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Measuring light levels

RogerS

RogerS

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We've been using 500w halogen security lights but as I got fed up with constantly going up the ladder to replace the bulbs (even though they are hardly on at all) I went to get some LED floods. My research and the listing details suggested that a 50W LED was equivalent to the light output from a 500W halogen - :eusa-snooty: Not a chance.

I quite fancy doing some empirical measurements and as I can't easily locate my trusty Weston, I was wondering if I could use the metering capability of my digital camera (Panasonic DMC-FZ20 Lumix) but lack the necessary knowledge. Point camera at white paper? Then what ?? !!

Can anyone help please ?
 
Dont know how to turn exposure details into absolute luminosity measurements, but simply comparing the exposures with the same subject, the camera on auto and the two light sources one after the other will give you a comparative measurement...
 
Many thanks for the suggestion but what am I looking for ? Am I looking for a comparative change in f number or speed? What setting would I put the camera on? I have four P A S M but as you can tell from my questions I am a bit at sea here!
 
Roger
If you have (or have access to someones) a smart phone, there are free light meter apps.
I haven't used one, have no idea if they work or are suitable for your purpose. But as they are free it might be worth a try

Mark
 
the bear":3fxh18xi said:
Roger
If you have (or have access to someones) a smart phone, there are free light meter apps.
I haven't used one, have no idea if they work or are suitable for your purpose. But as they are free it might be worth a try

Mark
Click to expand...

Oooh...that sounds more like it. Tanks for that...I shall go a wanderin'
 
Thanks David.

I'll try that for comparison purposes. I looked at the Google app store and indeed there are some nifty light meters...just like my Weston! But then :idea: and I looked for lumen meters which is really what I'm after and Bingo...several available. Pouring with rain last night but I'll try tonight.
 
The camera comparison would work but there are 3 variables to note - ISO, aperture and shutter speed. Use Manual and you can keep 2 of those variables constant and compare how the other changes for a correctly metered exposure.... but the app sounds a lot easier :)

I have a 30W LED floodlight in my drive. I'd compare it to 200W quality halogen lamp or 300W cheapo (cheap lamps have poor light output) in output but it is a very different quality and colour of light. The light itself is more intense to look at than a halogen and whilst you can squint at a halogen flood the LED is painful to look at. The light is very white and the throw is much wider than halogen. Think it is because it is so wide that the light on the ground only compares to 200W.

I like the light from the LED flood. Don't think I'd buy another halogen.
 
In mode A on a Nikon, the ISO value will not be changed by the camera, so while it is relevant (as Robert says) it will remain the same and so can be ignored for the purposes of a ratio.

Having said that, if you dont use mode A regularly, you probably dont know what ISO is set to, so I suggest you adjust it to 200 for this test.
 
And here are the results....

Using the lux meter on the phone and pointing it at the lights.

halogen 500w 77 lux unit

LED 50W 14 lux unit

roughly 6:1

I then wondered whether the very low light level of the LED (nothing like the ratio of 2:1 that desk research suggested) was because the spectrum from the LED was either (a) spikier that the halogen or (b) at a part of the spectrum, where the smartphone light sensing array was relatively insensitive compared to the halogen. So I looked at the spectrum display on the camera and found that both lights were peaky (which eliminates (a) but leaves (b) unquantified.

Next I set the camera to manual and 400ASA. The halogen needed 1/5sec at f2.8 to reach a known position on the exposure scale in the viewfinder. It needed 1.3 secs at f2.8 to reach the same position with the LED. Again roughly 6:1

Both verify what the Mk I eyeball suggested.

So they are going back and we've decided that we will stick with the halogen but only bother replacing bulbs on two sides of the house.
 
What distance are you measuring at?

Light level varies with distance. Out of curiosity I'll go measure the light output from my flood at a similar distance. I'm not disappointed by the light output of mine at all so it might be interesting to compare.

edit..

At about 5m my light meter (a meter not an app) reads 65 lux. Walk away to about 10m and it is 20 lux. distances judged by looking at the light and guessing how far away I was :)

The light is a 30W pir flood from Toolstation. Not sure they still sell the same model. It is very large for the wattage.

I now have green blobs in front of my eyes..
 
Robert":1pncznbq said:
What distance are you measuring at?

Light level varies with distance. Out of curiosity I'll go measure the light output from my flood at a similar distance. I'm not disappointed by the light output of mine at all so it might be interesting to compare.

edit..

At about 5m my light meter (a meter not an app) reads 65 lux. Walk away to about 10m and it is 20 lux. distances judged by looking at the light and guessing how far away I was :)

The light is a 30W pir flood from Toolstation. Not sure they still sell the same model. It is very large for the wattage.

I now have green blobs in front of my eyes..
Click to expand...

I would guess 4 - 5 m from each. Simple fact is that viewing the scene (which is what they are for, after all!!) is about 1/6 the brightness on the LED. My wife commented that when she is looking out of the kitchen window and I arrive in the car and trigger the sensor that she can quite clearly see who it is with the halogen light. With the LED, she says that it's impossible to see clearly.
 
Just been out with the Weston meter. Pointing it at a white piece of A4 gave about 1 dooberry with the halogen light. With the LED light, the needle didn't move a all.

Pointing the Weston directly at the halogen light ave about 6 dooberries and at the LED a mere 1.

No two ways about it. LED lights suck!

As an aside, we put some in our kitchen to replace the halogen downlighters....useless.
 
I've got some good LED's in our kitchen but it took testing about 20 types before I was happy, and they cost about £14 each!

Roger, I can send you some to test if you want to see whether they work for you, I've got loads of spares…

Cheers
Mark
 
Rod":3mj0l2sb said:
The new LED lights in our kitchen are great.

