# 1.2+km throw led spotlight?

#### GillBates

##### Newly Enlightened
1.2km+ led spotlight does something like this exist? If so what would be a good light to purchase?

#### Ryp

##### Flashlight Enthusiast
Check Vinhn's modded lights out.

#### Etsu

##### Enlightened
A green laser, perhaps?

More seriously, is 0.25 lux (the traditional measure of "throw") any use 1.2 km away? You're not going to see something that dimly lit at that distance. As you throw further, you're going to need progressively more lux in order to make a target visible to you. So you might need 4 lux at that distance, which means a "throw" 4x further than 1.2 km.

Yeah... green laser.

#### KeeblerElf

##### Enlightened
In order to make a recommendation, we would need to know what lux is required at the target distance. This can be estimated from what level of detail is required at that distance. Any info that you could provide about the application would be helpful!

##### Flashlight Enthusiast
you are looking for one of these
:thumbsup:

#### Mr Floppy

##### Flashlight Enthusiast
More seriously, is 0.25 lux

Which works out at 300000+ lux at 1 metre for 0.25 lux @ 1.2km. Inverse Square Law.

E1=(D1/D2)^2 * E2

Where E1 is output at D1 and E2 is output at D2.

#### Etsu

##### Enlightened
Which works out at 300000+ lux at 1 metre for 0.25 lux @ 1.2km. Inverse Square Law.

E1=(D1/D2)^2 * E2

Where E1 is output at D1 and E2 is output at D2.

That depends on the light source. A laser won't obey that law, nor will a light that uses a parabolic reflector (at least not if you measure it from the distance of the light).

#### sven_m

##### Newly Enlightened
That depends on the light source. A laser won't obey that law, nor will a light that uses a parabolic reflector (at least not if you measure it from the distance of the light).

It roughly applies if you are far away enough (which might mean several hundreds of meters, though).
But what issue do you have in mind for reflectors? (except, as always, if distance is too short). I never noticed any until now.

#### Etsu

##### Enlightened
It roughly applies if you are far away enough (which might mean several hundreds of meters, though).
But what issue do you have in mind for reflectors? (except, as always, if distance is too short). I never noticed any until now.

That was my main concern. Measuring lux at only 1 metre won't give you an accurate estimate at 1.2 km. For example, if a reflector is focusing the light at a distance of 20 meters, then lux will actually increase with distance out to 20 meters, then fall off at greater distances. The inverse-square rule may make some sense at long distances, as long as you use the focus point as the measurement and not the physical distance from the flashlight.

Also, if you have a very good parabolic reflector with a point light source properly positioned, you can get almost parallel light rays which theoretically focus at infinity. In that case, lux remains constant at all distances. Of course, you won't get that in reality, but it can still make the inverse square rule useless with a light that is specifically designed as a spot light.

It's a useful rule for most lights, but not for lights designed for extremes.