam is changed by using, e.g., an electrooptic material in accordance of a change in a signal to be detected, such as a voltage in an electronic device, and detected by a first photodetector. On the other hand, a second beam is detected by a second photodetector without being subjected to the intensity change. At least one of the outputs of the first and second photodetectors are adjusted so that they become equal to each other when no signal is applied to the light intensity changing means. The difference of the thus adjusted output signals is detected by, e.g., a lock-in amplifier in a narrow band at the chopping frequency.

Abstract: A photoluminescence sensor for detecting a photoluminescent light from a toluminescent material is disclosed. In a preferred embodiment the photoluminescence sensor comprises: a source of light; a concave mirror having at least one perforation for passing the source light through the at least one perforation; an optical waveguide having proximal and distal ends with the photoluminescent material being disposed at the distal end; an objective for directing the source light into the proximal end of the waveguide; an objective for receiving photoluminescent light and for focusing the photoluminescent light onto the perforated concave mirror; a liquid filter for passing the photoluminescent light reflected from the perforated concave mirror to a detector to detect the photoluminescent light.

Abstract: A pyroelectric detector is provided to measure the energy of pulsed light. To measure the power of continuous wave light (CW light), the pyroelectric detector is releasably attached to optical sampling apparatus which divides the CW light into pulsed light. Means are included to inform processor circuitry in the read-out electronics that the detector is attached to the optical chopper. The processor circuitry then calculates the power of the CW light given the energy of the pulsed light by the pyroelectric detector and the exposure time which is fixed by a crystal clock.

Abstract: Light photon detecting apparatus (10) comprises a photomultiplier tube (36) in a two-part housing (14), the underside (22) of which is open. The underside (22) is fitted with an elastomeric sealing ring (28) to make an airtight and hence light-tight seal with a surface (12). Two spring-loaded part (16, 18) of the housing (14) act as a pump to produce a partial vacuum which opens a shutter (60) to expose the photomultiplier tube (36).

Abstract: A combined optical power and noise meter for measuring both optical power and optical noise includes a detector housing and a photodiode which is disposed in the detector housing. The resistence of the photodiode changes continually in response to the application of optical power thereon. A current to voltage converter applies a bias voltage to the photodiode and transforms an output current therefrom in order to obtain a power voltage which is proportional to the optical power. A first voltage meter is electrically coupled to the current to voltage converter in order to display the power voltage. A noise measuring circuit measures noise in the output voltage of the current to voltage converter and processes the output voltage in order to obtain a noise voltage which is proportional to the optical noise. A divider divides the noise voltage by the power voltage in order to obtain an optical noise to optical power voltage which is proportional to the ratio of optical noise to optical power.

Abstract: In a method of measuring the flash duration of a flash unit, the momentary light intensity is converted into an electrical signal that is determined and stored, and that is delayed by at least that time interval that exists between the beginning of the flash curve and the maximum thereof. The stored maximum is compared with the delayed signal, with the flash duration being determined from the signal obtained thereby. As a result, at the maximum of the intensity/flash duration curve, the information concerning the rising leg of this curve is still available. The apparatus comprises a sensing element for the flash intensity, with the sensing element being followed by a time-lag element and a peak detector, the outputs of which are connected to a comparator, the output of which is connected to a time-interval measuring circuit.

Abstract: A device for measuring color temperature, having at least one measuring diffuser and at least one indicator for the color temperature, as well as operating elements for turning the device on and off, for resetting the measured values, and for initiating a measurement. To be able to easily and rapidly set or adjust the color temperature, the device has at least one adjustment control element for at least one remote control transmitter for adjusting the color temperature of a light source. A photographer can set or adjust the color temperature in a remotely controlled manner via the adjustment element and the remote control transmitter.

Abstract: The new compound diallyl selenide, or 3.3'-bis(1-propene)selenide, having the formula(CH.sub.2 .dbd.CH--CH.sub.2).sub.2 Se.

