Photoelectric Devices

Photoelectric transducers are devices capable of converting light energy into electrical energy. In this respect only a limited number of photoelectric components comply with such a definition—that is, photovoltaic devices, which specifically generate electricity when light falls on them. Other photoelectric devices are photoconductive only, meaning that a charge of illumination or light falling on them produces a change in their conductive properties in an electric circuit. Thus they work as transducers only when part of an electrical circuit.

One thing all photoconductive devices have in common is that they are basically diodes, even the phototransistor that combines the photoconductivity properties of photodiode with the amplification properties of a transistor (except that the “diode” part is a photoelectric device).

A photovoltaic device, on the other hand, works like a battery and is generally called a cell or photocell. This is the type used in “self-powered” light meters. Like batteries they can be connected in series to generate a higher voltage (but still normally in the low- voltage range) or in parallel to generate higher current (in the low- current range). Top performer in this range is the solar cell, which in average sunlight may be capable of generating a photovoltaic potential of the order of 510 mV or more per cell with an output current of 3 mA into a 100-ohm load. it's also capable of being connected in series and /or parallel; a multiple cell arrangement is called a solar battery.

Apart from obvious applications for powering electrical devices—where even powering a lightweight airplane by solar batteries has proved possible—solar cells can also be used to measure the strength of sunlight falling on a particular location. Only a single solar cell is needed for this. Because such a cell is quite small (typical size 1/2 in. square), it needs mainly a larger base panel (say about 5 in. square) with the sensitive (negative) side of the cell facing out wards. A cutout iii this panel can then accommodate a milliammeter facing the other way. Solar cell connections are then made directly to the meter via a resistor in one lead. Add a handle, and you have a portable radiometer (ill. 12-1).

ill. 12-1. A simple radiometer for measuring light intensity.

You can work out the resistor value and the meter range required in this way. In strong, average sunlight you can expect the cell to generate about 500 mV. Suppose you are going to use a 0-10-mA meter. A 500-mV signal thus has to develop a current of 10 mA through the meter for full-scale deflection. From Ohm’s law the corresponding circuit resistance required is:

ill. 12-1-1

ill. 12-2. An alternative wiring scheme for a radiometer using a meter with high internal resistance.

The resistance of the meter itself will be less than this, probably only a few ohms. So try a 39 ohm or 47 ohm resistor connected in series, and see if this gives near full-scale deflection. If not, use a smaller value for the resistor. Alternatively, if you can only get low readings on the meter in strong sunlight, this is because the effective meter resistance is higher than expected. In that case try connecting a low resistance (say 10 ohm less) directly across the meter terminals instead of in series with it (ill. 12-2).

Incidentally, a typical short-circuit current for a solar cell is 0-5 mA. Thus, if you used a 0-5 mA meter that had negligible resistance, you would not need any resistors in the circuit; the solar cell could be connected directly to the meter. However, the meter would still have some resistance, so to achieve full-scale deflection you would almost certainly need to short the meter with a small valve resistor connected across it.

MAKING A SOLAR BATTERY

A solar battery is a true transducer, which converts solar energy directly into electrical energy, just like a battery. Its effectiveness as a battery depends on the strength of the sunlight present and the number and arrangement of the cells used.

The starting point in the design of a solar battery is the volt- age and current required. We already have 0.5 V as a typical cell voltage in average sunlight. Thus, to generate a voltage of V, you need 2 x V cells connected in series. e.g., to generate 3 V, you need 2 x 3 = 6 cells (ill. 12-3).

ill. 12-3. Series connection of photovoltaic cells results in higher voltages than can be obtained with a single cell.

The short-circuit current of a typical solar cell is 5 mA, corresponding to a nominal cell resistance of 500/5 = 100 . With an external load we can thus anticipate the current I to be

I = 500 / (100+R) mA

where R is the external load resistance.

Thus, for a 100 external load resistance,

I = 500 / (100+100) mA

= 2.5 mA

Suppose we want to design the solar battery to work a 3-V de vice that has a load resistance of 25 ohm and needs to have a 12-mA current flowing through it to work properly. To generate 3 V, we need six cells connected in series, but the current through an external load of R = 50 ohm will only be

I = 500 / (100 + 25) mA

= 4mA

ill. 12-4. A series-parallel array of photovoltaic cells for obtaining 3 Vat 12 mA.

