bLu vs. tLu: IS confirms 65% conversion factor

js

js

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The time has come . . .

The time has come to talk of many things --but not in this case of cabbages and Kings and sealing wax and why the sea is boiling hot and so on,

but rather the time has come to talk of the CPF established 65 percent rule of thumb for converting bulb lumens from published specs and re-rating formulas, into torch lumens, or in other words, into lumens out the front of the light.

And the time has come for those of us who have used and defended the 65 percent figure all along to say a very loud and obnoxious "nya nya na nya na! I told you so!"

OK. Just kidding about that last part. But seriously, it does feel pretty good to find out that your back of the envelope calculations and amateur estimations turned out to be correct after all.

Now, I was one of the people who inherited the 65 percent figure. I did not derive it myself, but rather was given it by Ginseng, who suggested that it was more or less the best we could hope for and was in his experience as accurate as possible given our limited measurement capabilities. I am told that PaulW was the first person to come up with this rule, and also that NewBie/Jarhead did some calculations and basically said "Yup. That's about right."

But however that may be, I came to uphold and believe in the 65 percent transmission efficiency figure for lamps in reflectors in flashlights. It seems crazy at first glance that so much light would get lost, but there it is. It does. And after a lot of experience with incan modding and comparing lights and outputs, I no longer doubted it, but rather spent too much time and effort going around CPF trying to get people to start comparing apples to apples instead of to oranges.

Because, you might ask, "why use torch lumens at all? Why don't we all just talk bulb lumens?" and the reason is that SureFire lights are all quoted in terms of torch lumens, and the SF M6 seems to be the Lion whose tail is always being pulled. People just LOVE to talk about their 1,234 lumen Mag85's, and feel very cool when thinking that an M6 is only 500 lumens.

Now, I've given up my official position as CPF lumens ogre. I no longer care if people want to quote their bulb lumens and yet compare them to the torch lumens of other lights. More power to them! Those who know, know. And those who don't probably don't care anyway. And the whole thing just wastes my energy.

However, I still get sucked into arguments and most recently this happened in LuxLuthor's thread L2==>M4==> ? M6 Help a new member right around post #38 with Luna and his badgering about the 65 percent figure. (Not that I was faultless in these exchanges--far from it; check out the actual exchanges and judge for yourself).

So, now to the point --to wit, the evidence:

A smooth, vacuum aluminized reflector with a Welch Allyn 01274 lamp potted into it was sent to an integrating sphere to be measured for total beam lumens, or MSCP (=total lumens/4 pi). This lamp is rated at 553 lumens at 7.2 volts. 7.2 volts was applied at the pins and 391 lumens was measured in the sphere.

391/553 = 70.7 percent.

However, this was the lamp module only. Add in a lens and angular losses due to the lip of the reflector being recessed down into the head of a light and you probably have at least another 5 or 6 percent. Plus, if you consider that an OP or stippled reflector means more internal reflections of the light versus a smooth reflector, then you maybe have even more losses, but how much I couldn't say. But in any case, you end up with something close to --wait for it--

65 PERCENT

Taking a step back, I will readily admit that this is only a single piece of integrating sphere evidence. However, it still carries a lot of weight even so.

What really annoys me about all of the recent turmoil over this is the notion that if you don't have an IS, you can't say anything at all. I believe that our ingenuity and inventiveness here at CPF have proven otherwise. From the lumens estimator box, to our very own PaulW :bow: :bow: :bow: and his ceiling bounce tests, we have shown that there really is something in between an arbitrary, off the cuff uninformed random guess, and hard numbers from an integrating sphere.

So it's time for people like myself and Ginseng and PaulW and bwaites to pat ourselves on the back and do a little victory dance and (maybe) say "I told you so."

I would never do this, of course, being far too mature. *cough* I told you so! *cough*

Hmmm? What? Oh, no, I didn't say anything. Just a bad cough. Sorry. :devil:
 
Last edited:
G

Ginseng

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Well, at least for me it feels like a huge monkey has just been knocked off my back. PaulW was indeed the first person to present data suggesting the 65% figure and I have my PM's from several years ago to prove it. So to Paul, I would say that "well done!"

Wilkey
 
G

Ginseng

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Oh, and Jim, your tenacity and rigor has never ceased to amaze me. Nicely done.

Wilkey
 
bwaites

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Ginseng!!!

The master returns!!!

Very nice work Jim!!

Nice to have the empirical data to back up the long standing subjective evidence that we have been using.

Couldn't really be much better or closer.

I think flashlightlens.com quotes transmission as 98%, so knock of 2-3% and then an amount as necessary for the recess and js's numbers look close to perfect!!

65% is so close as to be the de facto standard!!