Abstract: The present invention is an apparatus and an associated method for determining the dominant scene illuminant which apparatus is more reliable and accurate than previous illuminant detectors. Mixed illuminant detection has been added to the choice of illuminant categories, to take care of the cases where one illuminant is not dominant and color correction is best handled by printing algorithms. A new boundary condition has been devised to eliminate detection errors seen when fluorescent illumination mixes with certain quantities of daylight causes a tungsten reading. The apparatus incorporates a thresholding technique to improve over existing art. The apparatus is comprised of a means for converting illuminate light into corresponding electrical signals. The electrical signals are then directed to a log amplifier wherein they are compressed to approximately form a signal having one term equal to the log of the DC term plus a ratio of the dominant AC components to the DC components.

Abstract: A laser warning receiver or detector for the detection of extraneous light not emanating from the laser to be detected. Thus, extraneous light is differentiated from incoming laser light. The laser radiation coming from a laser to be detected is detected on the basis of its time coherent radiation. Simultaneously, the wave length as well as the pulse form or shape are analyzed in spite of any presence of incoherent radiation components.

Abstract: Laser power is delivered at normal intensities or powers to the end of a laser guide such as an articulated arm from a laser console where the beam enters an attached attenuating unit. Inside the attenuator, the beam encounters a dual wavelength beam splitter that transmits an attenuated beam at, for example, 20 times reduction of power. The attenuated beam may be delivered to an area of medical treatment. The beam splitter is also reflective and reflects the remaining power to a power sensor where the thermal energy is absorbed and measured. The power of the attenuated beam is related to the power of the measured reflected beam. The beam splitter is coated to allow maximum transmission of a visible guide beam that always travels coaxially to the operating laser energy being attenuated. A signal from the power sensor is provided to a preferably battery powered read-out device that provides the surgeon with real time accurate readings of delivered power.

Abstract: An improved light measuring device for measuring flash light is provided, and includes a sensor for measuring the quantity of flash light to produce a flash measuring signal. A starting signal is produced upon a detection of the start of the flash emission to enable the sensor for measuring the quantity of flash light. The light measuring device is kept in an operative condition for a predetermined time period after it is manually turned on so that it is available to detect the start of any flash emission during that time period. A counter can be utilized to set the predetermined time period, and the counter can be reset in response to any light measuring operation.

Abstract: A photometric circuit comprises a switching circuit which causes a photo conductive element which responds to the luminance of an object of measurement to be selectively connected with an oscillation circuit so that the photo conductive element functions as a frequency determining element and a frequency ratio calculating circuit which calculates a ratio of the oscillation frequency obtained when the photo conductive element is in connection with the oscillation circuit to the oscillation frequency obtained when the photo conductive element is not connected with the oscillation circuit. Thus, it is possible to reduce the required capacitance of the capacitor operating as a frequency determining element, lower the voltage required to drive the circuit and yet achieve high measuring accuracy.

Abstract: In an optical signal generating apparatus of this invention, output light from a light source driven by a light source driver is externally output from an optical receptacle through a variable optical attenuator. An output monitor unit can be detachably connected to the optical receptacle through an optical connector. When an absolute level of the output light is to be set, a CPU compares a set value from an output level setting unit with an output value from the output monitor unit and controls the variable optical attenuator and the light source driver so that a comparison error becomes zero. The CPU maintains the above control of the variable optical adjuster and the light source driver so that the light is output to be the desired level set by the output level setting unit even after the output monitor unit is disconnected from the optical receptacle including the timing at which the comparison error becomes zero.

Abstract: From a data transmission device to a camera system including a camera and electronic flash device, operation instruction data such as set or calculated exposure control data, shutter release data instructing the operation of the camera system, flash emission testing data and the like are serially transmitted in the form of infrared light signal. When the reception of these data is completed, the camera starts its operation to control given operation of the camera system such as the exposure initiation of the camera, test light emission of the electronic flash device and the like in accordance with received data.

Abstract: A laser power and energy meter has a target surface provided with at least one pin hole for passing a small portion of the laser beam impinging thereon and further has a light detector positioned in the path of the laser light passing through the pin hole. The target surface is moved horizontally and vertically to cause the laser beam to scan the target surface and the center of the laser beam is aligned with the center of the target surface.

