Light Meter Optical Booster

First things first: is this possible? I'm thinking about the inverse squared law, if you collect light over area than the meter accepts and focus it down smaller, you can increase the measurement for that amount of light, right? Like any lens would when creating a focal point, magnifying glass in the sun being the most obvious, that light becomes concentrated. Or, well, an aperture is a simple enough comparison. Could an attachment be made that goes infront of a meter cell to let it read lower light values?

I used the term "booster" because I remembered devices that would attach to older light meters to increase the range, but those seem to be either for increasing the area of the measurement electronics by area (Leicameter Booster and Nikon F meter booster), or amplifying a CdS more with another battery (Canon meter booster). I'm interested if this can be done purely optically through an meter's existing sensor.

Yes, this will work. The directionality/angle of view of the meter will also change depending on the optics you put in front of it.

The short answer is no.
Generally the purpose of a light meter is to measure the /average/ brightness over some angular extent. Whether that extent is 40 degrees or 1degree.
There is a theorem in optics that states that a passive optical system can at best preserve the /brightness/ of the light going through it.
Given a fixed detector, what an optical system can do is improve the angular resolution. Have you noticed how so-called spotmeters have a large front lens? But the sensitivity to low light levels is at best preserved.
But, you may object, why then do astronomical telescopes are built with gigantic mirrors?
- to improve the sensitivity to /point sources/ i.e. stars
- to improve the angular resolution on spatially resolved objects e.g. nebulae or planets.

@all Before claiming me wrong make sure you understand the difference between Illuminance and Flux.

bernard_L said:

The short answer is no.
Generally the purpose of a light meter is to measure the /average/ brightness over some angular extent. Whether that extent is 40 degrees or 1degree.
There is a theorem in optics that states that a passive optical system can at best preserve the /brightness/ of the light going through it.
Given a fixed detector, what an optical system can do is improve the angular resolution. Have you noticed how so-called spotmeters have a large front lens? But the sensitivity to low light levels is at best preserved.

Click to expand...

I agree with this in terms of the OP's question. Practically, if you put a bigger lens in front of an existing light meter, what you will likely do is collect more flux but integrate over a smaller angle of view.

For more background on the conservation of brightness see for example https://www.cv.nrao.edu/~sransom/web/Ch2.html

bernard_L said:

But, you may object, why then do astronomical telescopes are built with gigantic mirrors?
- to improve the sensitivity to /point sources/ i.e. stars
- to improve the angular resolution on spatially resolved objects e.g. nebulae or planets.

@all Before claiming me wrong make sure you understand the difference between Illuminance and Flux.

Click to expand...

Somewhat off topic, I don't fully agree with your summary of why telescopes are large here (and I know that I am talking to an expert). I may be picking a nit, but improving the angular resolution improves sensitivity to even resolved objects, we just don't usually phrase it that way. The question is not only about true point sources, but the source size relative to the detector. So large telescopes make an improvement for objects such as nebulae that are larger than the atmospheric seeing, but smaller than the camera field of view. (This is different from the light meter case, where the detector integrates over the whole field of view, like an old single-channel detector.)

Suppose that you have the same camera and you take it to two telescopes, with the same f-ratio, but one has an aperture twice the diameter and thus twice the focal length. You take a picture of a galaxy that is say 10 arcseconds across, well resolved by the seeing, but let's say your detector has a much larger field of view - let's say it's a 2K x 2K detector with 0.4" pixels on the large telescope and 0.8" pixels on the small telescope (speaking digital, but the argument is the same for film). The physical flux per unit area (per pixel) on the detector is the same for the reasons you gave. So the surface brightness of the galaxy is the same per pixel. But the luminosity added up over each seeing disk, or over the whole galaxy, is 4x greater on the larger telescope, because you get to add up more pixels. If you had 1.2" seeing, that's 1.5 pixels diameter on the small and 3 pixels on the large. So the luminosity per seeing-resolution is greater on the large telescope.

Agreed.
Multi pixel detector aka camera introduces another dimension in the discussion.

