On Tue, 30 May 2017 12:42:03 +1000, Clifford Heath
wrote:

What's your experience with 14500 in single AA flashlights?

The Cree XP-E Q5 LED is designed to operate at about 350ma and will
generate about 120 lumens depending on type and bin selection:
http://www.cree.com/led-components/media/documents/XLampXPE-25A.pdf
That's about:
120 lm / (3.6v * 0.35A) = 95 lumens/watt
which is consistent with todays LEDs at room temp. There is no way
the XP-E Q5 LED will dissipate 7 watts or produce 600+ lumens.

Except in Chinese marketing materials

There are a few simple sanity checks for lumen claims. Unfortunately,
they have to be done after you've spent the money.

1. Measure the current drain with an ammeter. Assume that initial
lumens/watt luminous efficacy is about 75 lm/watt for consumer
lighting including some loss through the optics. So, if your
flashlight draws 1.0A at 3.6V, your flashlight will produce
approximately:
1.0A * 3.6v * 75 lm/watt = 263 lumens
You can also look up the specs for the LED, but that doesn't always
work because some lights claim to have the latest greatest high
efficiency LED, but actually use bin fallout, rejects, or
counterfeits.

2. Buy a cheap lux meter on eBay such as:
http://www.ebay.com/itm/381903904643
There are better models, but this one seems to track measurements with
my other more expensive meters quite well. Hang it on the wall and
project a round spot on the wall with the flashlight. Measure and
record the spot diameter and lux values.
1 Lux = 1 lumen/sq-meter
Calculate the area of the spot in square meters. Multiply the
measured lux value by the area and you have lumens. Notice that you
do NOT need to record the distance between the flashlight and the
meter.

Problem. The spot is not uniform brightness across its diameter. To
compensate, I like to measure the hot spot at the center, half way
between the center and the edge, and near the edge. I then calculate
(or guess) an average lux value.

No integrating sphere or tube required.

If you want to make calculations easy, a 1 square meter circle has a
diameter of 1.13 meters (44.5 in). Mark this on the wall and move the
flashlight back and forth until the edges of the spot hits the marks.
The lux meter will then read directly in lumens.

This measurement becomes difficult when measuring odd shaped spots,
such as with bicycle and automotive headlights, but I won't go there.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558