Abstract: An apparatus for monitoring the light energy power level of a beam of light, includes beamsplitters on the axis of the light beam for directing portions of the light beam in first and second paths respectively orthogonal to the axis while transmitting the remaining portion of the light beam therethrough. First and second photodetectors having similar spectral responsivity intercept the first and second light beam portions respectively, and detect and signal the light energy power level of the respective first and second light beam portions. Analysis circuitry coupled to the first and second photodetectors and responsive to the output signals therefrom to calculate the total light energy power level of the light beam. The apparatus resolves linearly polarized light into two orthogonal components while permitting most of the light to exit. This exit beam can be analyzed in terms of the incident beam or monitored for measurement or control purposes.

Abstract: From a data transmission device to a camera system including a camera and electronic flash device, operation instruction data such as set or calculated exposure control data, shutter release data instructing the operation of the camera system, flash emission testing data and the like are serially transmitted in the form of infrared light signal. When the reception of these data is completed, the camera starts its operation to control given operation of the camera system such as the exposure initiation of the camera, test light emission of the electronic flash device and the like in accordance with received data.

Abstract: A photometer tube for a microscope contains several switchable reflectors (5, 11). By the appropriate use of these, the ray path (c) for the back-illuminated measuring field stop (7) is reflected out of the photometry ray path (b) and is conducted to a second image inlet (32) of a beam splitter prism or prism slide. Various switchable elements (mirrors 5 and 11; stop 4; prism 2/22) in the tube are electrically driven and are centrally actuated from a control unit.

Abstract: An acousto-optic frequency shifter having a long interaction region is used as an optical analyzer. A variable frequency signal generator is used to drive an acoustic transducer to launch an acoustic wave in contact with an optical fiber. The acoustic frequency is varied over a known range to generate acoustic waves having known wavelengths. An optical signal having an unknown optical wavelength is introduced into one end of the optical fiber in a first polarization mode. The effect of the acoustic wave on the optical signal is to cause coupling of the optical signal from the first polarization mode to a second orthogonal polarization mode. The amount of the coupling is dependent upon the phase-matching between the acoustic wavelength and the optical beat length. The coupling between the polarization modes is maximum when the acoustic wavelength is equal to the optical beat length.

Abstract: An atmospheric optical calibration system is provided to compare actual atmospheric optical conditions to standard atmospheric optical conditions on the basis of aerosol optical depth, relative air mass, and diffuse horizontal skylight to global horizontal photon flux ratio. An indicator can show the extent to which the actual conditions vary from standard conditions. Aerosol scattering and absorption properties, diffuse horizontal skylight to global horizontal photon flux ratio, and precipitable water vapor determined on a real-time basis for optical and pressure measurements are also used to generate a computer spectral model and for correcting actual performance response of a photovoltaic device to standard atmospheric optical condition response on a real-time basis as the device is being tested in actual outdoor conditions.

Abstract: An instrument for measuring radiation including a probe for collecting light. The probe includes a diffusive reflective surface at the forward end of the probe and an aperture block containing a pair of apertures to direct light reflected from the reflector to an optical waveguide and then to a light sensor and display unit.

Abstract: A luminometer construction including means for supplying samples contained in respective cuvettes 20, in succession to be examined, the cuvettes being introduced into a carrier 10 having a cuvette gripper 19, the carrier 10 defining an examination chamber 21 into which the cuvettes are inserted, the carrier 10 being rotatable between a loading and a test position in which the examination chamber 21 is adjacent to a photo-multiplier device 25, whereby testing of a sample in a curvette 20 can be carried out, the examination chamber 21 having an edge in a plane inclined to the axis 11 of rotation of the carrier 10, and the photo-multiplier device 25 being arranged with its optical axis 26 perpendicular to such plane.

Abstract: A light source degradation detecting apparatus for a laser device monitors drive currents and output laser beams. The degradation detecting apparatus includes drivers for supplying three drive signals to the laser, a photodiode for detecting laser outputs produced in response to the drive signals, a memory for storing measured values and the drive signals, and a discriminator for calculating ratio P from storage data (laser outputs W.sub.n, W.sub.n+1, and W.sub.n+2 produced in response to drive currents I.sub.n, I.sub.n+1, and I.sub.n+2) as follows:P={(W.sub.n+2 -W.sub.n)/(I.sub.n+2 -I.sub.n)}/{(W.sub.n+1 -W.sub.n)/(I.sub.n+1 -I.sub.n)}and for discriminating that the laser has reached an end of its life when the ratio P falls outside a predetermined range.

