Abstract: Miniature light-activated lamp control apparatus and the like, including a thin disk-shaped switching unit positionable within a light bulb socket and a thin light conductor having one end attached thereto, the light conductor being extendable through the gap between the electrically conductive inner wall of the socket and the electrically conductive outer surface of the bulb base disposed in the socket, and terminating at the opposite end in a lens for gathering ambient or other light. The switching unit has a first contact on one side for engagement with the socket center terminal, and a second contact on an opposing side for engagement with the center terminal of a light bulb. The unit includes a thin circuit board having control circuitry mounted thereon, such circuitry being positioned to occupy available space between the conical bulb base and the circuit board.

Abstract: A rearview mirror system for a vehicle having a reflective element with a reflective surface and a variable light transmission element includes an optical display device behind the reflective element in order to produce a visual display to the driver. A control is provided having a light sensor that senses light conditions in the vicinity of the vehicle and produces a continuously variable light signal indicative of such sensed light conditions. The control further includes a drive circuit that is responsive to the light signal in order to supply a drive signal to the reflective element and thereby establish the light transmission level of the light transmission element. The control further includes an intensity control circuit that varies the intensity of the optical display as a function of the value of the sensed light in the vicinity of the vehicle in order to accommodate physiological changes in the driver's eyes.

Abstract: A mirror assembly is disclosed which includes a dichroic mirror which allows a predetermined narrow spectral band to pass through the mirror and be seen from a position forward of the mirror, and which further reflects a broad spectral band of light which originates from a direction forward of the mirror; a light source is positioned rearwardly of the mirror and emits light in the predetermined narrow spectral band passed by the dichroic mirror; and an electronic circuit is provided which is operable to step up, or step down the voltage supplied to the light source thereby maintaining a constant voltage source which is utilized to energize the light source.

Abstract: A mirror (10) with a pair of vertical dazzle light attenuation zones (24 and 26). Each zone consists of stripes (24a, 24b, . . . 24n, 26a, 26b, . . . 26n) capable of dynamically adjusting the widths and the attenuation intensity of the attenuation zones (24 and 26). The mirror (10) has a multi-layered structure containing a liquid crystal (32). The positions and widths of the stripes are controlled manually via adjustment pushbuttons (20 and 22), or automatically, in response to the intensity of the incident light. When a driver sees during night driving the reflection of the dazzling light in the non-attenuated areas of the mirror, the driver's head is slightly moved, or the driver manually shifts by pushbuttons (20 and 22) the groups of the attenuated stripes (24 and 26) to the position where the dazzling light (L) will be covered by these stripes. The localized attenuation of the mirror's surface will not disturb the overall view observed by the driver through the rearview mirror (10).

Abstract: A fully automatic and energy efficient lighting control or light switch comprises different settings, preset by a user, and activates all or a portion of at least one bank of lights upon detecting occupants within a room. In one embodiment, the light switch detects doppler-shifted, reflected ultrasonic waves caused by occupant motion within the room. In an automatic mode, the light switch is configured to automatically switch from an initial sensitivity level for detecting motion within a short range therefrom, to a higher sensitivity level for detecting motion anywhere within the room. After the room is empty or no motion is sensed, the light switch is configured to turn the lights off following a variable time delay. The light switch returns to its initial low sensitivity level following a predetermined grace period which begins once the lights are turned off.

Abstract: A rearview mirror system for a vehicle having a reflective element with a reflective surface and a variable light transmission element includes an optical display device behind the reflective element in order to produce a visual display to the driver. A control is provided having a light sensor that senses light conditions in the vicinity of the vehicle and produces a continuously variable light signal indicative of such sensed light conditions. The control further includes a drive circuit that is responsive to the light signal in order to supply a drive signal to the reflective element and thereby establish the light transmission level of the light transmission element. The control further includes an intensity control circuit that varies the intensity of the optical display as a function of the value of the sensed light in the vicinity of the vehicle in order to accommodate physiological changes in the driver's eyes.

Abstract: An electrical device such as a lamp is energized and deenergized remotely by a system in which pulses of different duration are transmitted to a receiver unit capable of discriminating between the length of the pulses in order to switch between an "on" mode and an "off" mode. A battery energizing the receiver unit is recharged by a photovoltaic cell. Advantageously associated with the battery charging circuit is an ambient lights sensing device capable of interrupting current from the battery to a load whenever the battery is being charged by the photovoltaic cell.

