Measurements of illuminance of my DBS. How should throw be defined?

[jenskh]

If we consider the light source as a point source then the illuminance at any distance will be:
q= q_0*(d_0/d)^2
where q is illuminance (lux) at distance d (m) and
q_0 (lux) is illuminance at distance d_0 (m)
If you come too close to the light, it is of course not correct to consider it as a point source, but at a certain distance, this model i quite accurate. In this region, you can calculate the illuminance at any distance by just measuring it at one point.

To find out where this formula is accurate for my DBS, I measured the max illuminance at different distances:

By jenskh

The red dots show the measurements. The yellow curve show for each point the calculated illuminance at 1 m distance. This curve show that this calculated flux varies substantially for the measuring points closest to the light. This means that in this region, it is not accurate to consider the light as a point source. At distance above 5- 6 m the calculated illuminance (@1m) seems independent of distance.

This means that to measure an illuminance representative for throw, we need to measure the illuminance at a distance greater than about 5 m from the light. We can then calculate the representative illuminance @1m which for my DBS seems to be about 30400 lux. Some of you may have seen that I earlier have given you a value of 32000 lux. The reason is that I measured it at 3 m and did not make any control at greater distance

Now that we have the representative illuminance @1m, we can calculate the illuminance at any distance. So, then the question is how should we define the throw.

I have earlier used the distance where I get 1 lux (moonlight). For my DBS that will be 174 m. In earlier threads where I have mentioned distances like this as the throw of my DBS, I get the reactions that this throw is far too short. I also agree that I normally can see the light longer than this. Many have mentioned the throw of the DBS to be about twice the distance I got above. If that is a normal perception of the throw distance, may be we should use 1/4 lux as the limit.

I do not know what would be best to use, but we should agree on a number for the limiting illuminance representative of what people perceive as the throw distance.

What do you think?

Regards from
Jens

 

[KeukaLite]

Wow Jens, you have done an amazing job of quantifying DBS performance way beyond the usual "Wow, that is really bright!" statements!

I'm going to take some time to really try and digest the information you have here. Awesome job putting it all together!

 

[asdalton]

This is a good experiment.

I have always thought that for a comparison of throw, it would be better to measure the lux at something like 5 meters (then multiply by 25 to standardize back to a hypothetical 1-meter reading) rather than measuring at 1 meter directly. The reason is exactly the same as yours: to work around the non-point characteristics of the beam that appear at short distances.

I don't think that there's a single lux value that you can use to define "the throw" of a particular light. Consider the effects of ambient light pollution. I am content to deal with standardized lux values at 1 m, and then make judgments based on comparison to the lights I've already used.

 

[jenskh]

Thank you both for positive comments.
I agree that what will be perceived as throw will vary with conditions, target and of course also how good your eyes are, but I think it would be an advantage to agree of a value representative for throw to try to get more comparable descriptions. For us flashaholics, lux values (representative illumination) is perhaps good enough. For less interested people, I think throw given as a distance gives more meaning.

 

[Justin Case]

Expanding on asdalton's comments, once you have your standardized methodology to back-calculate lux value at 1 meter, you can also compute relative throw. So what should be used as the baseline? Perhaps a SureFire 6P? Relative throw could be expressed as

sqrt[q1(1 meter)/qStd(1 meter)]

where

q1(1 meter) is the calculated 1 meter lux value for some flashlight of interest

qStd(1 meter) is the calculated 1 meter lux value for our baseline SureFire 6P

 

[saabluster]

In earlier threads where I have mentioned distances like this as the throw of my DBS, I get the reactions that this throw is far too short. I also agree that I normally can see the light longer than this. Many have mentioned the throw of the DBS to be about twice the distance I got above. If that is a normal perception of the throw distance, may be we should use 1/4 lux as the limit.

I do not know what would be best to use, but we should agree on a number for the limiting illuminance representative of what people perceive as the throw distance.

What do you think?

Regards from
Jens

This is excellent work Jens! You have my full backing. I think it will require field tests with a small group of people. Since what we most often would be illuminating would be trees and buildings from such distances use those or facsimiles thereof. I would do the test in an average large city on a clear night as the baseline for ambient light. We could have a plus/minus system which would reflect the ability to illuminate at the same relative levels for black night in the country and in the city on a cloudy night. This would also require the same panel of individuals to go and test. I propose that the point should not just be to be able to see that light is reaching an object but to be able to make out what you are actually seeing. This may mean making props that will act as a constant when changing environs from the country to the city with its higher ambient light.

 

[jenskh]

Thank you Saabluster. It was a response like that I was hoping for. I would have liked to be in a group with you, but I think the distance is to big. I will have to experiment on my own on illuminating different objects with my different lights. I will make up an opinion and share with you.
It is dark here in Norway now, so this is an ideal time for these experiments.

