FAQ’s

PHOTOMETRY

1. Q. What is the difference between Lumens, Lux, Candlepower and Peak
Beam Candlepower?

After compiling these notes and trying to get my head around all the
information, I decided to put the summary at the beginning so that
you can decide now if you want to torture yourself further.

Summary

The lumens rating provides a more meaningful measurement to consider when comparing and selecting a torch. Candlepower ratings may be impressively large numbers but they are misleading and you may be bitterly disappointed if you do not also take the lumen rating into account.

Peak Beam Candlepower is a measure of brightness at a specific point.
The lumens rating tells us the total amount of light present in a beam pattern.

I have had customers who have been bitterly disappointed with million candlepower torches they have purchased from their local hardware.

A. Lux is the unit of illuminance and is measured in Lumens/ sq M .
It quantifies the capacity of the torch to project light over a distance. Lux
is only measured at the very centre of the beam.
For example, a 1 watt luxeon LED with a tightly focussed beam could have a higher Lux reading than a 5 watt Xenon with a wide beam. The Xenon produces much more light but the tightly focussed LED will project light further.
Similarly, two torches, one tightly focussed and one with a wide beam, could have the same Lux rating but the wide beam would produce far more light. Another consideration is the area over which the Lux reading remains constant. Two torches may have the same lux rating but one may have a relatively uniform rating over the entire beam area whereas the other may have only a fractional rating at the beam perimeter.
The design of the lens and reflector greatly influences the Lux rating and can even vary between torches of the same model depending on manufacturing tolerances.
The Lux measurement alone should not decide what you purchase. In order for it to be an effective contribution you will need a beamshot showing the target area, its distance from the light source and the reduction in the lux reading as the periphery of the beam is approached.

Very few manufacturers provide Lux ratings. This may be because of the complexity of the measurements and the practicality of providing this information in a simple and meaningful manner.

Candlepower, or more particularly Mean Spherical Candlepower is a measurement of luminous intensity and is measured in candela which is lumens/steradian..
The candlepower rating does not tell you how much light will fall on the object you want to illuminate.
Some torch manufacturers refer to “Candlepower” ratings when they actually mean Peak Beam Candlepower ratings. This may appear less confusing than peak beam candlepower but also leads to misconceptions regarding the ability to convert candlepower to lumens.

Peak Beam Candlepower is another thing altogether.
Peak Beam Candlepower is a measure of the brightest spot in the focussed beam and is determined by both the output of the lamp and the efficiency of the reflector. Consequently, peak beam candlepower is a subjective measurement and each manufacturer has their own way of measuring it.
For this reason, peak beam candlepower ratings of torches from different manufacturers will not provide an accurate comparison of brightness.
Peak beam candlepower ratings can be very large (and very impressive from a marketing perspective) but can lead to disappointment and dissatisfaction if that is all you rely on to make your decision.
Peak beam candlepower is sometimes referred as centre beam candlepower.

The Lumen is the unit of luminous flux (also called luminous power), the flow of visible energy. It is the measurement of the rate of flow of light and unlike candlepower it is a measurement of light flux irrespective of direction, except for LEDs where lumens is the measurement of all the light inside the beam angle.
The lumens rating provides a more meaningful measurement to consider when comparing and selecting a torch. Candlepower is misleading if the lumen rating is not taken into account.

2. Q. Can I convert Candlepower into Lumens.

A. It cannot be done.
These have two different units of measurement.
Peak Beam Candlepower is focussed light and is measured in candela and Luminous flux (power), measured in lumens refers to unfocussed light.
It is absolutely impossible to convert the two as the focussing efficiency of the reflector is not taken into account.

You may see advice that candlepower (candela) can be multiplied by 12.57 to convert to lumens. This is actually referring to the conversion of Mean Spherical Candlepower to Lumens.
A light source that equally irradiates 1 candela of luminous intensity in all directions will produce a luminous flux of 1 lumen per steradian. A sphere has 4p steradians so the 1 candela source will produce 12.57 lumens.
This is a calculation only, not a conversion factor. It only applies to mean spherical candlepower ratings for point sources radiating light equally in all directions. It cannot be applied to a focussed light.

