thoughts on LED's

[xpatUSA]

Would be great if someone would just take a light meter reading with them so that we can objectively see what we're dealing with.

A light-meter reading is less useful than a lumen value because the distance must be known to compare readings.

Some Philips Data:
Item Ordering Wattage Base Voltage Rated Lumens CBCP Beam Power
Number Abbreviation (W) Type (VAC) Life (hrs.)1 (lm)2 CCT3 (cd) Angle R9 CRI4 Factor
78741 LED21PAR38/GU24/DIM/P/930/FL30 21 GU24 120 25,000 1150 3000K 3700 30° 62 95 0.7

My table-top lamps:

Model No. S8816:
Photometric Measurements at 25°C – Integrating Sphere Method

Sample No., CCT, CRI, CIE 31' (x), CIE 31’ (y), CIE 76’ (u’), CIE 76’ (v’).
S6129L 5143 87..6 0.341 0.343 0.212 0.480
S6130L 5125 87.15 0.341 0.344 0.212 0.480
S6131L 5131 87.28 0.341 0.343 0.212 0.480
Average 5133 87.22 0.341 0.343 0.212 0.480

Sample No., Base Orientation, Input Voltage (Vac), Input Current (mA), Input Power (Watts), Input Power Factor, Absolute Luminous Flux (Lumens), Lumen Efficacy (Lumens Per Watt).
S6129L UP 120.0 126.4 14.47 0.953 726.3 50.19
S6130L UP 120.0 127.2 14.58 0.954 760.5 52.16
S6131L UP 120.0 126.0 14.44 0.955 714.1 49.45
Average UP 120.0 126.5 14.50 0.954 733.6 50.60

... for the photo I was using nothing less than FOUR 500W Tungsten work lights for a total of two THOUSAND watts. So I'm sure you can imagine how well that lights up an average room. Would LED lights be any brighter than this?

We should note that LED lamps are much more efficient than incandescent lamps, so simply quoting lamp power consumption is meaningless for the purposes of comparison. For example, 500W of LED power would get you a whopping 25,300 lumens for my model S8816 (but you'd need 34 of them at 60 bucks a pop ;-). As opposed to approx 8000 lm from a halogen lamp.

Disclaimer: Not that I would use LED lamps for portraiture - or landscape work for that matter ;-).

[Inkanyezi]

From a layman's approach, lumens may be a bit difficult to understand, and how it relates to the light received from a scene, which in physicist's terms would be measured in candela per square metre. With the incident light meter, we actually do measure illuminance, which the physicist would measure in lux. Hence lux may be a fathomable entity.

But when we compare light sources, often their light output in lumen is compared, and for efficiency, lumens per watt, lm/W.

The relation is rather simple:

Incandescent "normal" lamps have a light output of approximately 10 to 15 lm/W, with a colour temperature between 2500 K and 3000 K. Higher wattage and lower voltage lamps are slightly more efficient than high voltage and low wattage lamps. The least efficient are so called "long life" lamps with a useful life span of >3000h. Normal lamps have about 1000h life span.

Incandescent "halogen" lamps have a light output in the region 10 to 25 lm/W. They are often more efficient than the "normal" lamps, particularly low voltage bulbs, that usually are in the region of 20 to 25 lm/W. Oh yes, what we usually refer to as "halogen lamps" are indeed incandescent tungsten lamps that burn with higher intensity and mostly higher colour temperature. Same thing there, lower voltage and higher wattage are slightly more efficient than the opposite.

Modern FL lamps have an efficiency from about 50 lm/W to 100 lm/W, about eight times as efficient as incandescent lamps. Fluorescent tubes often are declared for colour rendition, and the best FL lamps are comparable to daylight and may be mixed with daylight for photography.

High quality LED lamps also are as efficient as FL lamps, usually around 80 lm/W to 100 lm/W, but so far have poorer colour rendition, even if there are examples of CRI around 90 for some special types.

The most efficient lamps are gas discharge lamps, particularly sodium lamps, that regularly have an output of 160 lm/W, but their colour rendition is extremely poor, in fact, they are unsuitable for photography, as are various hybrid types, as metal halogen discharge lamps or mercury lamps.

Xenon gas discharge lamps, have a light output above 100 lm/W, and they can have extremely good colour rendition, in par with daylight. Modern car headlights, and flash tubes are in this category. Professional flash tubes, those used in studio flashes, are filtered for true colour rendition, while the cheap flash tubes in small strobes mostly have poorer colour rendition.

[ajohnw]

Following on from Manfred's post I do have some direct experience of this. There is a lot of interest in using power led's in microscopes. A photographic microscope typically collects a fair amount of the light from a 100w halogen bulb and concentrates into a very small illuminated circle when high powered objectives are used. The main reason for this level of lighting was film and medium format polaroid cameras but fast exposures are always nice. Power led's can easily match this sort of light level.

