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Fluorescents

The Fluorescent lamp is a useful compromise between competing factors not evident, or even significant, outside Machine Vision. The 9 important characteristics below should be taken into account for successful integration of Fluorescent lamps in Machine Vision designs.


Advantages in general:

✔ The Fluorescent lamp is a rugged, linear light-source that satisfies a multitude of lighting requirements for Machine Vision

•  Without complex reflectors, filters, or lenses
•  Without the heat control risks that often accompany other lamp types

✔ Fluorescents provide a soft-light ideal for Electronic Imaging
✔ When driven at high-power, Fluorecents generate a desirable light-field with:

•   A diffused source    •   Spectral Stability    •   Filter-free simplicity

•   Photopic purity    •   Excellent field-depth    •   Brilliant output

✔However, the successful use of Fluorescent Lamps is dependent upon precise, electronic control – with MERCRON's signature     regulation feature – which assures light that is continuous, ripple-free, and of constant brightness.


Due to the wide variety of Flourescent lamps, in terms of size and output, we recommend familiarizing yourself with important terms and notations – many of which can be found on our SPECs page.


Note: IF Fluorescents are the lamp of choice, seriously consider the advantages of Fluorescent apertures in particular.


IMPORTANT FLUORESCENT LIGHT CONTROL CONSIDERATIONS

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1) ACOUSTIC RESONANCE (Bubbles in the Light)

Fluorescent light-sources driven at high frequency have a tendency to resonate at acoustic frequencies. The smaller the lamp diameter, the greater the risk of resonance. The condition might come and go as a function of lamp current, gas temperature, lamp age, etc.

MERCRON believes that the risk of resonance is extremely great when a lamp with a diameter less than a T5 (5/8") is used. In many cases, the resonance is not visible to the human eye, but is easily seen by the camera.


To check for acoustic resonance...

Look just past the lamp and roll your eyes, keeping the lamp in view with peripheral vision.

If resonating, the lamp will appear to be a single row of balls - or bubbles - of light which are moving along the lamp. These bubbles might come and go, speed up or slow down, and might appear only on one end.

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2) COLOR TEMPERATURE STABILITY

Essentially, a fluorescent lamp is a mercury lamp that is doped with several phosphors for color. Both the mercury output-spectrum and the phosphor output-spectrum are visible in the output light. As the lamp ages, it is the phosphor that decays, changing the combined output spectrum.

The resultant color-temperature change follows the aging pattern (See Lamp Life, above) changing most rapidly when the lamp is new and slowing as the lamp ages. The temperature of the lamp affects the efficacy of the phosphor, so that a change in lamp temperature changes the color temperature. This is a good reason to keep the lamp temperature constant BOTH over time AND along the length of the physical lamp.

In recent years the Fluorescent lamp's universe has been flooded with high performance phosphors to market very efficient replacements for household tungsten lamps. The laptop computer market has also forced the production of this type of lamp. The problem is that some of the phosphors used are so sensitive to temperature that their use in Machine Vision lighting is risky. So new designs, especially T5's and smaller lamps, should be checked for color temperature stability.


To test for this...
• Ignite a cool lamp and hold your thumb on the lamp surface while the lamp warms to operating temperature
• When the lamp is warm, remove your thumb from the lamp and observe the surface uncovered
• IF it is a different color from the surrounding lamp (e.g., pink), RECONSIDER using this lamp.

The test shows that the several phosphors in the lamp respond very differently to a small change in surface temperature. This lamp will be very hard to regulate properly because the output spectrum will change radically to changes in surface temperature.

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3) ENVIRONMENTAL REQUIREMENTS


Fluorescent light-sources are unique in that they operate at - or near - room temperature; consequently, they are very dependent on their environment for proper operation.

Fluorescent light-sources operate best when the body temperature of the lamp is kept at 105° F. The closer the lamp's common body temperature is to 105°, the better efficacy and lamp life.