Abstract: A switching control circuit for energizing a lamp in response to a decrease in ambient light below a predetermined level includes a photosensor and a timer circuit that responds to the photosensor indicating that the ambient light level is below a predetermined level by gating a solid state switching device into conductance to energize the lamp. The photosensor is powered by the signal that gates the switching device in a manner that renders the photosensor operative only when the lamp is not energized. Therefore, the lamp is energized upon the onset of dusk for a predetermined period of time during which the photosensor is disabled. After the period of time has expired, the lamp is deenergized and the photosensor is again enabled for a brief period of time to determine whether ambient light is still below the predetermined level. If it is, the light is again energized for another period of time. If not, the light remains deenergized.

Abstract: A method and circuit arrangement for adapting the light intensity of the summation light (E.sub.i) of a room lit by internal light (E.sub.k) and external light (E'.sub.i) to the external light (E.sub.a), which varies with the time of day, in which the light intensity of the internal light is controlled in dependence on one or more control parameters according to a given function and the function can be varied according to individual preference, are to be arranged so as to provide means of making finer adjustments to the light intensity in a room. This is achieved by determining the function by a plurality of independently settable function values (11), each function value (11) being variable independently of other function values (11).

Abstract: An installation for the control of the level of illumination of premises provided with openings towards the outside and acting on means for blocking the openings or on means for variable artificial illumination associated with controllable operating devices, on the one hand by means of contacts which can be actuated manually and, on the other hand, by an automatic control comprising a logical processing unit depending on a reference value corresponding to a level of illumination established by the user and recorded by the LPU. The photosensitive means measuring the illumination are mounted on a mobile module in which the LPU is also preferably mounted. The reference value is established by means of the contacts and is recorded in the LPU either automatically or by actuating a third contact.

Abstract: Device for automatically controlling the lighting level of premises (A) having available one or more openings (B) to the outside. This device acts on means (V, S, E) of masking the openings and on means of artificial lighting with dimmer (L) as a function of a set point value corresponding to a lighting level established by the user and recorded instantaneously. The masking and lighting means are furthermore controlled in a specified order so as to limit the consumption of energy. In certain implementations, this order is determined as a function of the deviation between a measured temperature and a threshold temperature.

Abstract: A sensor system for recording the ambient conditions of an optical sensor having a transmitter unit and a receiver unit linked by an optical gap. An electronic sensor unit is provided that predetermines a basic setting of the optical sensor independent of the ambient conditions. The electronic sensor unit contains a signal processing section with comparators for comparing a reception signal of the optical sensor with threshold values corresponding to certain ambient conditions. The electronic sensor unit recognizes and evaluates changes in the ambient conditions as a function of the comparing, and restores the basic setting of the optical sensor such that the sensor system dependably reacts to subsequent changes in the ambient conditions.

Abstract: A vehicle accessory includes a reflective element for reflecting light to a photo-sensor positioned within the accessory's housing for controllably detecting an ambient light level. The vehicle accessory may also include a display source for generating vehicle information which is positioned in the housing to utilize the reflective element to also reflect an image from the display source to a driver of the vehicle.

Abstract: A solar powered lamp having a circuit for providing positive turn-on at low light levels causes a light source thereof to undergo a discrete transition to an "on" condition and prevents unnecessary drain on a battery thereof. The circuit comprises a positive feedback loop which amplifies current flow rapidly to a predetermined level at which the light source is illuminated. In doing so, it causes a plurality of solar powered lamps to illuminate at substantially the same time.

Abstract: A light transmitting system for a vehicle includes a window or mirror with variable transmittance. A first input device, which may comprise a user adjustable device or an ambient light sensor, provides a comfort setting. A second input device, which overrides the first, may be operated manually, or by the vehicle engaging reverse gear or switching on headlights, to increase or decrease the reflectivity and transmittance of mirror and window embodiments respectively. Where a mirror is used, a third input device, which may comprise a photosensor, may be actuated to provide lower reflectivity when experiencing glare.

Abstract: A mirror system for use on a vehicle includes one or more remotely actuated mirrors that may be mounted on the interior or exterior of the vehicle. Each mirror has a continuously variable reflective element such as a solid-state electrochromic device or an electrochemichromic cell. A first user actuated input device provides a comfort level-setting control that allows the driver to select a reflectance level for one or more mirrors at a comfortable level. A "panic button" overrides the first input device and lowers the reflectance level of one or more mirrors when an excessive glare condition is experienced. The lower reflectance level may be cancelled manually by a second actuation of the "panic button" or is automatically cancelled after a predetermined period of time to resume the normal comfort level of reflectance.