People tend to go for the highest light output versions but as Manfred mentions these emit incredibly large amounts of blue light. The reason for this is simple the blue they emit is at a wave length that our eyes aren't very sensitive too so apparently white light needs rather a lot of it. I converted one of my microscope to use a high efficiency daylight type and initially thought it was wonderful. No real change in colour temperature at varying even very low light levels. A 100w halogen bulb gives eye searing light levels so are often run for visual use with the filament producing and orange glow or a lot of neutral density filtering. After using the scope for a while though I started seeing a blue tint. The blue receptors in my eyes must have been overloaded and oddly enough there is an eye condition that can be caused by too much exposure to blue light. I didn't try photography with it but converse with some one who persisted with this type of led who did. So far any photo's I have seen have blue a tint. I suspect the excess blue is messing up the cameras sensors and leaving something that it can't cope with, Chances are that the blue sensors would be overload if the other colours were exposed correctly.

Warm white power leds are distinctly different. The blue peak on these is well below the level of the green emission. These leds use phosphors to generate 2 of the colour and some let through of the blue used to excite the phosphors. The spectrum of the warm white ones is sort of sun like. The colour temperature of these can be as low as typical tungsten lighting. The higher that is the more blue is emitted. I have also seen leds that emit 2 colours of green. It seems this improves the rendition of certain yellows.

It's important to not over run them if the colour temperature is to be retained. The red and green phosphors can saturate and just more and more blue comes out. In fact it's probably better to under run them slightly due to the way electronics parts are rated. They also need a heat sink of some sort. The types used on PC cpu cooling fans can be a cheap source of suitable heat sinks. It's also worth bearing in mind that looking directly at power leds can be an uncomfortable experience. 10w ones put out rather a lot of light from a very small surface area.

John
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[xpatUSA]

From a layman's approach, lumens may be a bit difficult to understand, and how it relates to the light received from a scene, which in physicist's terms would be measured in candela per square metre. With the incident light meter, we actually do measure illuminance, which the physicist would measure in lux. Hence lux may be a fathomable entity.

Quite so, as long as the layman realizes that the illuminance (in lux) is inversely proportional to the square of the distance from the source of light, e.g. a quarter of the lux at twice the distance. Same comment applies to candela per square metre. A possible advantage for the layman is that the lumens are usually noted on the package somewhere, making comparison a little easier.

We should note too that the lighting pattern of a lamp does affect the illuminance (lux). That is to say, all other things being equal, you'll get more illuminance from a 10 degree spotlamp that you would from a 40 degree floodlamp. 16X more!

[rpcrowe]

There is one niche in which I consider LED lights quite efficient and that is video work. I have a battery operated LED light that is great for a bit of fill for my video camera.



The little light is amazing as far as the weight and the length of time that batteries will power it. I can use either six AA batteries or a Sony video camera battery. I could also use my Panasonic camera battery to power this little light but, the Sony battery will power the light for a longer time per charge.

http://www.alzovideo.com/alzo_on_cam...0_dimmable.htm

I will use this little light both to add some light indoors and for fill outdoors. The 5600K color temperature is adequate for matching outdoor light. I have an amber filter and a green filter to modify the color temperature. The green is nice for providing fill when the ambient light is flourescent.

I also have a four bulb compact daylight temperature CFL softbox. The combination of the softbox and the little on camera LED light can provide enough light to shoot video quite satisfactory video in a normal size room. It actually looks a lot better than shooting using the available light. I like this combination when shooting video of my rescue dogs and puppies. Neither of these lights gets hot enough to cause any problems and the softbox is so light that it is quite stable on a standard light stand; although I will add a gravel bag to the bottom to make is even more secure in case a dog or puppy bumps into it while playing.

One night when I was having a bit of problem focusing for my shots of Maltese in my living room, I happened to have the softbox set up from some previous video work. I placed the softbox quite close and above the dog and turned it on. To my surprise it worked quite well, not just for focusing, but as a fill for my bounced 430EX with a Demb Flash Diffuser Pro. In fact I liked it so much, I now use the softbox + 430EX + DFD regularly for dog portraits...

[ajohnw]

Noticing the comment about Xenon reminded me of when I changed cars and lost HID lamps that are vastly superior to normal halogen. I fitted untinted xenon bulbs and they give out a lot more light than the standard halogen bulbs. It's also very white light. The blue tint often seen coming out of these types of light is actually down to the optics of the light not the bulb. These may make a suitable source of photographic lighting. Life is also rather long on headlamp bulbs.

An aside really but I recently read something by the IEEE on street lighting in the UK pointing out that low pressure sodium lighting is still by far the most efficient. Despite that and falling revenues and the need to go green councils are busy fitting high pressure sodium lighting and hi cri led lighting. It also pointed out that the high cir led lighting is less efficient than other forms of led street lighting presumably because of the extreme levels of blue that need to emitted to meet the cri which in real terms wont be that good really anyway. It's interesting to note that in some areas in the UK where taxes have been kept low due to local wage levels they use low pressure sodium for normal street lighting - about 1 every 100 yds or so. Hardly surprising that this provides adequate light levels as we are all perfectly capable of seeing well in even partial moon light.