Example - the 20 watt lamp used in the Streamlight SL-35X is rated at 450 lumens or 35.8 mean spherical candlepower but the torch is rated at 400 lumens and 40,000 peak beam candlepower.
Example - The Streamlight Litebox spot light and flood light both use the same 20 watt lamp and both are rated at 400 lumens but the spot light is 105,000 peak beam candlepower whereas the flood light is only 3,000 peak beam candlepower.
Example - A LED with a luminous intensity of 9200 mcd (millicandela), drawing 20ma current and a beam angle of 15° has a luminous flux of 0.4945 lumens, whereas a LED with a luminous intensity of 1800 mcd, also drawing 20ma and having a beam angle of 50°, has a luminous flux of 1.9505 lumens.

BATTERIES

2. Q. Do I need to fully discharge my lead acid battery before recharging it?

A. No. Routinely running the lamp until it extinguishes will drastically shorten the life of the battery.

3. Q. How long will lithium batteries last.

A. Non-rechargeable lithium batteries used in torches will have a shelf life of
5-10 years. The burn time will be similar to an equivalent alkaline but the discharge curve will be flatter providing “usable” light for a longer period than would be provided by an alkaline battery.

14. Q. Rechargeable or Non-Rechargeable Torches?

A. The operating costs of non-rechargeable torches are significantly higher and they are usually not as bright. Torches incorporating recoil LED technology may be the exception. The shelf life of alkaline and lithium batteries is excellent with the latter often rated to exceed 10 years.

Rechargeable torches have an extremely low operating cost and usually can accommodate more powerful lamps. However, the initial cost is higher and their self discharge rate is greater.

4. Q. Can I use rechargeable batteries in a torch that specifies the use of alkaline batteries?

A. The short answer is “NO”, unless recommended by the manufacturer.
I am not aware of any that do.

Manufacturers do not support the use of batteries other than those listed in the instructions. Normally the lamps and current regulators are designed for a specific type of battery and using other batteries often results in poor performance, either low output or extremely short lamp life, depending on the battery types involved.
For example, many LED torches are designed to take advantage of the slow chemical reaction in alkaline batteries which actually regulates the current to the LED. If the alkaline was replaced with a NiMH rechargeable battery which can supply energy at a far greater rate, the LED may burn itself out or become very bright and run for a very short time. I have also read of instances where the extra heat generated actually began to melt the reflector.

I have found contrary advice (not from torch manufacturers) but the suggested circumstances are very restrictive and conditional AND their implementation will void any warranty regardless of the accuracy of the advice.

5. Q. Is there any way to prevent Ni Cd batteries from developing a memory?

A. A few minutes research on the internet will produce many conflicting articles on this subject, all from well credentialed sources. The degree to which it occurs and the conditions, under which it occurs, are widely disputed.
I am not qualified to enter the debate so I can only advise you to operate your torch as per the manufacturers instructions and not void your warranty.
For example, Streamlight advises that the “memory effect” is almost non-existent in high-drain applications such as torches and provide these tips to help prolong the life of Ni Cd batteries:-

• The torch should be returned to the charger when it is not in use.
• It should never be run until the battery is completely exhausted.
• Return the torch to the charger when it begins to dim.

Rechargeable nickel metal hydride batteries (Ni-MH) do not suffer from this effect.

6. Q. If I replace my Steady Charger with a Fast Charger, will the fast charger shorten the life of my battery?

A. No. Batteries have an expected number of charge/discharge cycles (the number varies by the type of battery)

7. Q. Why does the brightness vary between LED torches from different manufacturers even though they use the same batteries?

A. The type of LED used may be the answer BUT, there are manufacturers who “over-drive” their LED’s. This results in a very bright light during the first hour (remember this is when you are making your decision to purchase) but with the battery losing power rapidly, the light dims fast. A true comparison can only be made after 1-2 hours of use.

8. Q. How is battery burn time measured and do all manufacturers use the same method?

A. Unfortunately the method of measurement is not uniform. For example, Lightwave considers the battery burn time to be the time elapsed until the light is just sufficient to allow a newspaper to be read from 150mm.
Consequently, the battery burn times provide a relative indication of performance rather than a standard measurement for comparison.
Having said that, a standard comparison would still be of limited use as what is “usable light” would depend on the application and the user’s expectations which may simply be a personal preference.

