Re-rated bulb life formula

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75RR
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Re-rated bulb life formula

#1 Post by 75RR » Fri Jan 13, 2017 4:13 pm

This includes the formula for calculating the new Rated Life and the calculator steps (for the rusty) to work it out.

Example based on the incandescent tungsten bulb that the Zeiss 12v 60w Lamphouse uses = (Zeiss 38 00 18-2520)
when limiting the voltage to 10v. Designed Rated Life is 100 hours at 12v.


Image

As can be seen the bulb life is extended to just over 1000 hours.

Image is an extract from an article in Micscape Issue 250 - January 2017: http://www.microscopy-uk.org.uk/mag/jan17ind.html
Article contains a graph (last page) that shows the penalty in light output in Candela that limiting output to 10v entails.
Last edited by 75RR on Mon Feb 27, 2017 12:39 pm, edited 2 times in total.
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Re: Re-rated bulb life formula

#2 Post by rnabholz » Fri Jan 13, 2017 9:51 pm

The bonus for running under voltage is pretty darn significant. For some of us using these older stands that rely on some incandescent bulbs that are getting harder and harder to find, it is a real consideration.

Congratulations on another great article and contribution to the community.

Rod

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Re: Re-rated bulb life formula

#3 Post by 75RR » Sat Jan 14, 2017 11:08 am

Many thanks rnabholz :)

As even a small reduction in voltage has very positive consequences on bulb life it is well worth playing
with every setting in order to find a combination that does not rely on max light intensity where possible.

Critical not to go over!

13v ... 35 hours
12.5v ... 58 hours
12v ... 100 hours
11v ... 300 hours
10v ... 1000 hours

Might be a good idea to confirm the accuracy of the voltmeter in the power supply!
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Re: Re-rated bulb life formula

#4 Post by billbillt » Sat Jan 14, 2017 4:05 pm

Hi 75rr,

Thanks for providing the above useful formula.. It is more important info that we can all use..

BillT

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Re: Re-rated bulb life formula

#5 Post by KurtM » Sat Jan 14, 2017 4:11 pm

I first learned about Kohler illumination using "research lamps" under the tutelage of Jim W, who goes by 'apatientspider' on this forum (or did, wonder where he is these days?).

Image

He taught me to use a hardware store corded dimmer switch between the mains and lamp, not only to reduce voltage a bit since full strength was usually too much, but also because giving bulbs a "soft start" increases bulb life. The dimmer works so well I still use 'em. And I can understand 'em too - those formulas are Greek to me.

Dimmer: http://tinyurl.com/z4lwqku
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Re: Re-rated bulb life formula

#6 Post by 75RR » Sat Jan 14, 2017 4:20 pm

Thanks BillT

I just checked my power supply output voltmeter and found it was outputting quite a bit lower that it indicated.
Assuming my no name digital multimeter is accurate then I am getting:

Power Supply........Digital
Voltmeter.............Multimeter
5v ...........................2.8v
6v ...........................3.6v
7v ...........................4.4v
8v ...........................5.3v
9v ...........................6.4v
10v .........................7.6v
11v .........................8.1v
12v .......................10.3v

Think I am going to have to find a way to test the accuracy of my multimeter.
Last edited by 75RR on Sat Jan 14, 2017 4:31 pm, edited 1 time in total.
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Re: Re-rated bulb life formula

#7 Post by 75RR » Sat Jan 14, 2017 4:29 pm

Hi KurtM
He taught me to use a hardware store corded dimmer switch between the mains and lamp, not only to reduce voltage a bit since full strength was usually too much, but also because giving bulbs a "soft start" increases bulb life.
Good advice. My power supply switches off rather like your dimmer does, when it reaches its lowest setting.
I consciously rotate it slowly when powering up and down.
... those formulas are Greek to me.
If you post the voltage rating and the designed rated life of your bulb I can work it out for you.
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Re: Re-rated bulb life formula

#8 Post by billbillt » Sat Jan 14, 2017 5:32 pm

75RR wrote:Thanks BillT

I am going to have to find a way to test the accuracy of my multimeter.
Yes, that is the age old dilemma.. Which meter is correct??.. Perhaps a somewhat crude, but probably accurate standard to use is a common dry cell... Simple dry cells are consistent with their output voltage when new.. You might calibrate your meter with a new 1.5 V battery?..