Mentioned as others on here may have an interest in astronomy and it's so easy to filter out low pressure sodium light pollution with hardly any loss of light at all. I have - not that I have an extreme view on this subject of course - what me, no never.

John
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[Inkanyezi]

I'll spin off a little more on a tangent here. Sodium lamps, particularly low pressure, are not so much of a help to bats as are mercury lamps, metal halogen, FL or LED. Not that the bat would actually need any light, but the whiter light sources attract insects, and bats feed on those insects. There is a considerably higher count of bats along roads with mercury vapour lamps.

[Colin Southern]

An aside really but I recently read something by the IEEE on street lighting in the UK pointing out that low pressure sodium lighting is still by far the most efficient. Despite that and falling revenues and the need to go green councils are busy fitting high pressure sodium lighting and hi cri led lighting.

What annoys me about our local council is that in an age where they're supposed to be trying to "save", why do they leave tens of thousands of the darned things still turned on a good 1/2 hour after they're not contributing anything significant to the ambient light in the morning? EVERY morning.

[Manfred M]

An aside really but I recently read something by the IEEE on street lighting in the UK pointing out that low pressure sodium lighting is still by far the most efficient. Despite that and falling revenues and the need to go green councils are busy fitting high pressure sodium lighting and hi cri led lighting. It also pointed out that the high cir led lighting is less efficient than other forms of led street lighting presumably because of the extreme levels of blue that need to emitted to meet the cri which in real terms wont be that good really anyway. It's interesting to note that in some areas in the UK where taxes have been kept low due to local wage levels they use low pressure sodium for normal street lighting - about 1 every 100 yds or so. Hardly surprising that this provides adequate light levels as we are all perfectly capable of seeing well in even partial moon light.
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I think the real issue is that people don't like the distictive yellow colour of the LP sodium lighting. Politicians that make the ultimate call on what lights to install by listening to what their constituents think, rather than what the experts say.

[Colin Southern]

Politicians that make the ultimate call on what lights to install by listening to what their constituents think, rather than what the experts say.

Politicians actually listening? Seldom happens in this part of the word. More a case of "doing what they like so long as they tell the sheeple what they wand to hear"

[Manfred M]

Politicians actually listening? Seldom happens in this part of the word. More a case of "doing what they like so long as they tell the sheeple what they wand to hear"

I've found that municipal politicians are particularly influenced by two groups; developers and constituents that make a lot of noise. They rarely seem to be influenced on making the right choice for the silent majority...

[ajohnw]

The street lighting thing started a long long time ago. Initially where I lived in the UK they all turned off at 11pm as all good workers should be in bed. I found I could get around easily in moon light but moon less nights were a bit of a problem. Then they decided to leave those on corners and road junctions on all night. That actually cured the moonless night problem as the light carries a long way. Then some noticed that accident statistics went down. No proof of any further improvement in that respect but the next step was to leave all on all night. This was egged on by the offer of cheap electricity as it's off peak.

Now the motor way lighting in particular tends to have much taller and more closely spaced lamp posts putting out a lot more light no doubt designed by lighting engineers in an attempt to get dead even lighting. All this while car headlights have improved dramatically since the introduction of moulded reflectors. When driving out of a long lit area on old headlamps it was like driving into a black hole. Not so bad now but lit sections keep getting extended.

:-) Some councils have begun to turn them off past midnight but only on motorways. Newly installed hp sodium. Maybe they have found that the running costs are higher than they thought. Same may happen with hi cri led with any luck. I remember my grandfather driving on side lights via moon light and also being not very keen on the drive on dipped beam campaign even when street lighting was available. A policeman too well aware of what caused accidents. Given the state of dynamo's and batteries in his day the reluctance was understandable really.

Personally I think a lot of this sort of thing is done on the basis of wouldn't it be nice and the fact that they are spending our not their own money. We are regarded as a bottomless pit with no control on where the money eventually goes.

Bats? The only places I have ever been aware of any in the UK is in dark areas usually by rivers. Good places for plenty of insect life. Their sonar doesn't seem to be able to detect fishing line and the collision seems like a very good bite. No harm to the bats though.

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[Bobobird]

Speaking of road lighting - I was once on a newly constructed major highway in India and guess what? The lighting was provided by long FL tubes hung off the usual highway style lamp posts. Cheaper this way?

[xpatUSA]

I remember my grandfather driving on side lights via moon light and also being not very keen on the drive on dipped beam campaign even when street lighting was available. A policeman too well aware of what caused accidents. Given the state of dynamo's and batteries in his day the reluctance was understandable really.
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Brings back memories of 1950's motor-cycling in deepest Berkshire - "to save the battery", of course. Hit a horse once. Ouch, bent the bike a bit . . .