Abstract: An apparatus for protecting the image recording media in a tracking camera rom destructive illumination. The apparatus includes a shutter assembly mounted between the lens and the camera and a sensor assembly mounted in juxtaposition with the camera so that a sensor at one end of the tube in the sensor assembly detects destructive illumination before it impinges on the lens. The sensor causes a signal to be passed through circuitry connecting to the shutter assembly, activating a rotary solenoid which moves a blade to an interrupting position between the lens and the camera until no further destructive illumination impinges on the sensor. The shutter blade return to a non-interrupting position when the sensor no longer receives destructive illumination.

Abstract: A remote indicator system for determining at a remote location when incoming mail is present in a mailbox. A light port in the mailbox admits outside ambient light onto a first photodetector on the bottom of the mailbox. When no mail is present, the first photodetector produces a first signal indicating that no mail is present. When incoming mail is present, the light to the first photodetector is interrupted, causing the first photodetector to produce a second signal, indicating that mail is present. A second photodetector positioned outside of the mailbox disables the first signal when the outside ambient light is below a given intensity, and a switching means associated with the manually operable flag disables the first signal to distinguish between incoming and outgoing mail.

Abstract: A constant-current light sensing system employs a constant-current diode in series with the power lead to a photoconductive cell. This diode provides a substantially fixed current through the cell and enables the system to be designed such that it can make maximum use of the linear portions of the cell's output response curve, and maximum use of the range of a potentiometer's settings. The cell used in conjunction with the system is received in a housing that is easily mountable to a wall or ceiling surface by an adhesive material.

Abstract: A method of controlling fluorescent lamps in accordance with any selected one of a family of curves plotting sensed natural and artificial light against lamp output, each curve having a steep portion and a more moderate portion and a circuit for providing such control.

Abstract: An illumination control apparatus includes a control slave unit, a master unit, a supervisory slave unit, and a photosensor slave unit. The supervisory slave unit has an operation switch, and outputs a first signal representing an operation change of the operation switch and the supervisory slave unit. The photosensor slave unit outputs a second signal representing a change of a level of an illuminance detected by a photosensor and the photosensor slave unit. The control slave unit controls at least one illumination load. In addition, the master unit controls the control slave unit assigned in a memory in response to the first signal from the supervisory slave unit and the second signal from the photosensor slave unit. When the master unit receives the first signal representing the supervisory slave unit, the illumination load is inverted.

Abstract: A system and method for transmitting an identification signal include a triggering device for periodically generating an enabling pulse signal at random times, whereby the periodic generation of the enabling pulse signal varies in each period in proportion to light incident on the triggering device and a transmitter for transmitting an identification signal in response to the enabling pulse signal. It is preferred that the triggering device include a timer for generating the enabling pulse signal in a defined time cycle in response to a control signal and a signal generator for variably generating the control signal within the defined cycle, wherein the signal generator includes a light sensitive component for varying in time the generation of the control signal in proportion to light incident on the light sensitive component for a portion of said defined cycle.

Abstract: Solar powered outdoor lighting and/or alarm systems are provided and include a light source or alarm, a passive infrared (PIR) sensor in conjunction with a battery recharged via solar cells, and a control circuit coupled to the light source or alarm, the PIR sensor, and the rechargeable battery. The control circuit guarantees that the light source or alarm is turned on by the battery only when the sensor senses the presence of a moving target. The systems may include a light detector which is utilized to prevent the system from activating in daylight. In the case of a lighting system, a timer is utilized to turn off a lamp after a desired period of time (e.g. 3 minutes). By limiting the "on-time" of the light source, a two to five watt bulb may be used. In the case of an alarm, the alarm may be hardwired to the control circuit; or if desired, a transmitter powered by the rechargeable battery may be used to transmit an alarm signal to a remote receiver.

Abstract: This invention discloses an accessory utilizing a light emitter, including the light emitter, arranged near a transparent body, for intermittently emitting light upon reception of an electrical signal, first pulse generator for generating a first pulse signal at a relatively short pulse interval, photo detector for detecting light incident on the transparent body and generating a light detection signal, second pulse generator for generating a second pulse signal at a relatively long pulse interval, and signal processor for receiving the first pulse signal, the second pulse signal, and the light detection signal and supplying the electrical signal to the light emitter at a predetermined timing.