 

[MrGman]

This is very good work. It seems to me that you can take readings at 10 meters for all the lights and get a much better feel for their true long distance performance without having to go out to where ever 1 lux is. Especially since it would have to be a very dark place (I would guess for accuracy sake ambient lighting would have to be less than 0.1 lux) to not interfere with your readings. You could calculate more accurately from the 10 meter readings what the decline would be to one lux yes?

 

[Ryanrpm]

This is very good work. It seems to me that you can take readings at 10 meters for all the lights and get a much better feel for their true long distance performance without having to go out to where ever 1 lux is. Especially since it would have to be a very dark place (I would guess for accuracy sake ambient lighting would have to be less than 0.1 lux) to not interfere with your readings. You could calculate more accurately from the 10 meter readings what the decline would be to one lux yes?

I agree that we should measure the ambient lighting before we do our tests, but I'm of the side that would rather walk that distance with my light meter so that I get an actual distance of throw from a light. I like the 1 lux standard rather than the 1/4 lux. Using calculations to figure it out may be accurate - I don't know, haven't done enough to verify one way or the other - but you know you can't go wrong when you are 174 meters away and your light meter says 1 lux. :shrug: Who knows....I'm still learning.

I like threads like these....keep the replies coming.

 

[jenskh]

At what distance we need to measure the representative illumination, will vary with the size of the reflector and the throw of the light. With more diffuse lights as for example my lf2x, it is more than good enough to measure directly at 1 m.
The advantage with using the shorter distances is that the measurements can be done indoor, and it is much easier to find the distance and also the center of the hot spot.
It would be nice to go out and verify the measurements at greater distances (max throw length), and I plan to do that, but everything becomes less controllable, so I would actually trust the measurements made at shorter distances most. I also think there is every reason to trust the formula for the point light source. The only thing I can think of that could alter that, would be if any light was absorbed by the air. Clean dry air absorbs very little, and I think also humidity in the air absorbs in the infrared range, and not of any significance (for this purpose) in the visible range.

 

[jenskh]

I have now done a little testing to find out what is usable light level for me. I often walk in the evening (in the dark) in some fields close to my house. Part of the normal route is on a road along a forest, and parts are in the open as shown in the picture.


The picture also show a wooden bridge over a fence. One of the use of my lights, are to find these bridges. So one very dark night, I did a little experimenting on how far I could discover it with my lights.

First stop was at 106 m distance. At this distance I could not discover the bridge with my CL1H 3SD Q2 A5 light. After I had used my DBS to discover it, I could barely see it with my CL1H, but I can not say that I could distinguish it from the poles. I could also see it quite clearly with my DBS at medium setting, but not at low.

I then moved closer, and my best judgment was that I would be able to discover it at around 80 m distance with my CL1H.

My calculations show the following illuminance in these cases:

CL1H 3SD Q2 A5: 0.63 lux @ 106 m, 1.1 lux @ 80 m
DBS 3SD Q5 WC high : 2.7 lux @ 106 m
DBS 3SD Q5 WC medium: 0,94 lux @ 106 m
DBS 3SD Q5 WC low : 0.16 lux @ 106 m

This seems to indicate that for this purpose, 1 lux is a very good measure for usable illuminance for me. Of course the required illuminance is dependent on the target, surroundings and distance, but I still thinks that this supports the use of 1 lux as the limit used for defining throw distance.

 

[Ryanrpm]

Good work.

Help me with the math....so if a light puts out 28,000 lux @ 1 meter, such as your DBS, how would you calculate the max throw distance?

 

[jenskh]

You use the formula in my first post in this tread and solve it for the distance d. If we put in d_0= 1 m and q= 1 lux then we end up with the very simple answer:
d=sqrt(q_0).
In your example the throw distance would be:
d= sqrt(28000)= 167.3 m.

 

[jenskh]

As can be seen above, I made a correction to my illumination test. I could clearly see the bridge at 106 m with the DBS at medium.

One interesting point is that the experiences described above, may indicate that I need less lux from the Q5 WC led than from the warmer Q2 A5 led for the bridge to be clearly visible. This is the opposite of what I thought. One explanation for this may be the different beam profiles from the CL1H and the DBS. May be the DBS makes the bridge stand out more since it illuminates less of my close surroundings. I shall see if I can switch the pills and do some further testing tonight.

 

[Ryanrpm]

I see you are coming to a conclusion that 1 lux should be a standard minimum of measurement, but are you still of the mind that throwers should have lux readings taken at a distance of 5 meters.....based on your 1st chart?

 

[jenskh]

It depends on the size of the reflector as well as the spreading angle of the light in the hot spot. For the DBS 5 m is a sufficient distance, and any distance above should be fine.

I think a good criteria for the required distance for getting a representative lux measurement, can be that the diameter of the hot spot has to be more than a certain number times the diameter of the reflector. For the DBS, this number seems to be about six. I would guess that this could be a criteria also for other throwers.