BillT

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Re: Re-rated bulb life formula

#9 Post by 75RR » Tue Jan 17, 2017 3:57 pm

You might calibrate your meter with a new 1.5 V battery?..
An internet search came up with this method which looks pretty effective ... now all I need is a friend with a quality multimeter!


https://www.youtube.com/watch?v=-izNaaLWG1k
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Re: Re-rated bulb life formula

#10 Post by gekko » Tue Jan 17, 2017 4:47 pm

For what it's worth: Most inexpensive voltmeters measuring AC voltage assume that the waveform is sinusoidal (at least that was the situation that obtained some years ago-- I'm not sure if the new advances in chips have changed the situation: hopefully someone would bring us up to date). If the waveform is not a sine wave, then one needs to use an AC voltmeter that measures rms voltage. The waveform of variable voltage supplied to a microscope lamp depends on the method of varying the voltage. Usually a triac switching circuit (or possibly in newer instruments, pulse width modulation) is used. These do not give a sinusoidal output. So for reasonably accurate measurement, an rms-reading AC voltmeter should be used, otherwise it may be prudent to give the power supply voltmeter more credence, I think.

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Re: Re-rated bulb life formula

#11 Post by 75RR » Tue Jan 17, 2017 5:15 pm

Hi gekko!
... it may be prudent to give the power supply voltmeter more credence, I think.
Will go with that advice! :) Thanks
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Re: Re-rated bulb life formula

#12 Post by rnabholz » Tue Jan 17, 2017 6:07 pm

At least it is putting out lower voltage than the power supply's meter is reporting, so you aren't using up your bulbs.

If I got to pick, that would be my preference, assuming you have enough light to get the job done of course.

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Re: Re-rated bulb life formula

#13 Post by apochronaut » Tue Jan 17, 2017 6:40 pm

There is a colour temp. change with both tungsten and halogen bulbs, if you drop the voltage too low. It can be annoying, affect resolution and affect photographic results.
Halogen need special attention, because low voltage, actually shortens bulb life. The voltage needs to be enough to maintain a threshold temp. in the bulb and if it drops too low , the critical temp. required for the redeposition of tungsten back onto the filament will not happen and the filament will burn out prematurely.
Going much below a 5% reduction, doesn't make a lot of sense, unless you are using the last bulb in existence or the bulb is an unusually rare and expensive one.

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Re: Re-rated bulb life formula

#14 Post by 75RR » Tue Jan 17, 2017 8:42 pm

A couple of quotes and respective links re dimming Tungsten-Halogen bulbs.

Effect of voltage on performance
"Tungsten halogen lamps behave in a similar manner to other incandescent lamps when run on a different voltage. For example, a bulb operated at 5% higher than its design voltage would produce about 15% more light, and the luminous efficacy would be about 6.5% higher, but would be expected to have only half the rated life."

https://en.wikipedia.org/wiki/Halogen_lamp


"In order to more clearly understand the electrical characteristics of tungsten-halogen lamps, the following generalizations can usually be applied: for every 5 percent change in the voltage applied to the lamp, the life span is either doubled or halved, depending upon whether voltage is decreased or increased. In addition, each 5 percent change in voltage is accompanied by a 15 percent change in luminous flux, an 8 percent change in power, a 3 percent change in current, and a 2 percent change in color temperature."

See section on Photometric Characteristics, end of second paragraph:

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Re: Re-rated bulb life formula

#15 Post by einman » Tue Jan 17, 2017 9:30 pm

apochronaut wrote:There is a colour temp. change with both tungsten and halogen bulbs, if you drop the voltage too low. It can be annoying, affect resolution and affect photographic results.
Halogen need special attention, because low voltage, actually shortens bulb life. The voltage needs to be enough to maintain a threshold temp. in the bulb and if it drops too low , the critical temp. required for the redeposition of tungsten back onto the filament will not happen and the filament will burn out prematurely.
Going much below a 5% reduction, doesn't make a lot of sense, unless you are using the last bulb in existence or the bulb is an unusually rare and expensive one.
Interestingly, Leitz research scopes have a voltimeter which is labeled in terms of color temp. There is an indicator where to set the ideal voltage/temp.

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Re: Re-rated bulb life formula

#16 Post by 75RR » Tue Jan 17, 2017 9:49 pm

Interestingly, Leitz research scopes have a voltimeter which is labeled in terms of color temp. There is an indicator where to set the ideal voltage/temp.
Would like to see a photo of that.