You might have just described a spotmeter

koraks said:

Yes, this will work. The directionality/angle of view of the meter will also change depending on the optics you put in front of it.

Click to expand...

bernard_L said:

The short answer is no.

Click to expand...

I feel conflicted!!

bernard_L said:

Generally the purpose of a light meter is to measure the /average/ brightness over some angular extent. Whether that extent is 40 degrees or 1degree.
There is a theorem in optics that states that a passive optical system can at best preserve the /brightness/ of the light going through it.
Given a fixed detector, what an optical system can do is improve the angular resolution. Have you noticed how so-called spotmeters have a large front lens? But the sensitivity to low light levels is at best preserved.
But, you may object, why then do astronomical telescopes are built with gigantic mirrors?
- to improve the sensitivity to /point sources/ i.e. stars
- to improve the angular resolution on spatially resolved objects e.g. nebulae or planets.

@all Before claiming me wrong make sure you understand the difference between Illuminance and Flux.

Click to expand...

This seems to be part of a reservation I had when writing the post. But then I thought of a magnifying glass producing a far more concentrated point of light and decided not to mention it.

The interplay between these two is confusing, to say the least. Why don't we have gigantic condenser lenses that take a bunch of light and shove it into a 5mm AFOV for our eye to see for some kind of night vision?? Because that light is captured by our eyes anyway? Then what's the difference with the magnifying glass making death rays??

All the photometric units get me mixed up haha

reddesert said:

I agree with this in terms of the OP's question. Practically, if you put a bigger lens in front of an existing light meter, what you will likely do is collect more flux but integrate over a smaller angle of view.

For more background on the conservation of brightness see for example https://www.cv.nrao.edu/~sransom/web/Ch2.html

Click to expand...

Fine by me! My goal is to just make weak, coupled meters a bit more sensitive. I don't even care if I need to use an "adjustment factor" so long as the read value is within the range instead of ambiguous by being at the bottom of the scale.

reddesert said:

Somewhat off topic, I don't fully agree with your summary of why telescopes are large here (and I know that I am talking to an expert). I may be picking a nit, but improving the angular resolution improves sensitivity to even resolved objects, we just don't usually phrase it that way. The question is not only about true point sources, but the source size relative to the detector. So large telescopes make an improvement for objects such as nebulae that are larger than the atmospheric seeing, but smaller than the camera field of view. (This is different from the light meter case, where the detector integrates over the whole field of view, like an old single-channel detector.)

Suppose that you have the same camera and you take it to two telescopes, with the same f-ratio, but one has an aperture twice the diameter and thus twice the focal length. You take a picture of a galaxy that is say 10 arcseconds across, well resolved by the seeing, but let's say your detector has a much larger field of view - let's say it's a 2K x 2K detector with 0.4" pixels on the large telescope and 0.8" pixels on the small telescope (speaking digital, but the argument is the same for film). The physical flux per unit area (per pixel) on the detector is the same for the reasons you gave. So the surface brightness of the galaxy is the same per pixel. But the luminosity added up over each seeing disk, or over the whole galaxy, is 4x greater on the larger telescope, because you get to add up more pixels. If you had 1.2" seeing, that's 1.5 pixels diameter on the small and 3 pixels on the large. So the luminosity per seeing-resolution is greater on the large telescope.

Click to expand...

bernard_L said:

Agreed.
Multi pixel detector aka camera introduces another dimension in the discussion.

Click to expand...

Bill Burk said:

You might have just described a spotmeter

Click to expand...

Well, it seems pretty relevant I think! Focusing a larger flux of light onto the same area, either a pixel OR a photocell, would produce higher measurements, right? Some cells limit the AOV of a cell, so you can foreseeably focus more light, that would otherwise not reach it, onto it.

Crysist said:

Fine by me! My goal is to just make weak, coupled meters a bit more sensitive. I don't even care if I need to use an "adjustment factor" so long as the read value is within the range instead of ambiguous by being at the bottom of the scale.

Click to expand...

