Lux meter value and film / paper iso?

[geirtbr]

I wonder if someone knows how to calculate the ISO-exposurevalue film of paper from measuring the light level that its the surface with a lux meter. So, not the light that goes through the lens, but the light that hits the surface. The practical application is to measure the lux-exposure on an enlarged photopaper, and then estimating the ISO of the paper or film. It strikes me that this should be an easy calculation but I didnt find any formula when I did a quick search on it.
Lets say that I have 600 lux for 10 seconds as exposure in the enlarger, what ISO can I estimate as equivalent?

 

[Mr Bill]

So, not the light that goes through the lens, but the light that hits the surface.

Well sure, the formal speed testing methods don't use a lens, etc., but simply the light required to produce a specified effect in the photo material (typically with some specific conditions).

For b&w photo paper, using the ANSI method, you need to know how many lux seconds are needed to produce a "density" (as measured with an instrument called a densitometer) of 0.60 above "base + fog," aka the plain unexposed paper. To a human this is gonna look real close to an official "gray card."

The formula is, paper speed = 1/H x 1,000. Where H is lux seconds exposure. .

So in your example, using 6,000 lux seconds, it's 1,000/6,000 = about 0.17.

B&w film requires development to a specified contrast, more or less, then find the exposure required to produce a result that is a specified small amount over blank film. The formula is different, and is essentially rounded off to the nearest official speed number.

I'm pretty sure you can find it in Wikipedia under "film speed" or similar.

 

[ic-racer]

Nice answer by MrBill, but are you interested in actually using the film in a camera?

 

[geirtbr]

Hello! Yes film or paper negative. So that I can have an estimate of the ISO for it in the respective situations.
MrBills formula might be a good starting poing, but I didnt go to the darkroom to make test yet.

 

[ic-racer]

Probably the easiest and most practical method of using photographic paper as a negative is to bracket some test exposures and see which paper negative makes the best print. That is the basis of ASA/ISO/DIN speed equations in the first place.