dive light output comparison

I am shopping for a new dive light for the Kona blackwater dive. Various lights are offered with various output and angles. So how do they compare? (Please leave out the difference between claimed and actual light output - this thread is related to measurement basis, not the difference between claims and reality.)

If one light puts out 500 lumens and has an angle of 50 degrees while another has 500 lumens and an angle of 25 degrees, what would I see in the water?

Would the 25 degree light be roughtly twice as bright in the smaller area of illumination?

Online definitions discuss a light shining in all directions with the measurement being the light in a defined angle. One could read this as a 500 lumen, 25 degree angle light would be as bright as a 50 degree light but over half the area.

So what is the position of people that have a clue... the clue meter here is running a little light - so to speak.

While technical by education, I am not so familiar with light measurements, etc. So I am fine with technical jargon as long as some definition is provided.

thanks,

dave

Generally speaking, a narrower beam will get you more punch through the water, especially in silty conditions. Lumens are useful for measuring light outputs over wattage, given the move from halogen to HID dive lights changed the metrics there.

Assuming you're looking for a decent primary light, some thoughts to consider:

Halogen

Use a lot of battery for not much light

Get very hot and can't be used out of the water for bigger lights

Moderately breakable

Now outdated technology

HID

Use much less battery for more light (my new 10W HID (3 hours on a smaller battery) was brighter than my old 50W Halogen(45 minutes on a huge lead acid battery pack))

Can have a highly focused output

Get hot and can't be used out of the water for more than 5 minutes

Very very breakable

LED

Use even less battery

Produce a more diffuse beam, reducing punch through the water. This is being refined through reflector development, with newer LED primaries being much better.

Don't generate much heat, and can be used out of the water

Can be dropped on concrete from waist height with no apparent problems...(oops)

In reality, numbers will only take you so far, and you may prefer different beam size, colour, etc depending on what you're using it for. The best way to judge output is to get some torches, get underwater, and compare.

Generally speaking, a narrower beam will get you more punch through the water, especially in silty conditions. Lumens are useful for measuring light outputs over wattage, given the move from halogen to HID dive lights changed the metrics there.

 

Assuming you're looking for a decent primary light, some thoughts to consider:

Halogen

Use a lot of battery for not much light

Get very hot and can't be used out of the water for bigger lights

Moderately breakable

Now outdated technology

 

HID

Use much less battery for more light (my new 10W HID (3 hours on a smaller battery) was brighter than my old 50W Halogen(45 minutes on a huge lead acid battery pack))

Can have a highly focused output

Get hot and can't be used out of the water for more than 5 minutes

Very very breakable

 

LED

Use even less battery

Produce a more diffuse beam, reducing punch through the water. This is being refined through reflector development, with newer LED primaries being much better.

Don't generate much heat, and can be used out of the water

Can be dropped on concrete from waist height with no apparent problems...(oops)

 

In reality, numbers will only take you so far, and you may prefer different beam size, colour, etc depending on what you're using it for. The best way to judge output is to get some torches, get underwater, and compare.

Also add that LED is the only one you can flick on and off without damage. Important if you want to save battery or you don't want t be blazing around with your lights on all the time.

For the same light output (in lumens) the illuminance will vary roughly as the ratio of the sin of half of the angles ie Sin(A/2) / Sin(B/2)

So yes comparing 50 deg with 25 deg the ratio is 1.95. Comparing 90 deg with 25 deg, the ratio is 3.26