Bill
 
PaulW

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Bill. Yeah, it's fun to see measurements support previous work. That would happen sometimes in my lab classes, although I have to admit there was a bit of fudge in my lab notebook in those days.

I hadn't thought that the 65% figure was a big deal. But you've stuck with the message. In reading some of those threads that discuss this, I learned that the figure can be important in estimating how much real light a planned project will emit. So it's a useful number for those that are willing to use it.

Wilkey. Great to see a post from you. I'm glad you're dropping in every now and then. I think of you and the Aurora2 often.

I haven't looked through those old posts but, as I recall, I never did glom onto the 65% number; I just presented raw data showing that SureFire lumens and calculated bulb lumens were different. I think it was Bill and Jim who gave life to the difference and the number itself.

Paul
 
KevinL

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Ginseng :thumbsup:

js: thanks for the proof. My take these days is that I let people believe whatever they believe. We haven't even gotten to talking about Surefire's ratings and their midpoint voltage, which they are rated for. A Surefire with fresh cells or even lithium ion cells puts out a lot more light than it's given credit for.
 
G

Ginseng

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PaulW said:
Bill. Yeah, it's fun to see measurements support previous work. That would happen sometimes in my lab classes, although I have to admit there was a bit of fudge in my lab notebook in those days.

I hadn't thought that the 65% figure was a big deal. But you've stuck with the message. In reading some of those threads that discuss this, I learned that the figure can be important in estimating how much real light a planned project will emit. So it's a useful number for those that are willing to use it.

Wilkey. Great to see a post from you. I'm glad you're dropping in every now and then. I think of you and the Aurora2 often.

I haven't looked through those old posts but, as I recall, I never did glom onto the 65% number; I just presented raw data showing that SureFire lumens and calculated bulb lumens were different. I think it was Bill and Jim who gave life to the difference and the number itself.

Paul

Hi Paul,
I actually carried out the final operation of calculating ratios from the data that you presented. So you're right, you did not publish the figure but it was just one step away from your raw numbers. You're also right that it is a useful figure for practical purposes. However, some people will always prefer to quote "crank horsepower" rather than "wheel horsepower" and that's fine, as long as the basis for comparison is equivalent.

BTW, let's get together for sushi sometime. I've had more philosophical investigations and I'd love to hear your perspective on a few things. :)

KevinL,
Wow, your ROP as become a legend! Very well done my friend.

Bill,
I see the USL is still trooping along. Man, you've got a lot more stamina than I have, that's for sure.

Wilkey
 
NewBie

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js said:
Now, I was one of the people who inherited the 65 percent figure. I did not derive it myself, but rather was given it by Ginseng, who suggested that it was more or less the best we could hope for and was in his experience as accurate as possible given our limited measurement capabilities. I am told that PaulW was the first person to come up with this rule, and also that NewBie/Jarhead did some calculations and basically said "Yup. That's about right."


Wow, you actually remember my post? OMGOSH!

What I get a kick out of these days is folks using the 25C die temperature lumen ratings on LEDs, and then claiming that on their flashlights, for lumen numbers....

Very nice js, thanks for doing the hard work.
 
js

js

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Hey everyone, thanks for posting!

If anyone wants to post the links to some of the groundbreaking threads (e.g. NewBie and PaulW and Ginseng), that would be great.

One thing I also wanted to bring up was something PaulW said to me at one point: he said that in his opinion the 65 percent figure was probably more reliable than the re-rating formulas themselves. And I think that is a very insightful observation.

In this case, the lamp module being tested was being driven AT exactly the rated voltage, so the re-rating formulas don't come into play. But when you over- or under- drive a lamp relative to the manufacturers ratings, then you must use the re-rating formulas. An ohmic approximation is just plain wrong. A heated filament is just about THE classic non-ohmic resistive element. It is the proverbial non-ohmic resistor, in other words. The steady state resistance is a function of the drive voltage, and is NOT constant, not even over a smallish range of voltages. Obviously, if you take a small enough range, then you can use an ohmic approximation, but that range is too small to be useful in modding work.

So you must use the re-rating formulas. These are as follows:

Ir = (Va/Vd)^0.55 * Id
Cr = (Va/Vd)^3.5 * Cd
Lr = (Vd/Va)^12 * Ld

the sub r and sub d mean "re-rated" and "design". I is current, L is hours of filament life, and C is Lumens or Mean Spherical Candle Power (MSCP), which is Lumens / 4 pi, or about equal to lumens / 12.5. If you use Lumens on one side of the re-rating forumla, you must use lumens on the other side.

OK. So take the ever popular Welch Allyn 01111-u, which by the way, was first used here on CPF by Ginseng, IIRC. It is rated at 6.0 volts, 3.35 amps, 464.95 lumens, and 100 hours of life. At this drive value, the heated filament has a resistance of 1.791 ohms (voltage / current).