The operator naturally expects the output of the fluorescent light-source to be linear or flat along its length, but the natural heating of the lamp will defeat the linear output-profile unless the lamp is deliberately cooled to restore both linearity and efficacy.

Since the electrodes are in the lamp's ends, the ends get hotter than the lamp's center. When the lamp gets hot, the output from the phosphor near the end of the lamp will produce less light than the phosphor nearer the center. This results in a "convex" light-profile rather than the linear or flat output-profile ordinarily expected.


To counter this effect...

The ends must be cooled more than the center. A good way to do this is to force cool air over each end of the lamp and exhaust it at the lamp's center.

Forced-air cooling is especially recommended for lamps

1) with small body diameters
2) with high output filaments: HO's
3) with very high output filaments: VHO's.

Turbulent air is best because it provides good coupling to the lampÂ’s body and causes the body temperature to be the same everywhere - which produces the best linearity.

NOTE: It is not sufficient to simply use a powered exhaust to evacuate a lamp box. Injected turbulent air is necessary to provide universal cooling to the lampÂ’s body.

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4) FILAMENTS


There are 3 basic filament-types in Fluorescent lamps...

1) Rapid Start filaments: These filaments are expected to be electrically-powered continuously during lamp operation. They are usually operated at 6-volts.

2) Preheat filaments: These filaments are electrically powered prior to lamp ignition and are not powered during lamp operation. Ballast current will usually drive these filaments at 8-to-10 volts.

3) Instant start, (single contact / cold cathode) filaments: These filaments are NOT electrically powered at all and depend on "anode bombardment" to become heated.

Usually, high-frequency operation of the fluorescent lamp does not provide enough anode bombardment for proper heating of this type filament. For this reason, MERCRON does not recommend the use of "Instant Start" (or cold-cathode or single contact) Fluorescent light-sources in Machine Vision lighting applications.


MERCRON light controllers are usually made for rapid start filaments. However, many T8 and T5 lamps will be found with preheat filaments, and oftentimes differences between the two begin to blur.

Since many different types of filaments are available, MERCRON recommends testing your light-sources to determine which type of filaments your lamps actually have - especially when dealing with custom lamps.

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5) LAMP LIFE


Published lamp life specifications are only half-life values ;i.e., half of the lamps will no longer function at the listed lifetime, while the other half that still do work are only half as bright as they were originally.  In Machine Vision applications where lumen maintenance is so important, usable lifetime is much less:

In the first 200-hours, the lamp will lose 15% of its efficacy.
In the next 800-hours, a 5-to-10% decline will occur.
In the next 1000-hours, another 5-to-10% loss will occur.

To preserve a constant output for a predictable number of hours, it is necessary to operate the lamp at less than full output so that a constant output can be obtained over the useful life.

Assuming a 20% decline for the first 100 hours of operation and allowing 10% for power line variation...

• Setting a new lamp at 70% of maximum initial output will provide about 1000 hours of constant use
• Setting the lamp at 60% of maximum initial output is recommended, if 2000 hours of use is needed
• Setting the lamp at 50% of maximum initial output will permit about 6 months of useful life with constant light level

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6) LAMP-OPERATING ARC VOLTAGE (Not filament voltage)


The operating voltage of a Fluorescent lamp is affected by lamp length and lamp diameter.

But first, recall: lamp diameters are classified in terms of "T" which equals 1/8" i.e., a T8 lamp = 1-inch in diameter, and a T12 = 1.5-inch diameter. Since smaller lamp diameters require proportionally greater operating voltage, a T8 lamp will require 12/8ths more voltage than a T12 lamp of the same length.


All MERCRON Lamp Controllers of 600 mA or greater are rated in terms of T12 lamps. Therefore, a MERCRON light controller - capable of driving 120" of T12 lamps, can regulate only 80" of T8 lamps. This is a handy formula to know when ordering a MERCRON Light Controller.