NB. Battery burn time is a useful tool in comparing the “real” costs of different torches. Then you need to consider the cost difference in batteries to have the unit with the lower burn time, run for the same time as the type with the longer burn time.

GENERAL

9. Q. Why don’t Lightwave LED torches have a protective lens?

A. Torches use a polycarbonate lens to protect the lamp but Lightwave claims these can rob 10% to 20% of available light and if the are scratched or yellow over time, even more light will be lost.
LED’s , by design, have their own built-in solid epoxy lens which is incredibly tough and even if a chip was broken off, they would continue giving light. Lightwave torches also have a protective edge (they call it a Bumper Head) around the LED’s. This absorbs the abuse and provides additional protection for the tough LED’s.

10. Q. Why don’t torches with multiple LED’s have reflectors?

A. Focussing reflectors are used with conventional lamps which give off light in 360degrees. LED’s have built-in lenses and so have light emitting properties similar to glass fibre optics. Reflectors offer little advantage and would have the disadvantage of requiring a form of protection such as a polycarbonate lens. This would cancel out any improvement gained with a reflector.

NB. These comment only apply to torches with two or more LED’s and using only LED’s.
Torches using Recoil LED’s or LEDs in combination with a xenon or halogen, still require reflectors.

11. Q. How can I compare the “true” costs of LED torches and those with conventional lamps?

A. Well, we have no knowledge of your application or personal preferences, that we cannot put a price on, so we are restricted to comparing operating costs.

Battery and lamp costs make the big difference.

For example if the chosen LED torch uses 3 “D” cells and has a battery burn time of 900 hours but you want to replace the batteries at 500 hours to provide sufficient light for your application.

Your total cost for the LED torch over 500 hours will be the price of the torch plus one set of batteries.

Now compare this with a conventional torch with a krypton or xenon lamp and having a 13 hour battery burn time. If we replace the batteries after 8 hours then you will need 189 batteries to achieve the same battery burn time of 500 hours.

Your total cost for the conventional torch over 500 hours will be the price of the torch plus 189 batteries, plus replacement lamps!

15. Q. What colour light source should I use to preserve my night vision?

A. Firstly, we are referring to the colour of the actual light source, not light
that has passed through a coloured lens.
Secondly our night vision capability can vary depending on the amount of sunlight we have been exposed to. I found one study of pilots that found that a full day in the sun significantly affected night vision and ten consecutive days of sunlight exposure reduced visual acuity, visibility range and contrast discrimination by half.

Red is the best light for preserving night vision provided you only use the lowest light level needed. However, if you are reading a map, then a red light will cause the red lines to disappear so a low level green light would be best.

Green is often favoured by hunters as apparently animals cannot see or are not bothered by a green light.

Some people assume green is preferable for night vision, referring to the green displays used in military night vision equipment. The display colour in these equipments has nothing to do with preserving night vision. Our eyes are most sensitive to green and as the images on such equipments are poor and “grainy”, the green allows users to see detail that would be lost on other colour displays.
When a serviceman is in a situation requiring night vision equipment, preserving night vision is of little importance. After all he is using night vision goggles because he has insufficient night vision.
These observations on night vision equipment were made by a 24 year veteran of the United States Marine Corp.

Q. Why are torches not rated in watts?

A. Light bulbs for the home are sold by their wattage because it is a practical (although inaccurate) way of comparing bulbs of the same technology. The issue would be too confusing and complex if reflector efficiency, beam angle and light frequency had to be considered.

Q. Why would I want to use LED’s instead of Xenon lamps?

A. The choice of LED’s or Xenon largely depends on the application. LED’s can be good for close work over long periods but xenon will provide illumination over long distance. Click on “Choosing a Torch” link on the homepage for further comment.

3 Q. How do I choose the correct light for my application?

A. We cannot make a decision for you. However, if you click on
“Choosing a Torch” link on the homepage, you will find suggestions as to what characteristics you may need to consider.