Abstract: A projection aligner wherein light from a light source is passed through a mask so as to focus an image of a pattern of the mask on a wafer, characterized in that at least one sensor for monitoring a luminosity and a distribution thereof is disposed in an optical path between the light source and the mask, whereby a luminosity and a distribution thereof on the wafer can be controlled to proper values. The projection aligner is effective for application to minute processing technologies for the production of semiconductor devices, etc.

Abstract: A high-sensitivity photometer/radiometer uses a single-pixel charge-injection-device (CID) light-sensitive sensor, and associated circuitry for: non-destructively reading-out the sensor charge level; for summing signals proportional to the charge level signal; and for then displaying the level. With cooling of the CID sensor, the sensitivity of the photometer can be sufficient to read incident flux levels on the order of several photons per second.

Abstract: An apparatus and method for measuring solar radiation received in a vegetative canopy. A multiplicity of sensors selectively generates electrical signals in response to impinging photosynthetically active radiation in sunlight. Each sensor is attached to a plant within the canopy and is electrically connected to a separate port in a junction box having a multiplicity of ports. Each port is connected to an operational amplifier. Each amplifier amplifies the signals generated by the sensors. Each amplifier is connected to an analog-to-digital convertor which digitizes each signal. A computer is connected to the convertors and accumulates and stores solar radiation data. A data output device such as a printer is connected to the computer and displays the data.

Abstract: An exposure meter includes a display device 16 on which is displayed a histogram 17. The x axis is divided into cells representing ranges of possible luminance values. The height (i.e., y value) of each cell of the histogram represents the number of individual areas within the scene to be photographed whose luminance falls within that cell. Alongside the histogram, a bar 18 is displayed representing the range of luminance values which can be photographed, this being calculated according to the film characteristics, shutter speed and aperture size. The information displayed at 16 is provided by a processor which receives information from a keyboard 12 and a color camera photosensor onto which is focused an image of the scene to be photographed.

Abstract: Light induced current from a photodiode is converted into a voltage proportional to the integral with respect to time of the current using two serially connected integrating operational amplifiers. In each case the integrating capacitors are connected so that the photodiode is part of the charge current for the capacitors. The amplifiers operate in sequence.

Abstract: The invention relates to measuring illuminating power of light incident on a light sensor without an outside power supply. The light is converted conventionally to a pulse train, the frequency of which varies as the illuminating power. During conversion the incident light illuminates a series connection (2) of a plurality of photo diodes, which thus generate a current. This is utilized as a power source for the converter and also as electrical input quantity for the conversion itself. The pulse train generated by the converter comprises light pulses, the frequency of which is detected at the converter or at the other end of an optical fibre (7) coupled to it.

Abstract: An improved light measuring device that is capable of measuring light upon initialization from either a flash of light or a manual switch is provided. A sensor assembly can produce a light intensity signal and a change in the intensity of the light measured can be used to produce a first signal. A second signal can be produced by a manually operable member. A counter can be used to count the number of signals and to produce a preparing signal upon an initial signal counted and a start signal upon a subsequent signal counted. Measurement of the amount of light in the circuit can be initially enabled when the counting means counts the first signal and the actual light measurement can be started subsequently upon the counting means initiating a start signal. Various sequences of producing the pair of signals can be utilized to control the operation of the light measuring device.

Abstract: Hand-held photographic light meters which may be of either the incident light or the spot variety are equipped with means of transmitting the light intensity measured to a suitably equipped automatic camera by means of a modulated beam of infra-red radiation. In this way the camera is set accurately regardless of the reflectance of the subject. Continuous or flash or a mixture of the two lights may be used. The camera has to be modified to receive the infra-red radiation in Type A of the invention while in Type B a special receiving unit which mounts on the hot-shoe is used to give a less flexible performance. More than one camera may be set simultaneously and most of the features of the automatic camera are retained.

