There is no such thing as "minimum illumination"


Back in the days of analog color video production, many manufacturers said their equipment complied with the RS-170A or EIA-170A standard.  Unfortunately, there was never any such standard.

Still, there was general agreement on what that standard would have said had it ever existed.  There is no such agreement about “minimum illumination,” whether it’s in footcandles (fc) or lux (lx).

What’s the least someone can be paid?  The smallest unit of U.S. currency is the penny, but nothing says someone needs to be paid in
currency.  And, even if the world’s smallest unit of currency were a penny, a penny per what?  Per second?  Minute?  Hour?  Day?  Week?  Month?  Year?  Decade?  Century?  Per person?  Family?  Town?  County?  State?  Country?  Continent?

Ridiculous, isn’t it?  So why do we allow the specification “minimum illumination?”

What is the illumination supposed to do?  Make a full-level image?  Make one pixel register one digital level above zero?  In how many bits?  A 10-bit level is four times smaller than an 8-bit level.  A 12-bit level is another four times smaller, and a 14-bit level is four times smaller than that – 64 times smaller than 8-bit.  That’s probably lower than some noise, so just how sure do you have to be that the “minimum illumination” has done anything?  “I think that maybe I saw something.”

How long can the exposure to the “minimum illumination” be?  With a still camera, if I open the shutter today and close it on the 10th anniversary of this date, does that count?  With a video camera, is it 60 frames per second or 24?  The latter can be 2.5 times more sensitive.  And some “minimum illumination” figures allow even longer video exposures.

Is there a lens on the camera?  What kind?  In theory, a lens could focus all of its light onto a single pixel, decreasing the “minimum illumination” by huge factors.  What is the iris setting?  How transmissive is the lens?

What kind of light is doing the illuminating?  If the camera’s image sensor is particularly sensitive to, say, green light of a wavelength of 510 nm, is that what’s doing the illuminating?  It’ll cause very different results from, say, violet light of 410 nm.

What is being illuminated?  A perfectly reflective surface?  That’s pretty different from a cat’s black fur.

Then there’s gain or amplification, the potential destroyer of images.  Every 6 dB of gain doubles sensitivity (halving “minimum illumination”) but increasing noise level.  If gain is increased 48 dB, “minimum illumination” can drop by a factor of 64 – and so can picture quality.

Here’s an actual “minimum illumination” specification from a major manufacturer of video cameras: 0.003 lux.  Wow!  But then the manufacturer provides a little more info:  +48 dB gain, slow-shutter mode at 64 frame accumulation (meaning the effective
video frame rate is less than half a frame per second).

Can we stop playing the “minimum illumination” game now?

Mark Schubin

Add new comment

 Nicely explained. 

Thats why it's so important to have adjustable gain in camcorders.