Abstract: A safety shut-off protection system is provided for a light responsive system such as video cameras, image intensifiers, thermal imagers, or the like. The light responsive system further includes several series-connected, normally-closed, light responsive photoelectric Darlington safety switches. These safety switches are located to receive light along with the light responsive systems. Light exceeding a threshold intensity impinging on any one of the Darlington safety switches causes such a switch to operate to deactivate the light sensitive system.

Abstract: A switching system operated from switching areas in a housing of an electronic device includes first and second sets of ambient light sensors and switch sensing means secured in the housing. An ambient sensing circuit and averaging circuit means connects to the first set of light sensors to produce a reference signal representing an average ambient light reaching the first set. A shielding sensing circuit means couples to the second set of light sensors to produce a switch sensing signal in response to an operator shielding ambient light therefrom. A comparison means coupled to the averaging and shielding sensing signal and the reference signal to provide an output after a predetermined delay; and a switch connected to said comparison means performs at least one switching function in response to the output from the comparison means.

Abstract: A system has individual infrared radiation transmitter assemblies which issue characteristics and coded identifying signals in infrared radiation, so that transmission and identification of a plurality of distinctively characteristic signals is accomplished by varying the timing of various periodic transmissions so as to avoid interference among the plurality of transmissions. A receiver assembly receives and decodes binary numbered codes sent from a plurality of the individual pocket-size transmitters for keeping track of the location of the transmitters within an area monitored by the receiver assembly. Each transmitter assembly has a timer which periodically variably issues electrical pulses within a predetermined time cycle. The timer actuates a clock oscillator to cause a code stored memory to be read out as coded electrical signals representing binary information and a light emitting diode converts the coded electrical signals into infrared radiation pulses representing the same binary information.

Abstract: A light fixture has a light-transparent cover at substantially grade level and a photocontrol device with a self-flushing outer surface for preventing accumulation of opaque material thereon. The light fixture is adapted to be positioned within a recess in the graded environment and includes a housing which encloses a lamp electrically connected to the photocontrol device. The self-flushing outer surface of the photocontrol device discourages the build-up of opaque material so that the photocontrol device can reliably detect changes in the ambient light conditions outside the light fixture and operate the lamp in response to the detected changes.

Abstract: A digitally controlled ambient light detection circuit for an infrared intrusion detector. A light dependent resistor is used as a charging circuit for pulse driven capacitor on the input of a digital buffer circuit so that the time of reaching the threshold voltage of the buffer varys with the light falling on the light dependent resistor. The buffer output is then compared to a reference pulse which is synchronized with, but delayed from, the capacitor charging pulse. If the buffer output occurs after the reference pulse it indicates low light, and an output signal is generated to activate the infrared intrusion detector.

Abstract: A loss of optical input detector is disclosed disposed to receive a signal voltage from a fiber-optic receiver indicating that the optical input has dropped below a preset threshold. The detector includes a voltage follower connected to the fiber-optic receiver which passes to the detector circuit the signal voltage. A comparator circuit receives the signal voltage and compares it to a reference voltage. When the signal voltage is greater than the reference voltage the comparator produces and applies to an alarm circuit a positive voltage signal. This positive voltage signal turns on the alarm circuit. The alarm circuit provides a visual alarm indication and a logic signal to a controller interface indicating a loss of the optical input signal.

Abstract: The invention provides an automatic rearview mirror system (10) for a vehicle comprising a reflective element (22) actuatable between at least two different reflective conditions. The system includes a forward light sensor (11) to detect light forward of the vehicle and a rear light sensor (12) to detect light rearward of the mirror. A control circuit means (14,16) is coupled to the reflective element (22), the forward light sensor (11) and the rear light sensor (12). The control circuit causes the reflective element to assume a reflective state dependent upon the output of the forward and rear light sensors. The control circuit includes means for modifying the output of the forward light sensor (11) to cause a reduction in the sensitivity of the mirror.

Abstract: An automated guided vehicle uses a scanning laser unit having a laser transmitter and receiver to transmit laser signals and receive them reflected back from a series of targets. The reflected signals are used to guide the vehicle along a desired path. The laser unit includes a photodetector mounted near the laser receiver such that any extraneous light, such as sunlight, that strikes the laser receiver will also strike the photodetector. The photodetector converts the extraneous light into an electrical signal proportional to the amount of such light. The photodetector signal is compared to a threshold value which generally is slightly below an amount of extraneous light sufficient to confuse the laser unit. If the threshold value is exceeded, the photodetector signal is used to stop the vehicle or take other appropriate action.