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7) PLASMA INSTABILITY (Snakes)


New lamps will sometimes display a winding arc that sweeps around inside, causing the lamp's brightness to pulsate on the lamp's surface. This is a so-called "snake", caused by impurities in the gas charge in the lamp.

To remove these snakes...

• Operate the lamp at full power for two hours; this is a "gettering" process.
• Allow the lamp to cool, then turn back on.
• With a MERCRON Light Controller, take the power to minimum, i.e., idle and look for snakes.

• IF the lamp is cured, no snakes will occur - even at idle.
• IF snakes are still present, DISCARD the lamp; the impurities cannot be corrected.

Note: When lamps are very cold, snakes during warm-up is a common occurrence. If a lamp cannot generate enough power to fully warm-up, use a clear plastic lamp "jacket" to help retain enough heat to achieve operational temperature.


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8) THERMAL RUNAWAY


Thermal runaway is a condition that occurs because the ability of the lamp's phosphor to generate light declines as the lamp temperature increases. It can occur either by insufficient heat removal or by normal lamp-aging.

Suppose that a Fluorescent light-source was installed in an environment that did not completely dispose of the heat generated by the lamp. As the lamp’s temperature climbed past 105º F, the light output would decline unless more power was applied to the lamp to maintain the desired light output; but - the extra power would further heat the lamp resulting in less light...so on and so on. This thermal runaway condition occurs when an increase in power results in a decrease in light.


Causes of Thermal Runaway


1) Ultimately each lamp installation is capable of disposing of a limited amount of heat; which determines the maximum power at which the lamp can operate. Operation above that power / heat-level will always result in thermal runaway - which actually results in less light.

2) Normal lamp-aging can also result in a thermal runaway condition. A new lamp can deliver the necessary light with relatively little power, perhaps satisfying the heat limits of the installation. As the lamp ages more power is applied to maintain the light level. If the increased power exceeds the heat limits of the installation, thermal runaway will occur.


How to Identify and Deal with Thermal Runaway

In this case, the All's Well circuit in the MERCRON Light Controller informs the operator when the lamp controller can no longer maintain the required light-level. If the All's Well signal is the result of a thermal runaway, the remedy is either an improved cooling system for the light-source, or possibly - more frequent lamp replacement.


THERMAL RUNAWAY is also a primary idiosyncrasy of high-pressure Sodium light-sources. Naturally, it has been addressed by MERCRON's Illumination Control technology.


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9) ILLUMINATED LENGTH


Both the lamp's diameter and the type of sockets used affect the usable illuminated length. Actual lamp-length is usually measured out-to-out, including the sockets.


•BiPin sockets are about ¼" thick, and are used with Standard Output, T8 & T12 lamps (1"& 1.5" in diameter respectively). These light-sources are cut slightly short to account for the sockets. In other words, an F40 /T12 lamp, expected to be 48" long, is actually only 47.25" long so as to fit in a 48" bay with its sockets.

Fall-off allowances need to be taken into account also. Fall-off in lamp light invariably occurs near the ends of a Fluorescent lamp. Therefore, 4-lamp diameters should be allowed on each end of a Fluorescent using either mini or medium biPin sockets.


Fall-off Allowances - for Bi-Pin sockets

An F40/T12 lamp - 48" long - will lose 6" on EACH end of the lamp (1½ x 4), therefore, can be used to scan a web only 36" wide
A T8-lamp would lose 4" on each end (1" x 4); and a T5-lamp would lose 2.5" on each end (5/8" x 4).


•RDC sockets are about 1½" thick, and are used with HO or VHO output, T12 lamps; i.e., an F48/T12/VHO lamp is actually only 44½" long.


Fall-off Allowances - for RDC sockets

Lamps with RDC sockets will lose 5-lamp diameters on each end as far as illuminated length is concerned.

Note: RDS sockets are similar to RDC sockets, but are not frequently used in high-frequency Fluorescent light-source applications.