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How to easily measure lumens
On Friday, March 24, 2017 at 10:30:13 AM UTC-7, Frank Krygowski wrote:
On 3/24/2017 2:22 AM, Jeff Liebermann wrote: A properly designed bicycle light or flashlight should have an even and equal brightness distribution across the spot on the wall, but this is rarely the case. A properly designed bicycle light should not have a round spot at all. See http://peterwhitecycles.com/plight.php for some information on headlight optics. Alternately, face your favorite motor vehicle toward a wall, turn on the lights (low beam) and observe the beam pattern. Frank, if you aim a light straight at something it should throw almost a perfect circle. When you aim you bike light up the road it strikes at an angle forming an oblong circle. If you try to form a light in any other manner you cannot have equal illumination within the beam. |
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How to easily measure lumens
On Friday, March 24, 2017 at 11:09:12 AM UTC-7, Sir Ridesalot wrote:
On Friday, March 24, 2017 at 2:22:08 AM UTC-4, Jeff Liebermann wrote: How to easily measure lumens without an integrating sphere. The correct way to measure total light output from a bicycle headlight or flashlight is with an integrating sphe https://www.google.com/search?q=integrating+sphere&tbm=isch https://www.youtube.com/watch?v=mvyptpA-BmY These are not cheap, although they can be home built. There are also other ways to do it (lumen tube): http://s1074.photobucket.com/user/mrsdnf/media/stuff/IMG_3226.jpg.html https://www.youtube.com/watch?v=xOE1ykJ5WAU My method is far from accurate, but good enough for estimating and comparing the light output of flashlights and bicycle headlights. To make it work, you'll need a tape measure and a lux meter. I'm using this one: http://www.ebay.com/itm/381903904643 It's main advantages are that it does auto ranging, has a max hold feature, and is cheap. First, the math: 1 lux = 1 lumen per square meter. That means if I project a circular spot on the wall, with an area of 1 square meter, the indicated brightness in lux equals the approximate lumens output, which can be read directly from the lux meter. Notice that it is NOT important to know the beam width or the distance between the light source and the wall. How big is a 1 square meter spot? Area = Pi * radius^2 For Area = 1 square meter the radius of the spot is: r = sqrt(1/Pi) = 0.564 meters The diameter of the spot is 1.12 meters (44 inches). Find a darkened room with a suitable wall, and put two pieced of masking tape on the wall separated by 1.12 meters (44 inches). Notice that the wall does not need to be flat or painted white. Half way between the two markers, hang the lux meter. To measure, turn on the lux meter and punch the max hold button. This will display and hold the highest reading. Start well back from the wall, turn on the flashlight, and slowly move towards the wall until the edge of the light spot lands on top of the two markers. Turn off the light and read the meter. The meter reading in lux will be equal to the lumens output of the flashlight or bicycle headlight. I bought various flashlights on eBay and tested them at maximum brightness with new batteries. This one claims 5000 lumens but delivers 200 lumens. http://www.ebay.com/itm/322447023467 This one claims 300 lumens but delivers 97 lumens. http://www.ebay.com/itm/391639378962 This one claims 6000 lumens, but delivers 212 lumens. http://www.ebay.com/itm/201457081072 There are plenty of problems with this method. Putting the lux meter at the center of the circle causes the meter to favor lights with hot spots in the middle. A properly designed bicycle light or flashlight should have an even and equal brightness distribution across the spot on the wall, but this is rarely the case. I'm working on a more accurate way to measure and calculate the average light output. Probably, it will be measuring the light in the center and along the edge, and taking an average or estimating the total based on a gaussian light distribution. Or maybe not putting the lux meter in the center of the circle. That's for later. Another error is the color temperature of the light. LED's come in a variety of color temperatures. The lux meter has a different sensitivity at each of these colors where the sensitivity curve follows the sensitivity of the human