To boost this current to the required 12 mA, we need 12/4 = 3 rows of cells connected in parallel (ill. 12-4). The total voltage generated is still the same (3 V), but each row contributes 4 mA of current, for a total of 3 x 4 = 12 mA. We now have the solar battery configuration required to power the device. Remember that when connecting them up and mounting them the sensitive side (negative side) of each cell must face directly towards the sun.

For maximum effect such a battery array should be mounted to point along the meridian (due south) and tilted away from the zenith at an angle of approximately the local latitude (ill. 12-5). This is, of course, only possible with a fixed installation. In a solar battery that is an integral part of a moving device, the optimum arrangement is normally with cells facing virtually upwards and mounted on top of the device. Solar cells for powering an electric, motor-driven model airplane, e.g., would need to be mounted on the top surface of the wing.

ill. 12-5. Optimum positioning of a solar battery array. The array should face due south and be slanted from the zenith by approximately the local latitude.

OTHER PHOTOVOLTAIC CELLS

Photovoltaic cells of the type used in light meters and other low-current devices consist basically of a thin film of semiconductor material such as gallium, selenium, or silicon deposited on a steel or copper plate. The surface of the semiconductor is then covered with a film of noble metal, such as gold, that is so thin that it's transparent to light. A metal ring over this transparent coating then forms one contact to the cell, and the steel or copper plate forms the other.

In practice, such cells normally look like metallic wafers and may be produced in a variety of shapes and sizes from about 3/32 in surface area up to 12 in surface area (selenium cells); or from about 3/32 in up to about 1 1/4 in. diameter (silicon cells). Thickness is of the order of .002 in. to .006 in.

ill. 12-6. A light meter for measuring low levels of illumination.

Photovoltaic cells work in the following way. Light falling on the photosensitive surface of such cells has the effect of liberating electrons in the boundary layer of the sandwich construction, with the result that if connected to an external circuit, an electric cur rent will flow in that circuit. Within limits, the amount of electricity generated is proportional to the amount of light falling on the cell. The current generated is quite small—only a few microamps—but this is sufficient to give a reading on a micro-ammeter (ill. 12-6) or to operate a sensitive moving-coil movement. This is the basis of the light meter. All you need is a photovoltaic cell and a micro-ammeter connected together, and you have a light meter capable of measuring the quantity of illumination falling on the face of the cell. It can only be calibrated, however, with reference to the specific performance curve for that cell, or, more simply, by comparing readings against those obtained from a standard light meter, such as a camera exposure meter.

Strictly speaking, provided the cell is not over-illuminated so that the cell is saturated, the microammeter reading will be almost directly proportional to the level of illumination. Most light meters, however, have a scale where the divisions get closer together at the higher levels of illumination. This is either a feature of the design of the poles of the permanent magnet used with the moving coil movement in the meter, or it's a direct result of saturation with increasing illumination. Often, both factors are responsible.

The polarity of the current developed will always be the same for a given device, but the actual polarities of selenium and silicon photocells are different. Thus, with a selenium cell the base or back of the cells is positive. With a silicon cell the sensitive face is the positive electrode.

Many photovoltaic cells show different responses for the same level of illumination produced by different light sources. Thus, al though a selenium-steel cell will give similar readings for daylight and tungsten-bulb light at the same levels of illumination it will under-indicate the light from discharge lamps. A copper/oxide- copper cell, on the other hand, will over-indicate daylight with respect to tungsten light and under-indicate with discharge lamps (but not fluorescent lamps).

In the case of converse meters gallium cells have been preferred until recently because of their lack of color bias (compared with silicon). Second-generation silicon cells don't have any marked color bias and are now largely replacing other types because of their superior transducer properties.

There is also a distinction to be drawn between photovoltaic cells that have a low internal resistance (a few thousand ohms only) and those that have a high internal resistance (of the order of megohms). Cells with low internal resistance can be classified as current generators; cells with high internal resistance, as voltage generators. Because it's also a feature of such cells that the response characteristics are affected by the load resistance, this can govern the choice of type for particular applications. Thus, although a low-resistance photovoltaic cell can generate useful current, both the value of this output current and the linearity of response will fall off with increasing external resistance. Thus, any meter movement employed in working the cell in a practical circuit, e.g., must have a very low resistance. A high-resistance cell, on the other hand, even though generating a much smaller current, is less affected by load resistance and can develop a useful voltage across a high load resistance, which can then be amplified as necessary.