After a little search I found what I think might be one on ebay: http://www.ebay.co.uk/itm/E-Leitz-Power ... Sw44BYSB0V

Image
Last edited by 75RR on Tue Jan 17, 2017 11:19 pm, edited 1 time in total.
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Re: Re-rated bulb life formula

#17 Post by apochronaut » Tue Jan 17, 2017 10:20 pm

einman wrote:
apochronaut wrote:There is a colour temp. change with both tungsten and halogen bulbs, if you drop the voltage too low. It can be annoying, affect resolution and affect photographic results.
Halogen need special attention, because low voltage, actually shortens bulb life. The voltage needs to be enough to maintain a threshold temp. in the bulb and if it drops too low , the critical temp. required for the redeposition of tungsten back onto the filament will not happen and the filament will burn out prematurely.
Going much below a 5% reduction, doesn't make a lot of sense, unless you are using the last bulb in existence or the bulb is an unusually rare and expensive one.
Interestingly, Leitz research scopes have a voltimeter which is labeled in terms of color temp. There is an indicator where to set the ideal voltage/temp.
That's a good way. I've seen that on other power supplies too, with a little green section on the voltage scale for optimal. It's kind of a logical anticipation of the eventuality of human error. Some makers took the time to use 6.5v or 6.8v. bulbs with a 6 v. power supply, in the anticipation that most unstabilized power supplies will over volt .
The unfortunate shortened life of tungsten halogen bulbs, when driven with low voltage , is more a consequence with the 100 watt versions of microscopes, than with lower wattage stands. Due to the high wattage on such stands, many people have a tendency to reduce the voltage when using low power objectives and an illumination technique, with low light demand; BF for instance. To increase bulb life, keeping the voltage in the lowest optimal range and using alternative dimming techniques ( neutral density filter, closing down iris(es), will keep the image within a comfortable viewing intensity. Most such instruments have multiple filters built in .

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Re: Re-rated bulb life formula

#18 Post by einman » Tue Jan 17, 2017 10:39 pm

apochronaut wrote:
einman wrote:
apochronaut wrote:There is a colour temp. change with both tungsten and halogen bulbs, if you drop the voltage too low. It can be annoying, affect resolution and affect photographic results.
Halogen need special attention, because low voltage, actually shortens bulb life. The voltage needs to be enough to maintain a threshold temp. in the bulb and if it drops too low , the critical temp. required for the redeposition of tungsten back onto the filament will not happen and the filament will burn out prematurely.
Going much below a 5% reduction, doesn't make a lot of sense, unless you are using the last bulb in existence or the bulb is an unusually rare and expensive one.
Interestingly, Leitz research scopes have a voltimeter which is labeled in terms of color temp. There is an indicator where to set the ideal voltage/temp.
That's a good way. I've seen that on other power supplies too, with a little green section on the voltage scale for optimal. It's kind of a logical anticipation of the eventuality of human error. Some makers took the time to use 6.5v or 6.8v. bulbs with a 6 v. power supply, in the anticipation that most unstabilized power supplies will over volt .
The unfortunate shortened life of tungsten halogen bulbs, when driven with low voltage , is more a consequence with the 100 watt versions of microscopes, than with lower wattage stands. Due to the high wattage on such stands, many people have a tendency to reduce the voltage when using low power objectives and an illumination technique, with low light demand; BF for instance. To increase bulb life, keeping the voltage in the lowest optimal range and using alternative dimming techniques ( neutral density filter, closing down iris(es), will keep the image within a comfortable viewing intensity. Most such instruments have multiple filters built in .
This is very interesting. Something I had not considered. Although I was familiar with the increased bulb life due to lower voltage I was not aware of the inverted relationship between voltage and bulb life with tungsten bulbs. It also answers a question I never took the time to research and that was the need and use of various density filters. I assumed they aided in softening the filament image from high intensity bulbs but never guessed they served to lengthen bulb life as you describe.

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Re: Re-rated bulb life formula

#19 Post by apochronaut » Tue Jan 17, 2017 10:57 pm

tungsten halogen, not tungsten. their normally extended life relies on the recycling of the burnt tungsten back to the filament but only if the temperature and therefore the voltage is high enough.

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Re: Re-rated bulb life formula

#20 Post by 75RR » Tue Jan 17, 2017 11:36 pm

Have been attempting to find the relationship between applied voltage and the commencement and maintenance of the halogen regenerative cycle.

See bold section in text:

"The halogen regenerative cycle can be dissected into three critical steps that are outlined in Figure 2. At the start of operation, the lamp's envelope, fill gas, vaporous halogen, and filament are initially in equilibrium at room temperature. When power is applied to the lamp, the filament temperature rises rapidly to its operating temperature (in the vicinity of 2500 to 3000° C), a sequence of events that also heats the fill gas and the envelope.Eventually, the envelope achieves its stable operating temperature, which ranges from 400 to 1000° C, depending upon the lamp parameters. The temperature differential between the filament and the envelope creates thermal gradients and convection currents in the fill gas. Once the envelope reaches a temperature of approximately 200 to 250° C (depending on the nature and amount of halogen vapor), the halogen regenerative cycle begins."