eye. https://image.slidesharecdn.com/ivanperrepresentationfor24-141008071626-conversion-gate02/95/pls-2014-is-measuring-led-illuminance-with-a-lux-meter-accurate-19-638.jpg Comparing lights with different color temperatures will be a problem. There is also a problem in dealing with the 1 square meter area when the spot is not a perfect circle, but rather an ellipse as in many bicycle headlights. Area = Pi * major_axis_radius * minor_axis_radius This can be easily measured, but will be different for each headlight with an elliptical beam pattern and will therefore be a bit more complicated to measure. I also do not yet have a calibration illuminance standard. Therefore, I don't know how accurate this method might be until after I get or build one. Good luck, have fun, enjoy, and please post some test results for various bicycle headlights. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 Much is made about different methods of measuring bicycle lights. I think the best thig is to asctually try the light in the conditions you ride in. If I'm going to or have to test a light only AFTER BUYING it then I'll just wait until night time, ride to where I need to know how the light works for me and then simply turn on the light. Does it light up the road enough and far enough ahead for me at the speeds I ride? Yes, then I'll keep the light. No, then I'll return it for something better. Cheers If you observe his beams you can see just as I stated - that there are bright and dark spots within the beam so that you can mistake potholes for dark spots in the beam. You should pay closer attention to what Jeff is trying to say and not what you are trying to think. |
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How to easily measure lumens
On Friday, March 24, 2017 at 6:17:30 PM UTC-7, Jeff Liebermann wrote:
On Fri, 24 Mar 2017 13:30:06 -0400, Frank Krygowski wrote: On 3/24/2017 2:22 AM, Jeff Liebermann wrote: A properly designed bicycle light or flashlight should have an even and equal brightness distribution across the spot on the wall, but this is rarely the case. Oops. That's wrong. I didn't think of the vertical gradient. A properly designed bicycle light should not have a round spot at all. See http://peterwhitecycles.com/plight.php for some information on headlight optics. True. A properly designed bicycle headlight should belch more light near the horizon than it does for light pointing downward to compensate for beam spreading at longer distances. When pointed at a vertical wall, the upper part of the spot should be brighter than the lower part. The horizontal light distribution should be even. At least, that's the theory. I've been playing with this cheap light: https://www.rei.com/product/769616/planet-bike-blaze-1-watt-front-bike-light which seems to produce a circular spot with no obvious vertical gradient. I'll see if I can borrow something better and see what it looks like on the wall. Meanwhile, I've been using cheap flashlights, which all produce roughly circular spots and have no need of a vertical gradient (unless one uses it on a bicycle). Even with a vertical gradient and an elliptical spot pattern, it should be possible to determine the average luminance by taking lux readings at the center, and at 0, 90, 180, and 270 degrees, and averaging the values (somehow). A straight line average would probably be sufficient, but I would like to try Gaussian for better accuracy: https://en.wikipedia.org/wiki/Gaussian_beam Alternately, face your favorite motor vehicle toward a wall, turn on the lights (low beam) and observe the beam pattern. https://www.google.com/search?tbm=isch&q=headlight+beam+pattern+on+wall Yep. Brighter at the top than at the bottom and roughly elliptical, but not very well controlled. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 You were correct the first time Jeff. With the tilted beam you get an oblong pattern that shows a wider pattern in the distance and a brighter smaller coverage up close. The photos showing the lighting patterns demonstrate what happens if you try to vary from that. Modern LED lighting is MUCH brighter per watt and so a good round light will give you the best and most understandable coverage. Remember that the rider does a very large part of understanding his pathway through peripheral vision so you cannot have dark and light spots randomly throughout your coverage. |
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How to easily measure lumens
On Sat, 25 Mar 2017 12:32:55 +0700, John B.
