In this video learn how to calculate the beam distance of a flashlight.

Finding the true range/distance capability of your long range flashlights. Using a digital light meter, Digital lux meter, or lux meter app/ light meter app on your phone.

## Step 1: Step 1: Gather Your Materials

You will need.

1. A lux meter or phone with a light meter app

2. Tape measure

3. Calculator

4. A flashlight for which you wish to measure range

## Step 2: Step 2. Setup Equipment.

Measure a set distance in meters away from a wall. I use 5 meters. You can go further out if you like, just make sure you record the distance because you will need to know it later.

## Step 3: Step 3: Check Your Distance

Make sure that the distance your using is far enough away that the flashlight beam is fully converged. This makes a particular difference if your using a light with an aspherical lens. The beam will have a cross over point as show above. This is less critical with reflector based lights as there is not the same cross over, however falsely high readings can still be made if your too close.

## Step 4: Step 4: Place Your Flashlight on a Tripod or Holding Device.

It will be easier to measure if you have the light in a stationary position and move the meter rather than the other way around.

## Step 5: Step 5: Measure the Light Output

Power the light on, and move the sensor around on the wall till you find the sweet spot. This will give a better result than using the peek hold function and moving the flashlight around.

## Step 6: Step 6: Record the Reading

Take the number you got from your light meter and record it. If you were on the higher settings of your meter be sure to add a zero or zeros to that number.

## Step 7: Step 7: Calculate the Lux Reading

Take the number you got in step 6 and multiply it by the distance you measured from twice. Reading times distance times distance.

## Step 8: Step 8;: Convert to Distance.

To convert this number to distance multiply it by 4 and then take the square root of that number as seen above.

## Step 9: Step 9: Have a Blast!

Using this method we can now extrapolate the beam distance of any flashlight. For this experiment I used the Streamlight Stinger LED. Measured 21,075 lux for a beam distance of 290+ meters!

If you have enjoyed this tutorial and would like to see more, please be sure to see my other instructables and also check out my youtube channel.

https://www.youtube.com/channel/UCn6sN4bKlFUF0YXNb...

Here you will find flashlight reviews and other tutorials about crafting great flashlights.

Thanks for watching!

## 3 Discussions

Reply 3 years ago

Your correct. You do need to make sure its very dark so no light is registering on the meter before you start. That is why I do mine inside the house so it can be very dark.

3 years ago

plz explain calculation clearly..i can't understand.

Reply 3 years ago

Hi RJS9, I would be glad to help you. Lets say that you had a lux reading of 425, and the distance you measured that number from was 10 meters. You would mutliply

425 x 10= 4,250 and then by 10 one more time 42,500 This means you have 42,500 lux @ one meter. Take this number and multiply it by 4. 42,000x 4= 168,000 then extrapolate the square root of that number. In this case is 409 which give you your range or distance number.

If you dont know how to extrapolate the square root of you number there are online calculators that can do it for you.

http://www.math.com/students/calculators/source/sq...

Let me know if I can assist you further :)