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:
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.
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.
[...]
--
Regards, Joerg
http://www.analogconsultants.com/