How can I measure actual lumen on a flashlight with a Luxmeter? How do I calculate Lux to Lumen?
I recently bought a lux meter from Amazon (http://www.amazon.com/gp/product/B00LYH0QKC?psc=1&redirect=true&ref\_=oh_aui_detailpage_o00_s00) and did some test trying to measure the number of lumen of my SK-68 flash light. I described the way I did below to share with everybody.
I adjusted the zoom of the flash light to make it show a light disk with a diameter of 2 feet. I then measured the lux at the center of the area and near the edge of the light disk. To simplify calculation, I take the average of the lux number at the edge and at the center. Assuming the area has the same lux number as the average, the number of lumen can be computed using the calculator given here.
For example, the best SK-68 I bought a few years ago from DX.com by Sipik has the lux numbers are as follows:
Center: 450, near edge 200. The average is 325 and the area is equal to 3.14 square feet. The calculated number of lumens is about 95.
The one I bought recently from DX.com, which is also marked as SK-68 by Sipik has the number of 380/180 corresponding to 82 lumens.
I understand the result such obtained is approximate. The total lumen really depends on the light distribution. However, I think it should be good enough to give a ballpark number for a hobbyist for evaluation of the flash lights on hand and for comparing different flashlights.
Any comments?