What is the lowest level of lumens a camera a camera can detect?

Question

In the past, I've asked about taking photos of luciferase. Now I'm curious how weak of a light source I can detect. From How many photons per second is one Lumen? on Physics Stack Exchange, I can determine how many photons/sec makes one lumen. What then is the minimum amount of lumens needed for a camera to detect?

Asked by bobthejoe

Answer

Short: About 5 picolumen per pixel with the best commercial DSLRs such as a Nikon D3s.

Long :-) :

Minimum detectable light source will depend on camera and how much of the image area the source occupies. For best detectability, a source will be "brightest" if all it's energy arrives in a one pixel area. The image will not be very interesting in most cases :-).

But, to attempt to put a very approximate empirical answer to the question:

I'll make various assumptions along the way and summarise them at the end so they can be adjusted as desired.

1 EV is a bit above bright Moonlight and is correctly exposed at ISO 100 at f1 for 1 second.

1 EV = 1 lux = 1 lumen per square meter.

I'll avoid the temptation here to leap into steradians and candela and stick with more intuitive empirical terms :-).

Let's assume you are using a Nikon D3s which has a 12 megapixel sensor that can just about see in the dark with no photons at all.
At about 100,000 ISO and an exposure of one second at f1 at 1 EV and dark field subtraction you may perhaps have difficulty detecting whether a given pixel was illuminated or not as even a D3s is getting somewhat noisy. At around 12800 ISO there would be little doubt.

If you set your camera to image 1 square metre then then the 1 EV lighting will be providing 1 lumen total so the 12 million pixel sensor will be accepting ~1/12,000,000 th of a lumen per pixel.

That's at f1 and ISO 100 and 1 second exposure.
Increase ISO to 12800 as above and you can detect 1/12800th less light again.
1/12 million x 1/12800 ~= 6.5 x 10^-12 lumen = 6.5 picolumen.
I don't think I've seen picolumen used before :-)

So, if, all of:

• You use an f1 lens

• Your camera can image at ISO 12800 for one second at 1 lux or 1 EV and produce a discernible change in a given pixel

• You have a 12 megapixel sensor

Then you can DETECT about 5 picolumen **in a single pixel area.
A Nikon D3S should do thus with relative ease.
Longer exposure times will produce increased sensitivities but in time noise will catch up with even a D3s.

Over the whole 12 megapixel sensor his corresponds to 78 microlumen which is 1/12800 th of a lumen total which is no surprise as it is just the inc=verse of the ISO setting when imaging a square metre at 1 lumen per squarre meter.

If you vary imaged area, aperture, ISO, sensor pixels, exposure time or camera capability then the answer will vary accordingly.

The biggest gain you can make with a given sensor is to cryo cool it.
And then there are advanced photo multiplying sensors that take the question away from the realm of "normal photography". eg Electron Multiplying CCD, Frame Transfer CCD, Intensified CCD, ...

See also:

Wikipedia astrophotography

Note: lumen is always 'singular'.

Answered by Russell McMahon

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