Abstract: A refrigerator ice dispenser light having a control including a photosensitive switch which applies half wave rectified line voltage when there is little or no ambient room light thereby dimly lighting the bulb at night. In response to the dispenser being activated, the photosensitive switch is shunted and line voltage is applied across the bulb.

Abstract: A device for measuring the intensity of light received by a photosensor comprises a photosensor for receiving light and producing a corresponding current and a circuit for integrating the current so that the intensity of the received light is indicated as the time required for the integration value of the integrating circuit to reach a predetermined value. If the intensity of the light is determined to be high at the start of the integration operation the integration value is set to the smaller of two predetermined values. If the intensity of the light is low, the integration value is set to a larger value.

Abstract: Light-sensitive switch structure for and method of turning a street light or other load on or off in response to variations in light level. The switch is designed to recognize a specific sequence of daily variations in light level and is capable of switching a load off at a light level lower than that at which the load is switched on. In a preferred embodiment, the switch consists of a photo-sensitive element, two operational amplifiers, a relay with its associated driver circuit, and a number of resistors forming voltage dividers. In accordance with the method of the invention, the photo-sensitive element, together with a resistor, establishes a signal voltage which varies in relation to the light level. Other resistors form voltage dividers which establish reference voltage levels at which desired switching is to occur. The operational amplifiers perform comparisons between the signal voltage and reference voltages and switch accordingly.

Abstract: An outdoor lighting controller including a light sensing element for automatically activating and de-activating an outdoor light source. A wiper is provided for removing accumulated dust from the light sensing element. The wiper is initiated when the light source is activated.

Abstract: A zoom lens system for use in microfilm projection apparatus in which the image projection is conducted in a condition of a definite object-image distance, the system comprising from the enlargement side to the reduction side, an aperture stop disposed fixedly, a first lens group of a positive refractive power located near the aperture stop at the longest focal length side, a second lens group of a negative refractive power, and a third lens group of a positive refractive power, wherein the first to third lens groups are moved along the optical axis while increasing a distance between the first and second lens groups and decreasing a distance between the second and third lens groups in an operation of zooming from the longest focal length side to the shortest focal length side.

Abstract: Improved gain control in a photomultiplier tube having a plurality of dynode stages is achieved through manual or automatic change of the bias voltage on at least one of the several dynodes between the anode and cathode of the tube. By such means, maximum tube gain change is obtained with a minimum of bias voltage swing.

Abstract: A power supply circuit for a light emitting diode may be adjusted automatically by the voltage of a solar cell which is a power source in the circuit by including in the circuit a series connected current regulator governed by said voltage of said solar cell.

Abstract: A photoelectric control unit for street lighting comprises control means including a photoelectric sensor mounted on a printed circuit board for switching on a street lamp when the intensity of light falling on the sensor falls below a preset value. Electrical connection means between the P.C.B. and the lamp is provided by three connector pins which are led through a cooling chamber or stem. The pins are cranked to closely follow the contour of the chamber to dissipate heat through the chamber walls. The chamber is also filled with a high thermal conductivity potting compound for heat transfer to the walls of the chamber.One controller for such a photoelectric control unit includes a switching means provided by a relay and a triac connected in parallel. The triac operates to prevent the relay being subjected to high voltages during switching and hence prevents pitting of the relay contacts.

Abstract: The present invention relates to an illuminated switch and in particular to one having a housing, switch contacts in the housing, an electrical device disposed in the housing and operatively associated with the switch contacts, a light transmissive colored filter mounted over a neon lamp of the electrical device, and a transparent member located over a cds photoresistor of the electrical device, whereby the neon lamp will light to provide indication of the switch location in darkness and will be extinguished when the switch is turned on.

Abstract: Light-sensitive switch structure for and method of turning a street light, or other load, on or off in response to varying light levels. The switch is designed to recognize the daily variations in light level and is capable of switching a load off at a light level lower than that at which the load is switched on. In a preferred embodiment the switch consists of a photo-sensitive element, two relays with time delay features, and two current limiting resistors to regulate the current flowing through the operating coils of the relays. In other embodiments wherein the values of the circuit components are properly selected one of the resistors regulating the current flowing through the operating coils of the relays may be eliminated. In accordance with the method of the invention one of the relays operates contacts which energize or de-energize the load, while the other relay operates contacts which connect or disconnect the resistors to or from the operating coils of the relays.