wrote: I would have to ask. Is it important to verify the advertising claims? Or is it important to verify that you can see sufficiently well to cycle? Are those the only choices available? How about: 1. Is it important to buy from a vendor that doesn't lie about the performance of their lights? 2. Is it important to verify lumens if the light tends to fade in output as it gets warm and as the battery becomes depleted. 3. Is it important to filter out ridiculous lumens claims so that selecting a light does not become an exercise in buy, try, return, but something else, try, return, ad infinitum[1]. 4. It is important to recognize that there might be a relationship between lumens output and visibility? 5. Is it important to correlate the relationship between lumens and selling price? 6. Is it important to also verify other advertised numbers, such as beam pattern, weight, battery life, and compatibility with available batteries and dynamos? I believe that you're asking me to choose between two extremes, neither of which is useful or optimum. At one extreme, we have the test ride as the sole determination of a suitable headlight accompanied by totally ignoring the data sheet. Yes, trial and error will eventually result in something worth buying, but methinks the process might be shortened if the buyer looks at the numbers first. At the other extreme, we have buying blindly, usually online and solely from the advertised claims. This is become more common thanks to online shopping, where the buy, try, return cycle is not very effective. The problem is that *ALL* the advertised specifications tend to be exaggerated well beyond reason. If online vendors can't handle the buy, try, return cycle, then 2nd best would be honest numbers. There's a third extreme in the form of buying from reviews. Most reviews simply recycle the advertised lumen claims from the manufacturers data sheet. A few reviewers attempt to make comparative measurements. A tiny number borrow an overpriced integrating sphere and actually make measurements. I would think that if the manufacturer or reviewer is willing to buy, rent, or borrow a $20,000 instrument just to deliver one accurate number, they would consider this number rather important. Buying from reviews also has its hazards. If the reviewer makes a procedural error, misinterprets the results, or misrepresents the products, it's quite easy to make a bad decision. In effect, the buyer is purchasing by proxy, where the reviewer actually makes the selection. What I've done is provide a possible procedure needed for mere mortals to verify advertised brightness claims. Whether you choose to use those tools is your decision. If it inspires Chinese manufacturers and resellers to provide honest light output figures, then I have achieved my goal and can then move on to dealing with inflated battery capacity claims. Drivel: [1] Buying a headlight without looking at the numbers is much like buying clothes without looking at the marked sizes and making the choice based on how it feels, looks, or fits. The somewhat standard clothing sizes cover wide ranges of size, fit, body shape, proportions, ethnicity, etc which require a fitting room to make the final decision. While it's still a trial and error process, without looking at the marked sizes, shopping for clothes would be rather tedious and time consuming without first narrowing down the available candidates. One runs the risk of buying something too large or too small (or in lighting too bright no too dim). -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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How to easily measure lumens
On Fri, 24 Mar 2017 14:12:52 -0700, Joerg
wrote: On 2017-03-24 11:13, Sir Ridesalot wrote: On Friday, March 24, 2017 at 2:09:12 PM UTC-4, Sir Ridesalot wrote: On Friday, March 24, 2017 at 2:22:08 AM UTC-4, Jeff Liebermann wrote: Another error is the color temperature of the light. LED's come in a variety of color temperatures. The lux meter has a different sensitivity at each of these colors where the sensitivity curve follows the sensitivity of the human eye. https://image.slidesharecdn.com/ivanperrepresentationfor24-141008071626-conversion-gate02/95/pls-2014-is-measuring-led-illuminance-with-a-lux-meter-accurate-19-638.jpg Comparing lights with different color temperatures will be a problem. It might be a problem, but I don't think so. The light meter follows the color sensitivity of the human eye. Lux (brightness) is also weighted for human eye response. Therefore, I believe that equal brightness (lux) numbers, of different color temperatures, would appear as identically bright to the human eye. I'm not an expert on optics and need to do some remedial reading first. Much is made about different methods of measuring bicycle lights. I think the best thig is to asctually try the light in the conditions you ride in. Yup! This is similar to what I have (2nd part, with diffuser lens) on both bikes: https://www.youtube.com/watch?v=5J9zIGTfgGw After installing them I never looked back. No need to measure any lumens. It simply does the job at night even north of 25mph and makes me very visible to motorists during the day. Only on singletrack I have to keep it below 15mph at night but that's fine. Perhaps we should consider changing the "tech" part of the newsgroup name to "feel"? No need for technology when we have expert opinions and impressions. I seem to recall that "tech" is partly an exercise in quantifying poorly understood phenomenon into repeatable tests and measurements so that meaninful comparisons can be made. I guess not. Does passing a road test now means that you can now throw away all the measurements and numbers? Grumble... I'm disappointed. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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How to easily measure lumens