Well then, one alternative would be to carry a "white card" along with you, and use it in the general manner of a gray card. Gray cards typically reflect about 18% of the light, whereas a white card will reflect about 90%. So a nominal 5x more light to hopefully bump your meters needle into a usable range. The adjustment factor, in f-stops, would be roughly a bit over 2 stops; say 2 to 2.5 stops. (Since the actual scene is darker than the meter "thinks" it is, your adjustment has gotta INCREASE exposure to the film.)

Crysist said:

The interplay between these two is confusing, to say the least. Why don't we have gigantic condenser lenses that take a bunch of light and shove it into a 5mm AFOV for our eye to see for some kind of night vision?? Because that light is captured by our eyes anyway? Then what's the difference with the magnifying glass making death rays??

Click to expand...

Here's a way to demonstrate the difference... first, use your "magnifying glass" to focus light from the sun and, as I'm fond of saying, try to "burn ants" or make dry leaves start to smoke. Ok, it works, right? Next, try the same thing using light reflected from your "scene" (to be photographed). No success, huh? I bet you already knew it wasn't gonna work either.

Mr Bill said:

Well then, one alternative would be to carry a "white card" along with you, and use it in the general manner of a gray card. Gray cards typically reflect about 18% of the light, whereas a white card will reflect about 90%. So a nominal 5x more light to hopefully bump your meters needle into a usable range. The adjustment factor, in f-stops, would be roughly a bit over 2 stops; say 2 to 2.5 stops. (Since the actual scene is darker than the meter "thinks" it is, your adjustment has gotta INCREASE exposure to the film.)

Click to expand...

I once used a white sheet of paper to meter one day at camp when scouts were practicing a flag ceremony at dusk.

The meter I was using was a Weston Master II. It helped just a little to have the needle move a little more.

It's still not the "right" meter to use in low light conditions.

Bill Burk said:

I once used a white sheet of paper to meter one day at camp when scouts were practicing a flag ceremony at dusk.

Click to expand...

I did the same but I was surrendering to the Girl Guides at the time They had invaded us

pentaxuser

koraks said:

Yes, this will work. The directionality/angle of view of the meter will also change depending on the optics you put in front of it.

Click to expand...

agreed but it should be simpler to do this electronically by just increasing the gain. An optical solution seems old-fashioned, even though it gives a bettersignal/noise ratio.

Crysist said:

The interplay between these two is confusing, to say the least. Why don't we have gigantic condenser lenses that take a bunch of light and shove it into a 5mm AFOV for our eye to see for some kind of night vision?? Because that light is captured by our eyes anyway? Then what's the difference with the magnifying glass making death rays??

Click to expand...

That (the condenser lens idea) is basically what a pair of binoculars does. A 10x 40mm pair of binoculars takes the light entering the 40mm aperture and produces a 4mm exit pupil, which is a good match to your eye's pupil. If you take them (or any pair of binoculars really) out at night and look up, you'll see way more stars than with the naked eye. However, if you point them at a night scene down the block, it won't be magically brighter. The improvement comes by the higher resolution the binoculars afford on stars or other small astronomical objects, which are unresolved objects to the eye. For a surface brightness, such as the night scene down the block, the binoculars just magnify it, but don't make it brighter. This is the point @bernard_L was making.

Crysist said:

Well, it seems pretty relevant I think! Focusing a larger flux of light onto the same area, either a pixel OR a photocell, would produce higher measurements, right? Some cells limit the AOV of a cell, so you can foreseeably focus more light, that would otherwise not reach it, onto it.

Click to expand...

You can try, but my guess is that for a roughly uniform scene brightness, it isn't going to help much, and will be somewhat tedious to calibrate how it responds to a scene of non-uniform brightness. It's sort of like metering through an SLR. Sticking a larger telephoto lens onto the SLR doesn't make the meter cell more sensitive to a lower LV, but it makes the meter cell subtend a smaller angle. The way to make a more sensitive meter would be to have more detector area: multiple meter cells.

The problem with light meters is not sensitivity at any price. The challenge is to do it in a practical way that is as simple as possible, long lasting, stable, and affordable.

