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View Full Version : HELP! Watts, Lumens, Candles & Lux; EVs, f/stops and flux


Graham_of_Rainham
12th March 2016, 04:35 PM
I'm playing with LEDs again with the aim of producing a continuous illumination panel. I could just buy one for a few quid off ebay and be totally disappointed by the faint glow it emits or spend a fortune and get something that can only be used when the model is wearing a welders hood...

Candles I understand and have used them quite successfully.
and set fire to a few things. :o

We all grew up with 60W lamps and knew that if you put a more powerful lamp in the slide projector, the slide melted. :eek:

Equally growing up "Lux" was soap and Lumens (The SI unit of luminous flux, equal to the amount of light emitted per second in a unit solid angle of one steradian from a uniform source of one candela.)
wasn't used at all

We all know EVs and what that means to our f/stops, but look at the specs for some of these LED panels and many seem determined to add confusion as well as exaggerated levels of output.

SO: Has anyone chanced across a web page that demystifies the rise of the LED and clearly presents its lighting ability.
Something that is perhaps "illuminating" :rolleyes:

Imageryone
12th March 2016, 04:43 PM
Found this, Graham, beyond me , but you will understand it ?

http://www.molex.com/mx_upload/family/lightemitting_diode/LED_Power.pdf

Walti
12th March 2016, 05:16 PM
If you buy an LED lamp you will need to look at a few numbers:

Lumens: can be simplified as the amount of light that comes out of the lamp

Lux: can be thought of as the number of Lumens per square meter once it's travelled to the surface being illuminated.

Colour temperature: defined in Kelvin, the higher the number the bluer or "cooler" the light looks, the lower the number the yellower or "warmer" the colour looks. An traditional lamp is in the 2700K region.

There are a few other bits and pieces to look at, but they are the important ones.

If you find a shop that sells Philips lamps, e.g. John Lewis, they print these numbers on the packs of all their lamps. So you can see the power in Watts against the Lumen output of each lamp type. Unfortunately they don't sell trad lamps any more for a full comparison.

The number of Lumens per Watt changes almost on a daily basis and bears little relation to the old numbers.

Harold Gough
13th March 2016, 07:41 AM
Flux is for solder!

Harold

Graham_of_Rainham
13th March 2016, 10:04 AM
Flux is for solder!

Harold

Back in the day, I made some "killed spirits of salts" by dissolving Zn in HCl

Still working on the "Flux Capacitor"... :cool:

shenstone
15th March 2016, 07:47 PM
Walti's description is spot on

Watts was a useless measure as it measures the amount of energy consumed in a bulb and depending on the efficiency the amount of light coming out is hugely variable

In terms of demystifying the LED just worry about the lumens figure little else matters to most people

Technically it's all about efficiency - in my caving lamps where brightness and hours matters I used to carry a belt mounted battery (made from a miners lamp, but boiled out the innards and put better batteries in) and it gave me about 20 hours at 40 lumens so not much better than a candle once you are over about 15 ft away. Now I can get up to 1500 lumens which I can actually take pictures by and if I rack it down to say 400 I can get tens and up to hundreds of hours and that is on a battery that is small enough to be on the back of my helmet - I love LED's

This page http://www.ledcaplamps.com/oldham.htm (http://www.ledcaplamps.com/oldham.htm) NOT my pictures gives a photographic comparison of LED vs Element based technology

Regards
Andy

Graham_of_Rainham
16th March 2016, 12:42 PM
I've mounted 9 panels of 48 LEDs on a board. Each panel pulls 0.17A at 12V and my bench PSU is only good for 1A continuous, but will let me run all 9 long enough to measure the light output.

Using a Sekonic meter, the EV of the window light reflected off the panel is 10.2 to 10.6 When I turn the power on the light reading rises to a steady 13EV :cool:

It's much brighter than I expected and not something you would want to look at.

Next will be illuminating a test subject, to get a colour temperature and playing with diffusers and gel filters to get what I want from it.

Cost so far is less than 10 :)

Walti
17th March 2016, 08:29 AM
Just been to the Light + Build exhibition in Frankfurt, highest output lamp I saw was at 170 lumens per Watt, which makes the old 100W lamp around 4W as an equivalent!

Graham_of_Rainham
17th March 2016, 10:19 AM
Just been to the Light + Build exhibition in Frankfurt, highest output lamp I saw was at 170 lumens per Watt, which makes the old 100W lamp around 4W as an equivalent!

That's the type of progress we need, but what price are they.

Once the mass production kicks in, in China, they will be very popular

*chr