Abstract: Six silicon solar cells are arranged in three pairs on three mutually perpendicular planes. One cell of each pair will be exposed to both the direct rays of the sun and the diffuse light radiation incident from the same direction, depending upon the orientation of the device and the time of day. The other cells of each pair will be exposed only to the diffuse radiation on their respective planes. The differences in the measured radiation on each plane are squared, summed, and the square root of the sum then taken to determine the actual value of the direct rays of the sun.

Abstract: This invention relates to a method and apparatus for measuring the exposure variation in the cross scan direction in an optical projector and scanning apparatus of the type that uses an annular aperture or slit. This measuring is achieved by measuring the light intensity from point to point along the arcuate illuminated area. Where the illuminated area is tapered so as to accommodate a pivoting carriage the system of this invention processes the intensity signal to yield a uniformity signal that only indicates departures from the ideal.

Abstract: An apparatus for measuring the scalar irradiance of a given vicinity produced by all sources over a given angular range. The apparatus includes a planar array of convex spherical surfaces, a detector oriented toward the array, and a shielding plate having holes which abut and secure the convex spherical surfaces.

Abstract: Device for measuring variations of the gain of a laser amplifier at a particular wavelength, as a function of a pumping energy, said device comprising a source supplying continuous light radiation having the said wavelength and a power Pe, said radiation traversing the amplifier with a given radius, a means for tapping part of the amplified radiation at the output of the amplifier, a photoreceiver receiving said part and measuring the power Ps thereof throughout the pumping time, a means for forming the quotient of Ps by Pe and supplying a signal G(t) representing the variations of the gain as a function of the pumping time, wherein the device also comprises means for supplying a signal E(t) representing the increase in the pumping energy used during pumping and means for extracting a signal G(E) representing the gain variations as a function of the pumping energy for the given radius from the two signals G(t) and E(t), by elimination of the time t.It also relates to a process which utilizes this device.

Abstract: A device is described using at least one polyconductor film as an active ment lens of an electrical circuit for detecting incident laser radiation. In several alternative embodiments, a polyconductor film is located at the focal point of at least one lens which focuses the incident laser radiation on the film. The polyconducting film acts as a variable resistor of a balanced bridge network and upon receipt of radiation causes an imbalance in the bridge for radiation detection. The film changes phase with increasing incident power such that it protects itself against high levels of radiation. The lenses can also be polyconducting films and another film shielded from the radiation may be included to compensate for temperature fluctuations and maintain circuit sensitivity.

Abstract: An exposure apparatus for production of ICs of the type that includes a stage on which is placed a semiconductor wafer to be exposed by illumination light projecting means, and means for two-dimensionally moving the stage within a plane intersecting the illumination light at substantially right angles. The improvement comprises illumination detection means provided with a photo reception surface, and means for mounting the illumination detection means on the stage in such a manner that the photo reception surface and the surface of the semiconductor wafer on the stage to be exposed are at substantially equal height relative to the stage.

Abstract: An illumination flicker measuring device having a photodiode detector producing a signal proportional to the illumination being measured. An input amplifier for increasing the level of the signal produced by the photodiode detector. A capacitor is inserted so as remove the average light output component of the signal produced by the photodiode detector and which is then entered into a rectifier and amplifier so as to remove the lower envelope of the signal and amplify the upper envelope of the signal. A low pass filter is then attached to the rectifier and amplifier so as to remove the 120 Hz ripple produced by the 60 Hz system powering the illumination system. The output of the low pass filter is then passed through a capacitor to remove DC components present and then rectified and amplified again to produce a signal which is proportional to the amplitude of the flicker present in the illumination system.

Abstract: An exposure apparatus for production of ICs of the type that includes a stage on which is placed a semiconductor wafer to be exposed by illumination light projecting means, and means for two-dimensionally moving the stage within a plane intersecting the illumination light at substantially right angles. The improvement comprises illumination detection means provided with a photo reception surface, and means for mounting the illumination detection means on the stage in such a manner that the photo reception surface and the surface of the semiconductor wafer on the stage to be exposed are at substantially equal height relative to the stage.