On 3/25/2017 12:57 PM, Jeff Liebermann wrote:
On Sat, 25 Mar 2017 12:32:55 +0700, John B. wrote: I would have to ask. Is it important to verify the advertising claims? Or is it important to verify that you can see sufficiently well to cycle? Are those the only choices available? How about: 1. Is it important to buy from a vendor that doesn't lie about the performance of their lights? 2. Is it important to verify lumens if the light tends to fade in output as it gets warm and as the battery becomes depleted. 3. Is it important to filter out ridiculous lumens claims so that selecting a light does not become an exercise in buy, try, return, but something else, try, return, ad infinitum[1]. 4. It is important to recognize that there might be a relationship between lumens output and visibility? 5. Is it important to correlate the relationship between lumens and selling price? 6. Is it important to also verify other advertised numbers, such as beam pattern, weight, battery life, and compatibility with available batteries and dynamos? I believe that you're asking me to choose between two extremes, neither of which is useful or optimum. At one extreme, we have the test ride as the sole determination of a suitable headlight accompanied by totally ignoring the data sheet. Yes, trial and error will eventually result in something worth buying, but methinks the process might be shortened if the buyer looks at the numbers first. At the other extreme, we have buying blindly, usually online and solely from the advertised claims. This is become more common thanks to online shopping, where the buy, try, return cycle is not very effective. The problem is that *ALL* the advertised specifications tend to be exaggerated well beyond reason. If online vendors can't handle the buy, try, return cycle, then 2nd best would be honest numbers. There's a third extreme in the form of buying from reviews. Most reviews simply recycle the advertised lumen claims from the manufacturers data sheet. A few reviewers attempt to make comparative measurements. A tiny number borrow an overpriced integrating sphere and actually make measurements. I would think that if the manufacturer or reviewer is willing to buy, rent, or borrow a $20,000 instrument just to deliver one accurate number, they would consider this number rather important. Buying from reviews also has its hazards. If the reviewer makes a procedural error, misinterprets the results, or misrepresents the products, it's quite easy to make a bad decision. In effect, the buyer is purchasing by proxy, where the reviewer actually makes the selection. What I've done is provide a possible procedure needed for mere mortals to verify advertised brightness claims. Whether you choose to use those tools is your decision. If it inspires Chinese manufacturers and resellers to provide honest light output figures, then I have achieved my goal and can then move on to dealing with inflated battery capacity claims. You make some good points above. Two things I'd like to see come out of your experiments: 1) A comparison of your methods results with those of an integrating sphere for several different bike lights. Obviously, it would be good to show your method is reasonably accurate. 2) Some examination of the correlation between lumen output and usefulness as a road lighting bike headlight. (That's as opposed to a "be seen" light.) The point of #2 is that I believe there is far too much emphasis on lumen count. IME a light with relatively low lumen count can do an excellent job of lighting the road, and often do a better job than lights with higher lumen outputs. And IME any headlight that adequately lights the road is perfectly capable as a "be seen" light. -- - Frank Krygowski |
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How to easily measure lumens
On 2017-03-25 10:18, Jeff Liebermann wrote:
