My poor Light Meter- Experiment

Hi,

I have no light meter, so I try to know that which flashlight is brighter or have more light intensity.

I know that LDR (light dependent resitor) might help me to know that.

LDR will give us resistance level at each light intensity.

I connected LDR to my old multimeter which I not use it for a long time.





Thanks Norm to correct my word.

The resistance value will reverse to light intensity. More light is show low resistance value and less light intensity will show higher resistance number.

Now I can know that which light is more light intensity from my poor light meter. How about that? my $10 light meter.

I wonder how well it works Krit?
I think you might find that rather than a diode what you have is an LDR (light dependent resitor).

Norm

Now figure out the math to convert those number to LUX and... Viola! Light Meter! nice work

Norm said:

I wonder how well it works Krit?
I think you might find that rather than a diode what you have is an LDR (light dependent resitor).

Norm

Click to expand...

Sorry, I will correct my word. It is LDR not diode. Thank for tell me.

I think If we have/borrow light meter, we can make a standard curve and might convert it for lux level from resistance value.

Hi Krit,

Might I suggest that if you want to measure total light output (and convert it to CP's..) you'll have to measure over larger distance: 8 inch is way to close.

Seen from the LDR the reflector of the torch must be fully lit by the filament of the lamp! Only then you can compare torches. Because of the fact that filaments and reflectors of different torches have different sizes, you cannot expect a reliable measurement at that distance!

In your case a well collimated 1 watt luxeon could blast away a Maxabeam!!

And If you increase the distance, lightlosses can be compensated by using a lens to concentrate more of the light at the LDR.

But when taken care of those prob's, it should work

BTW: Take note of the fact that this way you can only measure the output of a torch at the 'hottest' part of the beam (candlepowers) You cannot measure the lumens output of a torch with your setup !!


Ra.

Hi Ra,

Yes, I would light to compare hot spot than over all out put only. Because when I chose emitter in to my flashlight, I want to know which emitter is the best out put or chose better reflecter for my light. may be 3 watt and 1 watt produce similar out put.

Thanks, for info.

I have used something similar to this before and I think it works well for comparing relative light outputs. Some of those LDRs are chromobiased though, which means they're much more sensitive to some colors than others. They should have a frequency response curve on the package though. I read on Astronomy magazine that the human eye is more sensitve to green, with response tapering off in either direction toward blue and red.