On 2/21/2018 12:21 AM, Jeff Liebermann wrote:

Also from several revious rants, if you draw a 1.13meter (44.5inch)
diameter circle on the wall, and shine your bicycle head light or
flashlight on the wall until it fills the circle, the lux meter will
read directly in lumens. It works because the area of the circle is 1
square-meter and:
1 lux = 1 lumen/sq-meter
If you light produces a non-circular spot on the wall or an uneven
pattern, just guess(tm) until you have a spot that looks roughly like
1 square-meter. No warranty for accuracy expressed or implied.

You need to build an integrating sphere.
http://forums.mtbr.com/lights-diy-do-yourself/diy-integrated-sphere-780122.html

Or buy one. http://www.pro-lite.co.uk/File/general_purpose_spheres.php