On Fri, 24 Mar 2017 14:12:52 -0700, Joerg wrote: On 2017-03-24 11:13, Sir Ridesalot wrote: On Friday, March 24, 2017 at 2:09:12 PM UTC-4, Sir Ridesalot wrote: On Friday, March 24, 2017 at 2:22:08 AM UTC-4, Jeff Liebermann wrote: Another error is the color temperature of the light. LED's come in a variety of color temperatures. The lux meter has a different sensitivity at each of these colors where the sensitivity curve follows the sensitivity of the human eye. https://image.slidesharecdn.com/ivanperrepresentationfor24-141008071626-conversion-gate02/95/pls-2014-is-measuring-led-illuminance-with-a-lux-meter-accurate-19-638.jpg Comparing lights with different color temperatures will be a problem. It might be a problem, but I don't think so. The light meter follows the color sensitivity of the human eye. Lux (brightness) is also weighted for human eye response. Therefore, I believe that equal brightness (lux) numbers, of different color temperatures, would appear as identically bright to the human eye. I'm not an expert on optics and need to do some remedial reading first. I think Sir wrote that. I do not know much about color temps and stuff but have a relative who is an expert in that field (not for lighting but cameras). Much is made about different methods of measuring bicycle lights. I think the best thig is to asctually try the light in the conditions you ride in. Yup! This is similar to what I have (2nd part, with diffuser lens) on both bikes: https://www.youtube.com/watch?v=5J9zIGTfgGw After installing them I never looked back. No need to measure any lumens. It simply does the job at night even north of 25mph and makes me very visible to motorists during the day. Only on singletrack I have to keep it below 15mph at night but that's fine. Perhaps we should consider changing the "tech" part of the newsgroup name to "feel"? No need for technology when we have expert opinions and impressions. I seem to recall that "tech" is partly an exercise in quantifying poorly understood phenomenon into repeatable tests and measurements so that meaninful comparisons can be made. I guess not. Does passing a road test now means that you can now throw away all the measurements and numbers? Grumble... I'm disappointed. Tech does not always mean everything is best described by hard numbers. Take audio noise, for example. Shannon's theorem and measurements may show that information in a certain noisy environment cannot possibly be deciphered yet then people can do it. With light there are just too many factors. Spectral distribution, illumination to the side and slightly up, and so on. For example, a StVZO light which gets stellar reviews in scientific testing can perform horridly on a bicycle with a suspension. In a nutshell, when it comes to lighting I strongly believe there is nothing better than to actually try it out. I have, and I had found a light I really like that way (after a slight mod). -- Regards, Joerg http://www.analogconsultants.com/ |
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How to easily measure lumens
On Sat, 25 Mar 2017 09:57:15 -0700, Jeff Liebermann
wrote: On Sat, 25 Mar 2017 12:32:55 +0700, John B. wrote: I would have to ask. Is it important to verify the advertising claims? Or is it important to verify that you can see sufficiently well to cycle? Are those the only choices available? How about: 1. Is it important to buy from a vendor that doesn't lie about the performance of their lights? 2. Is it important to verify lumens if the light tends to fade in output as it gets warm and as the battery becomes depleted. 3. Is it important to filter out ridiculous lumens claims so that selecting a light does not become an exercise in buy, try, return, but something else, try, return, ad infinitum[1]. 4. It is important to recognize that there might be a relationship between lumens output and visibility? 5. Is it important to correlate the relationship between lumens and selling price? 6. Is it important to also verify other advertised numbers, such as beam pattern, weight, battery life, and compatibility with available batteries and dynamos? Ultimately the only test that makes sense is "does it work", and I would even add the amendment "does it work"+"for me!" James, for one, and probably many others, ride faster than I do and likely need see the road much further ahead than I do so logically they probably need a stronger light than I do and a light that fulfills my requirements may well not suit their needs. Thus a light that fulfils my needs may not fulfil theirs. As for claims by makers or distributors I generally disregard them as I am fully aware that they are intent on selling as much of their product as possible. I am, this moment, looking at a smallish, black, LED flashlight equipped with the usual 3.7V battery that states, in black and white, right there on the body of the light "UltraFire XB 8000W". But that was the cheap one. I have another that says "CREE T6 15000W". I might comment that I did not purchase either of these two, very handy, lights based on the maker's claims or data sheet :-) In fact after reading some claims made by various testers or testing institutes I tend to disregard them to a great extent. I remember one magazine downgrading a Winchester single shot .22 rifle.... because it was a "single shot". Sort of like downgrading a Bi-cycle because it only has two wheels :-) I believe that you're asking me to choose between two extremes, neither of which is useful or optimum. At one extreme, we have the test ride as the sole determination of a suitable headlight accompanied by totally ignoring the data sheet. Yes, trial and error will eventually result in something worth buying, but methinks the process might be shortened if the buyer looks at the numbers first. I'm not asking you