PHOTOCONDUCTIVE CELLS

Photoconductive cells are quite different in their working properties, for they need to be supplied with an external source of electricity. The effort of varying the level of illumination falling on the cell is to give it a variable electrical resistance, the effective resistance changing in proportion to the light level.

Earlier photocells were of two main types: cadmium sulphide cells responsive to visible light, and cesium sulfide cells highly sensitive to light in the infrared spectrum. For general use they have largely been replaced by photodiodes and phototransistors. With both of these (photodiodes in particular), current charges realized are small compared with photoconductive cells, so in practical circuits they normally have to be used with one or more stages of amplification.

PHOTODIODES

If any semiconductor diode is reverse biased and the junction illuminated, the reverse current flow will vary in proportion to the amount of light. This effect is utilized in the photodiode, which has a clear window through which light can fall on one side of the crystal and across the junction of the p and n zones.

In effect, such a diode will work in a circuit as a variable resistance, the amount of resistance offered by the diode being dependent on the amount of light falling on the diode. In the dark the photodiode will have normal reverse working characteristics; that is, it will provide almost infinitely high resistance with no current flow. At increasing levels of illumination, resistance will become proportionately reduced, thus allowing increasing current to flow through the diode. The actual amount of current is proportionate to the illumination only, provided there is sufficient reverse volt age. In other words, once past the “knee” of the curve (ill. 12-7), the diode current at any level of illumination will not increase substantially with increasing reverse voltage.

ill. 12-7. The reverse current through a photodiode is a function of the relative illumination falling on it.

Photodiodes are very useful for working as light-operated switches (and can be made sensitive to infrared as well as visible light). They also have quite a high switching speed, so they can be used for counter circuits counting the interruption to a light beam.

A basic photodiode light switch circuit is shown in ill. 12-8, where the diode, together with the resistor, controls the current flowing through a relay coil. The relay pulls in when the diode is illuminated and drops out when no light falls on the diode. The sensitivity of the circuit can be adjusted if a variable resistor is used in the primary circuit. No amplification is needed in this circuit because any battery voltage less than the breakdown voltage of the diode can be used to power the relay directly.

ill. 12-8. A simple photodiode-actuated relay circuit.

PHOTOTRANSISTORS

The phototransistor is much more sensitive than the photodiode to changes in level of illumination, thus making a better “switching” device where fairly small changes of level of illumination are present and must be detected. It works both as a photoconductive de vice and an amplifier of the current generated by incident light.

A basic phototransistor light switch is shown in ill. 12-9, again using a relay (which could be replaced by a solid-state switching circuit). If a OCP71 phototransistor is used, a sensitive relay with a coil resistance of about 2000 ohm would be suitable, which is capable of pulling in at about 2 mA. The variable resistor connected across the base and emitter of the transistor provides a sensitivity control for adjusting the pull-in point of the relay. Performance will be improved if a diode is connected across the relay.

ill. 12-9. A phototransistor-actuated relay circuit. The relay pulls in when the illumination increases to a level determined by the setting of the potentiometer.

No reset function is incorporated because the actual alarm circuit is quite independent of the main circuit, having its own separate battery, the circuit being completed by the opening of the relay contacts when the relay drops out at a low level of illumination. This alarm circuit will be switched off again as soon as illumination is restored to the phototransistor, and thus the bell will ring only during the period the light beam is interrupted—just momentarily if anything passes through the light beam.

The sensitivity of this circuit can be greatly improved by adding a second conventional transistor to provide one stage of amplification following the phototransistor (ill. 12-10). A second potentiometer (R2) is included to adjust the bias applied to the base of the second transistor, which in turn affects the relay current and , thus, the pull-in of the relay. In practice, R2 is adjusted with the phototransistor shielded or covered up so that the relay does not pull in with circuit switched on. The phototransistor is then un covered, and R1 is adjusted to set the level of illumination at which the relay pulls in.

ill. 12-10. A more sensitive phototransistor-actuated relay circuit. Transistor Q2 amplifies the output from phototransistor Q1. Sensitivity is adjusted via R1 and R2. Resistor R3 limits the current through the relay.

ill. 12-11. A circuit using a photoconductive cell for actuating a relay. Sensitivity is adjusted via R1; R2 limits the current through the relay. In this circuit the relay is open with illumination and closes when the light intensity falls below a certain level.

