How do I measure Lux, Lumens, etc?

I see many flashlight dealers advertising in Lumens output, some in Lux. Lumen is typically meaning Lumen per square foot right?

I wanted to know what I needed to test, how to test.

Also web-search shows that Lux to Lumen per sq.ft. is divide by .09 (and vice versa multiply by 10.76)

Is this correct?

plc,

when I was trying to figure this stuff out, I made a summary of the whole thing: here it is, for better or for worse:

A lumen radiates one candela of luminous flux uniformly in all directions (a lumen is the total output of one source in all directions). Placed in the center of a sphere with a diameter of one foot, one lumen produces one footcandle uniformly over a square foot at one foot away. Each point on that surface measures one footcandle. Placed in the center of a sphere with a diameter of one meter, one lumen produces one luxuniformly over a square meter at one meter away. Each point on that surface measures one lux.

Candela and candlepower measure intensity of a beam, the amount of energy in the light radiated in a particular direction; footcandle and lux measure illumination on a surface. Lumen measures the amount of energy in the light radiated in all directions.

One footcandle of luminance at one foot is 10.76 times weaker at one meter, and is called lux; it would take 10.76 footcandles to get a reading of one footcandle at one meter. One footcandle thus equals 10.76 lux; one lux is 10.76 times weaker than one footcandle.

"Throw for one footcandle": a source will measure one footcandle on a footcandle meter at this number of feet.

The lumen measures the total amount of energy in the light radiated in all directions, and the candela measures the amount radiated in a particular direction.

The lumen can be defined in terms of a source that radiates one candela uniformly in all directions. If a sphere with a radius of one foot were centered on the light source, then one square foot of the inside surface of the sphere would be illuminated with a flux of one lumen. Flux means the rate at which light energy is falling on the surface. The illumination, or luminance, of that one square foot is defined to be one foot-candle.

The illumination at a different distance from a source can be calculated from the inverse square law: One lumen of flux spreads out over an area that increases as the square of the distance from the center of the source. This means that the light per square foot decreases as the inverse square of the distance from the source. For instance, if 1 square foot of a surface that is 1 foot away from a source has an illumination of 1 foot-candle, then 1 square foot of a surface that is 4 feet away will have an illumination of 1/16 foot-candle. This is because 4 feet away from the source, the 1 lumen of flux landing on 1 square foot has had to spread out over 16 square feet. In the metric system, the unit of luminous flux is also called the lumen, and the unit of illumination is defined in meters and is called the lux.

Hello Plc,

Welcome to CPF.

In general the higher value will be brighter. The exception is that advertisers quite often fudge on their numbers...

To measure lumens you need an integrating sphere which costs in the neighborhood of $15000. To measure lux or foot candles, you just need a light meter (they run $50-$250).

Carmel has given a pretty good defination of how light works.

The conversion you are looking at (0.09 and 10.76) has to do with converting between foot candles and lux.

1 lux at 1 meter = 1 foot candle at 1 foot.

1 lux at 1 meter = 0.0929 foot candles at 1 foot.

1 foot candle at 1 foot = 10.764 lux at 1 meter.

Doug (Quickbeam) has throw and total output charts for several lights in the reviews section. As Carmel has mentioned, light falls off with the square of distance. Doug, in the throw charts, shows the distance (in meters) that the light will drop to 1 lux. To find the lux rating at 1 meter, you need to square his throw number. The total output chart is an effort to get close to the values from an integrating sphere without spending the money for one. Doug came up with a device utilizing a milk carton and it gives a close approximation.

If you have a very focused light beam, it can have a long throw. If you have a broad difused light beam, it will have a high total output.

The best way to understand the charts is to compare lights that you already have. Then you can evaluate a light you want to get by reading the reviews, doing a search on CPF, and checking the charts to get some idea of what the beam is like.

Hope this helps.

Tom