to do anything. I am commenting that the ultimate test is "does it work for you"? Regardless of what the data sheet says or does not say. I have from time to time commented on my bike riding in Japan. In the 1960's I was a lowly Airman at an airbase outside of Tokyo and resided (illegally) off base with a convivial young lady. I bought a 2nd/3rd/who knows what/hand bicycle fitted with a bottle generator and a rather anemic incandescent light. I rode that bicycle back and forth every day for a year or more with no problems what so ever. No maker's data sheet, no advertisement. No nothing. Just get on and pedal. Had I purchased a super, dooper, very powerful, magnificent, bicycle light, would I been better off? At the other extreme, we have buying blindly, usually online and solely from the advertised claims. This is become more common thanks to online shopping, where the buy, try, return cycle is not very effective. The problem is that *ALL* the advertised specifications tend to be exaggerated well beyond reason. If online vendors can't handle the buy, try, return cycle, then 2nd best would be honest numbers. There is a very old adage that states "a fool and his money are soon parted" that I have always taken to heart. The only unknown quality item I would even think of buying on line would be from an individual that I trust to tell me the truth, of a specific make that I already know is of sufficient quality to suit my needs. But of course, living in a relatively primitive country I am not restricted to buying on line. I have the advantage of a "whole bunch" of shops that I can visit and compare prices and quality. There's a third extreme in the form of buying from reviews. Most reviews simply recycle the advertised lumen claims from the manufacturers data sheet. A few reviewers attempt to make comparative measurements. A tiny number borrow an overpriced integrating sphere and actually make measurements. I would think that if the manufacturer or reviewer is willing to buy, rent, or borrow a $20,000 instrument just to deliver one accurate number, they would consider this number rather important. Buying from reviews also has its hazards. If the reviewer makes a procedural error, misinterprets the results, or misrepresents the products, it's quite easy to make a bad decision. In effect, the buyer is purchasing by proxy, where the reviewer actually makes the selection. What I've done is provide a possible procedure needed for mere mortals to verify advertised brightness claims. Whether you choose to use those tools is your decision. If it inspires Chinese manufacturers and resellers to provide honest light output figures, then I have achieved my goal and can then move on to dealing with inflated battery capacity claims. Drivel: [1] Buying a headlight without looking at the numbers is much like buying clothes without looking at the marked sizes and making the choice based on how it feels, looks, or fits. The somewhat standard clothing sizes cover wide ranges of size, fit, body shape, proportions, ethnicity, etc which require a fitting room to make the final decision. While it's still a trial and error process, without looking at the marked sizes, shopping for clothes would be rather tedious and time consuming without first narrowing down the available candidates. One runs the risk of buying something too large or too small (or in lighting too bright no too dim). -- Cheers, John B. |
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How to easily measure lumens
On Thursday, March 23, 2017 at 11:22:08 PM UTC-7, Jeff Liebermann wrote:
How to easily measure lumens without an integrating sphere. The correct way to measure total light output from a bicycle headlight or flashlight is with an integrating sphe https://www.google.com/search?q=integrating+sphere&tbm=isch https://www.youtube.com/watch?v=mvyptpA-BmY These are not cheap, although they can be home built. There are also other ways to do it (lumen tube): http://s1074.photobucket.com/user/mrsdnf/media/stuff/IMG_3226.jpg.html https://www.youtube.com/watch?v=xOE1ykJ5WAU My method is far from accurate, but good enough for estimating and comparing the light output of flashlights and bicycle headlights. To make it work, you'll need a tape measure and a lux meter. I'm using this one: http://www.ebay.com/itm/381903904643 It's main advantages are that it does auto ranging, has a max hold feature, and is cheap. First, the math: 1 lux = 1 lumen per square meter. That means if I project a circular spot on the wall, with an area of 1 square meter, the indicated brightness in lux equals the approximate lumens output, which can be read directly from the lux meter. Notice that it is NOT important to know the beam width or the distance between the light source and the wall. How big is a 1 square meter spot? Area = Pi * radius^2 For Area = 1 square meter the radius of the spot is: r = sqrt(1/Pi) = 0.564 meters The diameter of the spot is 1.12 meters (44 inches). Find a darkened room with a suitable wall, and put two pieced of masking tape on the wall separated by 1.12 meters (44 inches). Notice that the wall does not need to be flat or painted white. Half way between