"Thus for a measured filament temperature of 3000° C, the color temperature is approximately 3080 K. The limit for tungsten color temperature is determined by the melting point, which is just over 3350° C or approximately 3550 K."

If we look then at the graph on the side of the Leitz power supply (see image), it appears that a voltage range of about 6v to 12v (which provides a colour temperature of between 2600 and 3400 K˚) would seem to be due to the filament operating within a temperature range of 2500 to 3000° C which is enough to maintain the halogen regenerative cycle.

The range may in practice not quite reach down to 6v, but it nevertheless would seem to be wider than has been suggested.

Image

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Re: Re-rated bulb life formula

#21 Post by einman » Wed Jan 18, 2017 1:23 am

75RR wrote:
Interestingly, Leitz research scopes have a voltimeter which is labeled in terms of color temp. There is an indicator where to set the ideal voltage/temp.
Would like to see a photo of that.

After a little search I found what I think might be one on ebay: http://www.ebay.co.uk/itm/E-Leitz-Power ... Sw44BYSB0V

Image

Actually I was referring to this on my Dialux:


Image

Notice the vertical line just before the 6V which represents the optimum setting for 3100K.

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Re: Re-rated bulb life formula

#22 Post by 75RR » Wed Jan 18, 2017 1:30 am

Can see the line but can't see the text - I imagine it is that faint writing at the bottom of the window that is hidden by the horizontal parallel lines?

Finding the one I did, even if the hunt for it was due to crossed wires is a big plus. So thanks. :)
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Re: Re-rated bulb life formula

#23 Post by einman » Wed Jan 18, 2017 1:34 am

75RR wrote:Can see the line but can't see the text - I imagine it is that faint writing at the bottom of the window that is hidden by the horizontal parallel lines?

Finding the one I did, even if the hunt for it was due to crossed wires is a big plus. So thanks. :)
Sorry for the bad photo. Yes it is labeled just below the line and reads 3100K.

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Re: Re-rated bulb life formula

#24 Post by apochronaut » Wed Jan 18, 2017 2:48 am

The drop in colour temp., at voltages under about 9, in. A 12 v. system, is quite noticeable. I tend to stay above 10v., at least , for that reason alone but also to keep the bulb healthy. It's hard to see that voltages much lower than that ,especially once the bulb has been used a little are not going lower life.
I have had a couple of premature burnouts, on 12v. halogens. Each time, it was after a fair amount of low magnification work, when I took the easy road and just dimmed the lamp rather than carefully filtering and adjusting. The lamp, in both cases was very dark and internally fouled.
No doubt, once halides get left in the envelope, binding tungsten , the efficiency of the bulb starts to slide. Maintaining low voltages, can only exacerbate, it's condition.

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Re: Re-rated bulb life formula

#25 Post by 75RR » Wed Jan 18, 2017 2:51 am

Each time, it was after a fair amount of low magnification work, when I took the easy road and just dimmed the lamp rather than carefully filtering and adjusting.
How low did you dim? Low magnification requires surprisingly little light.
It would be interesting and useful to have confirmation as to the transition point (actual numbers in volts viewtopic.php?f=18&t=4437&p=40680#p40670 ) of the halogen regenerative cycle, even though halogen bulbs are cheap and plentiful.

The main advantage of the formula lies in extending and particularly in not shortening the life of increasingly rare incandescent tungsten bulbs.
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Re: Re-rated bulb life formula

#26 Post by apochronaut » Wed Jan 18, 2017 12:12 pm

Lowering the voltage, probably down as low as 7 or so. I figured the problem out eventually, There was a lot of blackening on the quartz, way more than with a normal burnout. The microscope has a digital readout for the voltage but I didn't take that much notice, at the time. First bulb was a Riluma(Switzerland) and the second was an Osram( U.K.) but the results were the same.

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Re: Re-rated bulb life formula

#27 Post by gekko » Wed Jan 18, 2017 6:54 pm

For what it's worth, I usually run my 12-V 50-W tungsten halogen lamp at 75% to 90% of rated voltage (according to the microscope's voltmeter). I had it burn out once only, and when I replaced it I noticed no blackening at all. I myself don't care about the color temperature (but maybe I should) because I reset the camera's white balance whenever I change the lamp voltage, and I think the eye is pretty tolerant to relatively small changes in color temperature. If one is interested in identifying minerals using a polarizing microscope, then the color temperature of the light would be critical (same if one were using color film to record the images). When using low power objectives I use partially crossed polarizing filters over the field lens to reduce the light intensity for visual work (and I remove them before taking pictures**). These provide nice, continuously variable illumination without changing the voltage and having to redo the camera's white balance.
Of course I do not regard what I do as "best practice", only that I find it works for me and I find it convenient.
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