ANNUNCIATOR PHOTORELAY

The circuit shown in ill. 12-11 employs a conventional photoconductive cell with straightforward amplification, giving a simple design, again with a minimum of components. Virtually any type of relay can be used with a coil resistance between 1 ohm 10 ohm that can pull in at about 3 mA or less. A potentiometer can be used to adjust the relay current to the required operating level. Alternatively, with a relay coil resistance of 5000 ohm, this potentiometer can be dispensed with. A sensitivity control for the whole circuit is provided by R1, which can be adjusted to establish the pull-in of the delay at the desired level of illumination. Ideally, this should be with as much of R1 left “in” as possible (corresponding, that is, to a fairly high level of illumination) so that the current drawn when the light is on is very low. The circuit can thus be left set with very little drain on the battery. When the light is interrupted or removed from the photocell, the current will rise to between 3 to 5 mA, causing the relay to pull in and closing the contacts to complete the alarm circuit.

ANNUNCIATOR RELAY WITH “HOLD”

The circuit just described can, with slight modifications, be made to work with a “hold” function. That is, when the relay drops out in response to an interruption of the light, it continues to hold out until the circuit is reset.

This is accomplished by completing the circuit through the relay contacts, as shown in ill. 12-12. In this case only a single battery is required, and the circuit is reset by a pushbutton switch. it's similar to the burglar alarm circuit previously described.

ill. 12-12. A variation of the circuit of ill. 12-11, incorporating a “hold” function.

The circuit works as follows. With the photocell illuminated, R1 is adjusted so that the relay is not pulled in. When the light level falls and the photocell resistance rises, the balance of the potential- divider circuit shifts sufficiently for the current, amplified by the transistor, to operate the relay. The relay contacts now change over, removing the photocell from the circuit and providing a large positive bias on the base of the transistor. This maintains the cur rent through the relay to keep it held in. At the same time the changeover of the relay contacts completes the alarm circuit. Pressing the reset button de-energizes the relay and restores the photocell to the circuit once more.

EXTRA-SENSITIVE ANNUNCIATOR RELAY

The addition of a second stage of transistor amplification to the type of circuit already described produces a circuit with extreme sensitivity to light changes. Such a circuit is shown in ill. 12-13, where the potentiometer R4 controls the overall sensitivity by tap ping off a voltage drop applied to the base of the second transistor as bias.

ill. 12-13. A more sensitive version of the circuit.

This again is a current-rise circuit: an almost negligible current is drawn from the battery when the photocell is strongly illuminated, and the relay is not operated. A fall in illumination produces a change in circuit conditions, which triggers the amplifier circuits so that sufficient current is then passed to pull the relay in. The current drain on the battery then rises from a few microamps to the order of 5 mA.

THE OPTOISOLATOR

A phototransistor and a light-emitting diode (LED) may be combined in a single envelope, such a device being known as an optoisolator. In this case the LED provides the source of illumination to which the phototransistor reacts. It can be used in two working modes: either as a photodiode with the emitter of the transistor part left disconnected; or as a phototransistor. In both cases the operation is governed by the current flowing through the LED section. Further variations on this device are the opto-Darlington-Isolator and the opto-triac-isolator. The former comprises, typically, an optically coupled, gallium-arsenide, infrared-emitting LED and an npn silicon photo-Darlington transistor in a six-pin IC package, capable

of collector-current switching from 100 mA (off-state) to 100 mA (on-state). The opto-triac-isolation is essentially similar, with a triac replacing the transistor pair, and capable of being worked directly from household ac voltage.

PHOTOELECTRIC CONTROLS

Photoelectric controls are used in numerous industrial and commercial applications for sensing, detecting, counting, and similar functions. They are commonly interfaced with logic capabilities to cope specifically with individual applications. Most photoelectric controls consist of a light source/photoreceiver combination pro viding a signal to a control base, which then amplifies this signal and applies the signal logic to transform it into a usable electrical component.

There are two main types of controls. The self-contained control includes the light source, photoreceiver, and the control base function, and the modular control uses a light source/photoreceiver combination or reflective scanner separate from the control base. Self-contained retroreflective controls require less wiring and are less susceptible to alignment problems, whereas modular controls are more flexible in allowing remote positioning of the control base from the input components and are more easily customized.

Modulated LED controls respond only to a narrow frequency band in the infrared. Consequently, they don't recognize bright, visible ambient light.

