what happens when you go to say 4 degree spot, a tad more usable beam for most?
can i assume that it will be .25 lux, since we did the x4 thing ?
This is common question we get as far as what happens to your luminous intensity as you widen the beam from spot, but the 4x thing your referring to comes from somewhere else.
First off I want to define a few things and I'll try to balance between simplifying for ease of explanation and getting overly technical. There may be some shortcuts here or some oversimplifications there, but for most people I think this will help understand the basics of what is going on with any searchlight and what you can and cannot take away from basic specs.
Luminous Flux/Lumens - Total light. It is always good to clarify when you see a lumen spec if those are lumens of the lamp or of the fixture, sometimes called "Out the front" or OTF Lumens. ANSI spec is OTF Lumens. Lumens will tell you how much total light you are dealing with but nothing about how well that light can be focused or concentrated.
Illuminance/Lux - Incident light on a surface. This is what you would often measure with a light meter. This is basically what you are trying to achieve with any searchlight. You are trying to put light on a surface often times far away. (lux= lumens/m
2)
Luminous Intensity/Candela/CandlePower - How intensely focussed or concentrated the light is in a particular direction. CandlePower and the ANSI standard take few shortcuts/make a few assumptions here, but generally speaking those shortcuts don't make a huge difference especially when making comparisons. Luminous intensity as a spec is always calculated by measuring lux at a specific distance, where (Lux) x (Meters)
2 = CandlePower.
So if you have any two of those variables in the above equation you can solve for the third. This is how the ANSI standard has you determine the range of a light/searchlight. Once you have determined the Candlepower, it asks at what distance will that same light/searchlight put 0.25 lux on target. If you reduce the required illuminance (Lux) required by a factor of 4 (1 lux ---> 0.25 lux), due to the square on the distance you extend the range by a factor of 2.
If you want to double the light on target you need to double your CandlePower. If you want to double your range you need to quadruple your CandlePower.
When it comes to the focusing of the light and different beam angles you are going to see a significant drop off in range. Remember your total light (lumens) for all intents and purposes is not changing, you are just adjusting how it is distributed. At long distances the difference between a 1 degree beam and 4 degree beam as far as area of illumination can be huge. If my quick calculations are right, at a 1,000m your area of illumination would be 16 times larger.