the two markers, hang the lux meter. To measure, turn on the lux meter and punch the max hold button. This will display and hold the highest reading. Start well back from the wall, turn on the flashlight, and slowly move towards the wall until the edge of the light spot lands on top of the two markers. Turn off the light and read the meter. The meter reading in lux will be equal to the lumens output of the flashlight or bicycle headlight. I bought various flashlights on eBay and tested them at maximum brightness with new batteries. This one claims 5000 lumens but delivers 200 lumens. http://www.ebay.com/itm/322447023467 This one claims 300 lumens but delivers 97 lumens. http://www.ebay.com/itm/391639378962 This one claims 6000 lumens, but delivers 212 lumens. http://www.ebay.com/itm/201457081072 There are plenty of problems with this method. Putting the lux meter at the center of the circle causes the meter to favor lights with hot spots in the middle. A properly designed bicycle light or flashlight should have an even and equal brightness distribution across the spot on the wall, but this is rarely the case. I'm working on a more accurate way to measure and calculate the average light output. Probably, it will be measuring the light in the center and along the edge, and taking an average or estimating the total based on a gaussian light distribution. Or maybe not putting the lux meter in the center of the circle. That's for later. Another error is the color temperature of the light. LED's come in a variety of color temperatures. The lux meter has a different sensitivity at each of these colors where the sensitivity curve follows the sensitivity of the human eye. https://image.slidesharecdn.com/ivanperrepresentationfor24-141008071626-conversion-gate02/95/pls-2014-is-measuring-led-illuminance-with-a-lux-meter-accurate-19-638.jpg Comparing lights with different color temperatures will be a problem. There is also a problem in dealing with the 1 square meter area when the spot is not a perfect circle, but rather an ellipse as in many bicycle headlights. Area = Pi * major_axis_radius * minor_axis_radius This can be easily measured, but will be different for each headlight with an elliptical beam pattern and will therefore be a bit more complicated to measure. I also do not yet have a calibration illuminance standard. Therefore, I don't know how accurate this method might be until after I get or build one. Good luck, have fun, enjoy, and please post some test results for various bicycle headlights. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 This is the first time I've ever seen a thread about lighting speak about the taboo truth, that the eye does _not perceive brightness based solely on raw lumens, but also by the chromaticity and evenness of the beam. When designing the Oculus, I made identical lights of the same lumen output (+/- Cree's 7% standard tolerance for production LEDs) using identical LEDs but from from different bins, in order to test what color temperatures people considered brighter. Without fail, the cool/cold white on the ANSI curve is seen as brightest. Above or below the line, where the output may appear with a blue or red tint, is not seen as bright, nor as easy on the eye, despite being the same overall color temperature as the bins right on the ANSI curve. Lux is a frivolous measurement, because it can only measure how bright the light is at a given point. There is not a spec for the distance lux should be measured at. This a light claiming 80 lux is a meaningless value. See the Bush and Muller website for useless irrelevant overstated claims made in lux, without any distance given, nor lumen for their lights. Evenness is the most often overlooked element of usable visibility. Bright spots cause the eye to stop down, lowering the sensitivity for everywhere dimmer. The Oculus beam in the lights I currently sell actually has a slight measurable dimmer region where other lights would have a bright center spot. The evenness all around makes the eye able to see better everywhere in the beam, because the eye doesn't need to stop down to filter overly bright spots. Other ray trace images from other lights all have hot red areas. My beam has none, yet also has very high efficiency. Simulations in Zemax showed 1 watt of light into the optic results in .97 watts out. That's independent of color temperatures or wavelengths, based only on ray traces. That DIY lumen tube can be improved greatly by spraying the inside with a bright white reflective coating. Try Rustoleum white gloss protective enamel. Still not close to an accurate measurement. Some light test labs use stopped down filters with spheres too small to get a good measurement of higher powered beams. Cutting the light in half doubles the tolerance of the measurement. Bike light type beams are most accurately measured in an integrating tube, not an integrating sphere. I got some free testing from Gamma Scientific at the Strategies in Light LED trade show. Much better accuracy at capturing projected beams. Spheres are best for a omnidirectional beam such as a light bulb. |
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