Non-modulated controls respond to the intensity of visible light. Therefore, to maintain control reliability, they should not be used where the photo-sensor is subject to bright ambient light, such as sunlight.

Controls typically respond to a change in light intensity above or below a certain value or threshold response. However, certain plug-in amplifier/logic circuits cause controls to respond to the rate of light change (transition response), rather than to the intensity. Thus, the control responds only if the change in intensity or brightness occurs very quickly, not gradually.

Both modulated and non-modulated controls energize an out put in response to

- a light signal at the photosensor when the beam is not blocked (light operated or L.O.)

- a dark signal at the photosensor when the beam is blocked (dark operated or D.O.)

Although some controls have built-in circuitry that determines a fixed operating mode, most controls accept a plug-in logic card or module with a mode-selector switch that permits either light or dark operation.

Therefore, much depends on the actual design of light source/photoreceiver combination, amplifier and electrical output device interfaced with logic level circuitry. The following is quoted from Honeywell and applies specifically to their Micro Switch photoelectric controls, where you can select a self-contained control or a control base plus discrete light source and photoreceiver units (or a scanner, which combines light source and photoreceiver in one housing).

SCANNING TECHNIQUES

There are several scanning techniques (ways to set up the light source and photoreceiver to detect objects). The best technique to use is the one that yields the highest signal ratio for the particular object to be detected, subject to scanning distance and mounting restrictions.

Characteristics of the objects to be detected that have a bearing on scan technique include whether the objects are (1) opaque or translucent; (2) highly, or only slightly, reflective; (3) in the same position or randomly positioned as they pass the sensor. Detecting change in color is a special consideration.

Scanning techniques fall into two broad categories: through scan and reflective scan.

THROUGH SCAN

ill. 12.14. Through (direct) scanning. The source is located some distance from the sensor.

In through (direct) scan the light source and photoreceiver are positioned opposite each other, so light from the source shines directly at the sensor. The object to be detected passes between the two. If the object is opaque, direct scan will usually yield the highest signal ratio and , therefore, should be your first choice.

As long as an object blocks enough light as it interrupts the light beam, it may be skewed or tipped in any manner. As a rule of thumb, object size should be at least 50 percent of the diameter of the photoreceiver lens. To block enough light when detecting small objects, special converging lenses for the light source and photoreceiver can be used to focus the light in a small bright spot (where the object should be made to pass), thereby eliminating the need for the object to be half the diameter of the lens. An alter native is to place an aperture over the photoreceiver lens in order to reduce its diameter. Detecting small objects typically requires direct scan.

Because direct scan does not rely on the reflectiveness of the object to be detected (or a permanent reflector) for light to reach the photosensor, no light is lost at a reflecting surface. Therefore, the direct-scan technique lets you scan farther than reflective scanning.

Direct scan, however, is not without limitations. Alignment is critical and difficult to maintain where vibration is a factor. Also, with separate light source and photoreceiver, there is additional wiring, which may be inconvenient if the application is difficult to reach.

REFLECTIVE SCAN

ill. 12-15. Reflective scanning. At A, a permanent, fixed reflector is used. At B, the object itself reflects light from the source.

In reflective scan the light source and photoreceiver are placed on the same side of the object to be detected. Limited space or mounting restrictions may prevent aiming the light source directly at the photoreceiver, so the light beam is reflected either from a permanent reflective target or surface, or from the object to be detected, back to the photoreceiver. There are three types of reflective scan: retroflective scan, specular scan, and diffuse scan.

RETROREFLECTIVE SCAN

ill. 12-16. Retroreflective scanning. The source and sensor are in the same enclosure. A tri-cornered reflector makes alignment less critical than a flat reflector.

With retroreflective scan, light source and photosensor occupy a common housing. The light beam is directed at a retroreflective target (acrylic disc, tape, or chalk) which returns the light along the same path it was sent. Perhaps the most commonly used retro target is the familiar bicycle-type (tricorner) reflector. A larger reflector returns more light to the photosensor and thus allows you to scan farther. With retro targets, alignment is not critical. The light source/photosensor can be as much as 15 degrees to either side of the perpendicular to the target. Also, since alignment need not be exact, retroreflective scan is an excellent way to counteract vibration.