Abstract: A hot body activates, such as by a person or a car moving into an area scanned through a lens by a passive infrared detector, a light control system having an adjustable length turn on duration for each hot body intrusion turn on cycle. The light control system also includes a photocell deactivation sensor that is in a daylight deactivation circuit for the light control system with the light sensing photocell positioned behind a multifaceted scan zone lens, along with the infrared detector. A manually switch activated circuit is also provided with the light control system for an emergency blinking light mode of operation as an alarm.

Abstract: A self-contained light sensor head (10) has a photodiode (18) operable to produce an electrical response proportional to light intensity, and an amplifier (20) that produces an output signal from the photodiode's response. The head (10) further includes a gain control circuit, for suitably controlling the gain of the output signal, and has noise and oscillation control circuits. The latter circuits permit the sensor to generate a low voltage output signal that is compatible for input directly to an analog computer.

Abstract: A light sensitive switching circuit for automatically controlling the supply of electric power from a power source to a load, such as a lantern, is disclosed. When it becomes darker than a predetermined brightness the power supply starts and when it becomes bright again, the power supply is cut. The light sensitive switching circuit comprises first and second lines for connecting the power source and the load in series between the first and second lines. A first thyristor is connected between the first and second lines, and a second thyristor connected is to the first thyristor such that the anode of the second thyristor is connected to the gate of the first thyristor and the cathode of the second thyristor is connected to the cathode of the first thyristor. A photocell is connected between the gate and the cathode of the second thyristor.

Abstract: A power adapter for timed control of auxiliary electrical loads is provided for connection between a streetlight fixture or the like and a control module of the fixture. The adapter has a weather-resistant housing and an electrical jumper in the housing extending a power source connection to an input of the module, another jumper returning a control output from the module to a main load of the fixture. An auxiliary socket at the side of the housing powers the auxiliary load, which can be a holiday lighting display. In one version, the adapter has a clock timer for controlling the auxiliary load independently of the main load. In another version, a programmable delay timer, synchronized with commencement of main load operation, terminates operation of the auxiliary load following a predetermined interval of main load operation.

Abstract: An electric switch has an infrared radiation detector and a visible light detector. The switch is activated when both a change in the detected infrared radiation is sensed and the sensed visible light has a given relationship to a predetermined intensity level. An adjustment of the predetermined level is provided whereby the switch is activated when the level is set to the visible light intensity at the time the adjustment is made.

Abstract: The average video level of a video camera is corrected for film-to-video conversion by having a film aperture plate with a light-passing opening that is slightly enlarged with respect to the image area of the film. Non-image light passes through a peripheral region of the film, as defined by the enlarged opening, and strikes a part of the camera sensor not receiving image-wise illumination. By enlarging the aperture plate opening just enough that the video signal from the non-imaged part of the sensor approximates a desired average video level, the automatic video level correction performed by the camera will pertain only to the image. The aperture plate is part of a film gate that is rotatably mounted so that the film may be supported in either a "horizontal" or "vertical" orientation, depending on how the film image was initially exposed.

Abstract: A photoelectric control device having a photoelectric sensing unit for delivering an output voltage in response to changes of the intensity of light, the output voltage being applied to a capacitor for operating a SCR. When the output voltage reaches a predetermined level the SCR is caused to become conductive or non-conductive. The SCR is connected to a bridge rectifier which causes a triac to become conductive or non-conductive when the SCR becomes conductive or non-conductive, so as to allow electric power to be supplied to a load or to be cut off in response to changes in the intensity of light.

Abstract: A system for monitoring the level of light at a distant location from a central location. A light sensor at the location to be monitored is comprised essentially of a photodiode and an operational amplifier and produces a high-level, analog output signal on an output signal line, the signal being linearly proportional to the level of light at the location. A control unit at a central location is electrically coupled to the sensor and comprises a power supply for providing electrical power to the sensor and a calibration device connected to the output signal line for providing a feedback signal to the sensor. The calibration device may be adjusted so that the output signal at the control unit bears a selected ratio to the light detected. A control unit having a single and power supply may be coupled to a plurality of sensors.