Now, push it to 7.2 volts. The re-rating formulas will yield the following:

7.2 volts, 3.70 amps, 880 lumens, 11 hours of life.

However, pretend that the resistance stays constant at 1.791 ohms. The ohmic approximation predicts the following:

7.2 volts, 4.02 amps, ??? lumens.

Now, I have EXTENSIVE experience driving the 1111 at and around 7.2 volts, and I have measured current vs. voltage, and I measured 3.68 amps at 7.2 volts, which, as you can see, is more or less exactly what the re-rating formula predicts.

The ohmic approximation is just plain way off.

Before a certain CPFer attacked me for using the re-rating formulas, I would have thought that they were so well established as to be common ground here on CPF. But I learned that that was not the case. So I have inserted this post to the thread to support the use of the re-rating formulas.

Back to the main point, however: the re-rating formulas are only approximate, and the farther you take them from the design voltage, the less accurate they probably are. ESPECIALLY the hours of life re-rating formula. That has got 12th power in it, and is HIGHLY non-linear. If you are pushing lamps right to the edge, the re-rating forumlas are not going to be accurate enough. For example, both the 1111 at 7.2 volts, and the 1185 at 10.8 volts are supposed to have 11 hours of life (12 in the case of the 1185), but the 1185 on 9 (10.8 volts nominal) cells is MUCH more prone to flashing than the 1111 on 6 (7.2 volts nominal), all other factors being equal, such as the potency and capacity of the cells, and their state of charge. Plus the 1185 has a shorter life span there as well, than the 1111 on 6 cells.

As a rule of thumb I would say that when you go more the 15-20 percent away from design voltage, or if you get into sub 15-20 hour life spans, take your results with a grain of salt, especially the hours of life figure.
 
SilverFox

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I have been involved in testing for a number of years. There is always a bit of a celebration when the theoretical numbers actually match the numbers as tested.

I have always found it easier to celebrate a close correlation, than to comment on the possible sources of inconsistencies.

At any rate, this qualifies as a time for celebration... :D

Tom
 
Lurveleven

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Great work Jim! Good to see theory and reality coming together.

The reflector you were using, can it be compared to a 15mm opening reflector or 8mm opening reflector? I assume the latter so the 15mm opening reflectors most are currently using will probably have less than 65%, how much is not good to say, but I think it will be best to use 65% for both types of reflectors to reduce confusion.

Sidenote:
I have found the geometry of the beam to be more important than the lumen number. The Gen4 TL LA is a fine example of this, it has the same lumens as Gen3, but it gives you a much better light than the Gen3. For flood lights I can see the lumens number may give a very good indication of the performance of the light, but for throwers I think it should be used cautiously.

Sigbjoern
 
Kiessling

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:D :thumbsup:
I love it when my pessimism about true output is correct :green:
bernie
 
L

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Excellent work Jim -

When I first joined CPF, the arrival at 65% light loss was one of the threads (amongst others) I copy/pasted into a word document to keep for reference. The posters names were removed so I don't remember who posted the data, but the analysis was very insightful.

It's nice to have it reconfirmed via another means.

The threads which I find most insightful are ones like this one which expand the knowledge base.

Everyone who has posted in this thread so far, has made contributions which have increased the knowledge of many, me included.

I wish more people would take the time to review the wealth of information that exists here on CPF. Doing a "thread started by" search plugging in any of the above key folks will get you started on a reading adventure.
 
bwaites

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Though I have posted on CPF literally THOUSANDS of times now, I KNOW that it wasn't me who came up with the number!!

However, with all the lights I have looked at in the last 2+ years, I became convinced that the number was close enough that it made sense.

Having read literally EVERY Ginseng and js post, as well as most of Litho123's, PaulW's, Newbies, and Silverfox's, (I still do weekly/biweekly searches of the names to make sure I don't miss any), I became almost certain that the number was so close as to be essentially the same.

The empirical evidence makes me very happy with the data we gathered with much less sophisticated gear and our own eyeballs!

I still am interested to see how the Light Meter testing turns out, but since almost all my lights, (and all that I design) are hotwires it means less than if I was doing the colored light thing. Most of them are reasonably close on the white lights, (with a few outliers, admittedly).

Bill
 
PlayboyJoeShmoe

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I have nothing ground breaking or earth shattering to say here. But....

I THINK a small hole reflector puts a BIT more out the front....