Retroreflection from a stationary target normally provides a high signal ratio as long as the object passing between scanner and target is not highly reflective and passing very near the scanner. Retroreflective scan is a preferred technique to detect translucent objects and assures a higher signal ratio than is obtainable with direct scan. With direct scan the “dark” signal may not register very dark at the photosensor because some light will pass through the object. With retroreflective scan, however, any light that passes through the translucent object on the way to the reflector is diminished again as it returns from the reflector.

Another way to use retroreflective scan is to apply retroreflective tape or chalk coding to cartons or other items that must be sorted.

Retroreflective scan can normally be used at distances up to 30 feet in clear air conditions. As the distance to the target in creases, a larger retro target should be used to intercept and re turn as much light as possible.

Single-unit wiring and maintenance are secondary advantages of retroreflective scanning.

SPECULAR SCAN

The specular-scan technique employs a very shiny surface, such as rolled or polished metal, shiny plastic, or a mirror, to reflect light to the photosensor.

With a shiny surface the angle at which light strikes the reflecting surface equals the angle at which it reflects from the surface. positioning of the light source and photoreceiver must be precise (mounting brackets that fix the light source/photoreceiver relation ship are available), and the distance of the reflecting surface from the light source and photoreceiver must be consistently controlled. The size of the angle between light source and photoreceiver deter mines the depth of scanning field. With a narrower angle there is more depth of field. With a wider angle, there is less depth of field. In a fill-level detection application, e.g., this means that a wider angle between light source and photoreceiver enables you to detect fill level more precisely.

Specular scan can give a good signal ratio when required to distinguish between shiny and nonshiny (matte) surfaces, or when using depth of field to reflect selectively off shiny surfaces of a certain height. When monitoring a nonflat shiny surface with high and /or low points that fall outside the depth of field, these points will appear as dark signals to the photosensor.

DIFFUSE SCAN

Non-shiny surfaces such as kraft paper, rubber, and cork reflect only a small amount of light directly. Light is reflected or scattered nearly equally in all directions. In diffuse scan the light source is positioned perpendicularly to a dull surface. Emitted light is reflected back from the target to operate the photoreceiver. Because the light is scattered, only a small percentage returns. Therefore, scanning distance is limited (except with some high-intensity modulated LED controls), even with very bright light sources. it's often difficult to get a sufficient signal ratio with diffuse scan when the surface to be detected is almost the same distance from the sensor as another surface (for instance, a nearly flat or low- profile cork liner moving along a conveyor belt). When you are trying to distinguish between the reflection from two unlike surfaces, signal ratio can be improved considerably where the unlike surfaces have contrasting colors.

Diffuse scan is used in registration control and to detect material (corrugated metal, e.g.,) with a slight vertical flutter, which might prevent a consistent signal with specular scan. Alignment is not critical in picking up diffuse reflection.

COLOR DIFFERENTIATION

In distinguishing color, as in registration mark detection, contrast is the key. High contrast (dark color on light, or vice versa) provides the best signal ratio and control reliability. Therefore, if possible, plan early to use bright, well-defined, contrasting colors in your operation.

Diffuse scan is normally used to detect color change. The chart gives some of the common color combinations that must be distinguished in registration control, plus the most suitable type of photosensor and scan technique.

When the background is clear (transparent), the best method is to detect any color mark with direct scan. When the background is a second color, contrasts such as black against white usually assure sufficient signal ratio (difference between dark and light signals) to be handled routinely with diffuse scan. Red, or a color that contains much red pigment (yellow, orange, brown) on a white or light background is a special case. You should use a photoreceiver with a cadmium sulfide (CdS) cell to detect red marks because it makes red appear dark on a light background.

A retroreflective scanner with a short-focal-length lens (but without a retro target) can be used to detect registration marks. it's placed near the mark and is actually used in the diffuse-scan technique. If you use a retro scanner to detect marks on a shiny surface, cock the scanner somewhat off the perpendicular to make certain you pick up only diffuse reflection. Otherwise the shiny surface of the mark could mirror-reflect so brightly it would overcome the dark signal a CdS cell normally gets from red. This would mean a light signal from both background and mark. In detecting colors a rule of thumb is to use diffuse (weakened) rather than specular (mirror) reflection.

SENSITIVITY ADJUSTMENT

Most photoelectric controls have a sensitivity adjustment to determine the light level at which the control will respond.

Conditions that could require the sensitivity to be adjusted to less than fully clockwise (maximum) include:

- Detecting translucent objects

- High-speed response

- High cyclic rate

- Line voltage variation

- High electrical noise atmosphere

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