It's been more than half a century since a well fettled photo-multiplier detector could register the arrival of a single photon. Actual quantum uncertainty requires detecting two photons
to be pretty sure of finding one and detecting three means the existence one is as good as dead certain. There's no sensitivity beyond that, even in theory.

Mr Bill said:

Well then, one alternative would be to carry a "white card" along with you, and use it in the general manner of a gray card. Gray cards typically reflect about 18% of the light, whereas a white card will reflect about 90%. So a nominal 5x more light to hopefully bump your meters needle into a usable range. The adjustment factor, in f-stops, would be roughly a bit over 2 stops; say 2 to 2.5 stops. (Since the actual scene is darker than the meter "thinks" it is, your adjustment has gotta INCREASE exposure to the film.)

Here's a way to demonstrate the difference... first, use your "magnifying glass" to focus light from the sun and, as I'm fond of saying, try to "burn ants" or make dry leaves start to smoke. Ok, it works, right? Next, try the same thing using light reflected from your "scene" (to be photographed). No success, huh? I bet you already knew it wasn't gonna work either.

Click to expand...

Yup, white reflects more, but I don't know how much that would help in a dark scene the same way.

I understand the demonstration and that it would happen that way, still there's the question of why that I haven't wrapped my head around. Is it because one is taking the bright object of the sun and shrinking that light down to the size of a pin, certain to make a large effect? But focusing a bunch of light at night is probably collecting a whole lot of not-much-light in the scene onto a point producing... not much light still. The effect is only drastic with a large input, is that right?

RalphLambrecht said:

agreed but it should be simpler to do this electronically by just increasing the gain. An optical solution seems old-fashioned, even though it gives a bettersignal/noise ratio.

Click to expand...

If you know an easy mod I can do to increase the sensitivity of my CdS meters (or just adjust it up or down either way), let me know! That is my ultimate goal, I just was thinking optically because it seemed non destructive, and maybe possible by an amateur like me, and kinda a fun idea.

Is it just providing proportionally more voltage? If I jam a 3V cell into the slot for the 1.3V cell on most older meters..... it wouldn't fit, but if I did do that, will I read a bit over a stop faster?

Or, for the non-powered selenium cell, is there a tiny tiny amp I can put between the cell and the meter??

reddesert said:

That (the condenser lens idea) is basically what a pair of binoculars does. A 10x 40mm pair of binoculars takes the light entering the 40mm aperture and produces a 4mm exit pupil, which is a good match to your eye's pupil. If you take them (or any pair of binoculars really) out at night and look up, you'll see way more stars than with the naked eye. However, if you point them at a night scene down the block, it won't be magically brighter. The improvement comes by the higher resolution the binoculars afford on stars or other small astronomical objects, which are unresolved objects to the eye. For a surface brightness, such as the night scene down the block, the binoculars just magnify it, but don't make it brighter. This is the point @bernard_L was making.

You can try, but my guess is that for a roughly uniform scene brightness, it isn't going to help much, and will be somewhat tedious to calibrate how it responds to a scene of non-uniform brightness. It's sort of like metering through an SLR. Sticking a larger telephoto lens onto the SLR doesn't make the meter cell more sensitive to a lower LV, but it makes the meter cell subtend a smaller angle. The way to make a more sensitive meter would be to have more detector area: multiple meter cells.

Click to expand...

Wait, but doesn't it actually do so, a larger telephoto lens capturing more light? I mean, after all, they get wider/bigger the larger the aperture is.

If you can have multiple detector areas having a larger effect, why can't it be condensed onto one using a lens? x.x

No idea about all the technical background, but if you put a lens in front of your light meter to have it respond to fewer light - you`re basically putting a wide-angle converter onto your light meter.
You expand the angle the light meter "sees", but then you`re also measuring objects outside of the angle your taking lens has.
That's probably why boosters of older light meters did increase the area of the selenium cell, so there will be more light "catched" by the light meter - light from the same direction/part of the subject. With a lens you also should be able to collect more light, but light above and below your subject being irrelevant for the measurement.