1 candela is 1/60 of the light intensity in the perpendicular direction of a black body with a surface of 1 cm², at the solidification point of liquid platinum (2046 K) under a pressure of 101 325 N/m² (comes together with 1 atmosphere) .

I just pointed two lights at a lux meter from fixed positions. One measured 3800 lux, the other measured 4000 lux. When I turned both on, I got 7700 lux.

Looks like candela is approximately cumulative to me.

Thanks.. so it's close, but not exactly the same.
I wonder how much influence 2 throwers would have..

Ah.. maybe candela does add up... (at least close), but definitely not the distance.
I was kind of thinking about distance in the back of my mind.. but yeah, candela might add up...

I think the difference between Grizzly’s and Yokiami’s answers is that Yokiami assumes the lights are being held perpendicular as per the OP while Grizzly is pointing them both at the meter, which, while not perpendicular, may be closer to what the OP was really asking.

There seems to be confounding between candela (lumens per unit solid angle) and lux (lumens per unit area).

If you hold two identical flashlights close and pointing at the same object (a slightly stronger condition than side-by-side, which could happen without the beams overlapping), both measures should double. By pointing at the same object (ideally the two beams should completely overlap), the lumens double and amount of solid angle stays the same, so candela doubles. The unit area of the object stays the same, so lux also doubles.

I might argue that lux is a better measurement in the context of combining two flashlights, since candela is defined based on a single point source emitter, which a flashlight (with its large reflector) is not, not to mention two flashlights. Lux, on the other hand, is a measure based on the target receiving light, independent of the shape of the light source/beam and not requiring the point source assumption.

A difference of 100 lux is good enough for me. I opt for striking the word “approximately”. Before we are shifting our attention to the accuracy of your meter over it’s total range. Remember: even the accuracy of a DMM isn’t lineair. There is also a sensor to consider.

No, it isn’t. Two lights is double intensity, and √2 throw.

EDIT: (minute) addition. It is possible to place one light perpendicular to the sensor. With two lights that is impossible. Because in real life we are not dealing with a light source concentrated in one single point.

Intensity is additive when you aim two lights at the same spot: if each light contributes an intensity of x and y respectively, the combined intensity is x+y.

For distance, the relationship is not quite additive but behaves in a Pythagorean manner: if the lights have distances x, y respectively, the combined distance is sqrt(x^2+y^2). Note that it still behaves kinda like addition, being commutative, associative, and satisfies the distributive law with multiplication.

Thanks for the clarification Henk4U2, you are right the sum of the launch of two sources with the same launch is equal to the launch of one of the two sources multiplied by √2.