And whatever my M*g85, M*g74 etc. REALLY put out, they are dang bright to me!
 
andrewwynn

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Really nice to have some I.S. data to back up the renowned figure of 65%. I will continue to use the .65 fraction.. and btw.. my Mag100 figures do include the 65% calculation.. the osram 64625 calculates to 3000L at 12.9V.. and i believe that lamp can actually be pushed to closer to 13.5V... which will bring the output up to 3500L.. (5300 bulb lumen!)..

I nowdays most often put the lumen rating like so:

The Mag100 i have sitting next to my keyboard right now is dialed into 13.2V which means it generates 3909/2541 Lumen.

It is extremely confusing to read a description of a lamp that says it is an '880L' lamp or a '700L' lamp.. when those numbers are re-rated to battery voltage and not factoring in resistive losses (the particular "700L" lamp, which is unnamed to offend as little as possible esp. since it's one of my absolutely favorite lights.. ACTUALLY is closer to 491/319 in stock configuration.. and even with a KIU kit it's 547/347.

I did a lot of learning about how much output is affected by slight voltage changes, due to resistance, and was fairly quickly introduced, probably by JS to the conversion factor here..

Thanks for the update and going through the effort to get a known light into an IS.. very nice.. and good to see all these times we've been on the right track.

Oh.. esp. with the fact that SF does rate (conservatively i might add) with torch lumen.. my estimate was over 600L with the SF M6 i was working on.. it had more output than my M66 that is generating 900/585L... at least initially.. maybe the 500L is to consider average over battery life or something.. nice to see a company use conservative values and also to use the 'out the front' output and not the bulb values.

Originally i only used bulb values because that is what is being generated and what can be calculated.. but now, esp. with the hard-evidence.. so can 'torch lumen' as well.. the problem i'm seeing.. is that I believe that the reflector size is directly responsible for a signficant portion of that efficiency.. so.. maybe with a mag reflector it's 65%.. and maybe with the 3" reflector it might be 70.. even 75%.. it would be very nice to know what ratio to use for bigger reflectors.

That said, thanks once more for your solid and continuous input to improving illumination, Jim.

-awr
 
NewBie

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js said:
Now, I was one of the people who inherited the 65 percent figure. I did not derive it myself, but rather was given it by Ginseng, who suggested that it was more or less the best we could hope for and was in his experience as accurate as possible given our limited measurement capabilities. I am told that PaulW was the first person to come up with this rule, and also that NewBie/Jarhead did some calculations and basically said "Yup. That's about right."


If anyone ever finds that thread in the archives, I'd sure appreciate it if you'd PM the link to it!

I sure miss the search function *alot*. There is such an incredible amount of information posted here on cpf that for all practical purposes is now gone (not searchable).
 
js

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AWR,

Oops! Sorry. I actually wondered about that the other day. I'll edit my post to remove that reference. Thanks for your post!

Lurveleven,

The reflector was actually a Gen2/3/4 TigerLight stock reflector, which has an inner bore, IIRC, of about 12 mm, so it is right in between the 9.6 mm Carley stock 1940 bore, and the 15 mm Mag pedestal compatible bore. Oh, and so this was a 2" diameter, smooth surface, fairly tight parabolic reflector.

I'm going to see what kind of a deal I can get on 3 or 4 more tests, and I'm thinking of testing a couple 1940 with two different ring-potted WA lamps installed (1160 at 6.0 volts and 1111 at 7.2 volts probably)--and this would test the 9.6 mm bore--as well as a Carley 2102 3" reflector (with same bore), as well as maybe a SureFire M6 with MN20 installed, or a SF M6-R with MN21, and maybe an A2.

But it all depends on how much it will cost. I'm just toying with the idea at present.

As for the note on beam profile which Lurveleven mentions, I am right on board with that. A lumens total is only one part of what a beam is, and for some beams, it really doesn't tell you the whole story. The Gen4 TL LA is a great example of this. It is "only" about 275 lumens, but I just love that beam. It is far more powerful than the lumens total indicates.

But still, a lumens total is good information to have, and if we're going to talk lumens, we might as well talk sensibly about them (i.e. use the correct conversion factor for a more realistic estimate of what comes out the front of the light.)
 
M

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AWR,
regarding 2" vs 3", I don't think you will get better efficiency with the 3". I think it actually will be less. The reason I think this is because you will have a larger reflector area that means you will have a larger area that "steals" light. But will there be a significant difference, I don't think so, and the difference would also have to be pretty large for anyone to notice in a ceiling bounce test. However, even if the 3" should be less efficient it will stomp all over the 2" when it comes to throw. This is another example that lumen numbers should not be used blindly.
 
G

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I had removed the opening posts of many of my informational posts so that I could update them. Unfortunately, I have found that the time to update them never really materialized. So, over the next few days, I will reinstate those posts. I will also look for Paul's landmark thread. That is, if someone else doesn't find it first.

Wilkey
 

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