Rod
Click to expand...

Rod, I hope you have a spare from the same manufacturer because when one of them goes chances are that you will find it difficult to colour-temperature match a replacement.
 
Interesting result that confirms what my Mk1 eyeball thought too when looking at some LEDS recently.

However, I thought it was now mandatory to quote a light output figure on the packaging ? Having thought that the ones I looked at were a bit dim, I checked the output figures for the halogens they were suposed to be equivalent to and sure enough there was a huge difference.

LEDS are not there yet I think (or at least horribly expensive if you compare like with like).
 
That's a good idea Roger - I will contact our electrician and buy a spare from him. I know he only uses one particular type/manufacturer after trying out others.

I was reading about OLEDs (for TVs) but they cost £100/1" of screen at the moment!
My trusty Weston has died - cannot be batteries as it doesn't use one [emoji20] I bought it over 50 yrs ago though.

Rod
 
Oh I forgot about them - my small Fenix (450 lumens) is fantastic and the LED conversion kit I used to upgrade my Maglite is just amazing!

Rod
 
RogerS":3g4s3zml said:
No two ways about it. LED lights suck!
Click to expand...

Think I'd say many LED lights not all.

My 30w flood with 65 lux at 5m is not far off your 500W halogen if the light meters are anywhere near similar calibration.

As was said earlier in the thread all LEDs are not created equal and many don't even run at the wattage stated. Lumen output as given by a maker or seller you can trust is the best guide short of trying them first.
 
Robert, do you have a link to the one you have as all the ones I can see on Toolstation are only 2400 lumen for 30W (I don't think 65 lux = 2400 lumen :eusa-think: )
 
Mark, thanks for the offer ...what fitting and wattage are they?
 
RogerS":17ylng8t said:
Mark, thanks for the offer ...what fitting and wattage are they?
Click to expand...
GU10, 24LED SMD, warm white (2700k), 50mm dia, 55mm length (from face of bulb to end of prongs), 330lumens, 3 watt, dimmable, 120deg beam angle.

Think that covers everything :)

I chose these specifically because of two factors, dimmability and the 120deg beam angle which means they give what is at first an odd kind of diffused light and do not have the spotlight effect that a lot of them seem to.

Unfortunately it appears that the site I bought them from has gone bump, but luckily I bought more than required, well more than I needed at the time, but if I ever get round to fitting the ones in the cinema room then I probably won't have any spares :(

But given the purported :eusa-pray: lifespan of 50,000 hours then I can't see them needing changing for a good while… :eusa-whistle:

Cheers
Mark
 
RogerS":1au9loxg said:
Robert, do you have a link to the one you have as all the ones I can see on Toolstation are only 2400 lumen for 30W (I don't think 65 lux = 2400 lumen :eusa-think: )
Click to expand...

It is code 86366 which is stated as 2180 Lumen. They have the wrong picture for the 30W it is quite big.

flood-1.jpg


flood-2.jpg


I'm not saying it is going to compare to 500w halogen but the nature of the light source means it must be more efficient at getting the light out of the light fitting as most light is direct rather than reflected as in a linear halogen. Hence I'd compare it to 300W halogen.

Lumens are the quantity of light coming out of a light source. intensity in Lux is lumens per square meter so the better aimed the light from the lamp is, the greater the intensity where it is focussed. Inverse square law means it falls off as distance increases. In other words design of the fitting and lamp has an effect on useful light as well as the amount of lumens the lamp puts out.

Maybe try one if you go near Toolstation. They have always been no problem with returns on anything I've taken back.
 
Hi Mark, thanks for the info. Ideally I'm after 6W ones for the workshop and so I think perhaps 3W won;t be bright enough.

Robert...I might well give the Toolstation one a try.
 
RogerS":g0fh85h8 said:
Hi Mark, thanks for the info. Ideally I'm after 6W ones for the workshop and so I think perhaps 3W won;t be bright enough.
Click to expand...

Again with LED GU10 lamps compare the lumen output not the wattage...
 
Sorry...my bad posting perhaps. We have got some in the kitchen and like you tried several different wattages and colour temperatures. I preferred a brighter wattage although admit they looked harsh but SWMBO preferred a lower light output. In the end I just gave the magic words "Yes, dear"

Robert - good point. I'll check the label on the boxes of the three LEDs I have earmarked for the workshop to replace the halogens.

Now why is he replacing halogens with LEDs if he doesn't like them, I hear you ask? Quite simple....when the halogens blow (which seems often) they usually take the breaker out. If that happens at night then I am plunged into darkness and not a good idea with spinny things. I then have to grope my way to the door where I keep a torch (ahem, a LED one :oops: ) thence to the lockup where the consumer unit is. But usually SWMBO needs the floor for her gardening stuff so it is a right old palaver! Then I have to replace the blown halogen which involved precarious extending ladder manoeuvring in the workshop..very unstable and scary. So LEDs.
 
RogerS":3dm9h3ob said:
when the halogens blow (which seems often) they usually take the breaker out.
Click to expand...

Ok, so here is the stupid question for the day .................

Why would a light (halogen or other?) take out a breaker when they blow?

Is the problem not related to your electrical setup? For example - turn on the lathe at the same time as the halogen switches on?

Cheers
Phil - confused
 
We had a couple of 40W incandescent bulbs go within a couple of days of each other - tripped the breaker each time. Perhaps a mini power surge each time?
Fortunately I've got a reasonable stock of them.

Rod
 
As the filament ruptures, there is a short duration, very low impedance across the bulb of vaporised metal which will take out most MCBs - old fashioned wire fuses could normally withstand this and so it 'appears' to be a modern phenomenon. It does not indicate a fault other than hypersensitivity of type B MCBs

Bob
 
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