I had a couple of years of experience in commercial and retail lighting so i had to have a very basic understanding of real-world info on theoretical light measurements.
Ill try and give a brief and probably misguided rundown and im sure theres things here that are completely misunderstood or mixed up so ill gladly take corrections if ive got it wrong. I know Ondra and Juraj have talked about this in various threads.
AFAIK Each of the measurements have a specific function and isnt always just a personal preference thing.
Lumens is a measure of how much total light a light will emit at the source (luminous power) so it isnt dependent on how big the light is because its an absolute and in my opinion is the correct unit to use if youre going for realism. Because its an absolute value, it spreads that absolute measure of light across the surface and gets brighter or dimmer as the light gets smaller or larger but the total amount of emitted light stays the same. Lumens output is on most commercial lighting spec sheets because there are minimum required lux levels across various spaces by law. This info is almost always available for every light you could need to include in your scene if its based on a real light as more often than not its on the manufacturers spec sheet. I like working with this one because you can be sure that if your bulb is the correct size and if you know the lumens value, you can be confident its light output is pretty accurate, espeically if you have an IES too.
If you run IR and set the intensity units to something like 2000 w/sr.m^2 and change the radius, youll see the scene get brighter. Whereas lumens, if you make the light bigger, the same amount of light output is spread across a larger area like when you focus and defocus a lens on a torch or a headlamp. (UPDATE 2023: Corona 9 you need to stop and restart IR to see the effect as lights dont seem to update when you change their units.
Lux The measure of light output as percieved by your eye ( measures exitance at the source and illuminance at the surface). Its a photometric measurement of how lumens are spread across a surface (UPDATE 2023: Im told the lux in corona is a measurement at 1000mm from the light source which is pretty much standard).
You could have 1 really small bright light in your house and get a high lux level on your floor. Whereas if you wanted to light a warehouse with those same lights, you would need alot more of them to achieve the same lux level despite the lumens output of the light being the same. 1 lux is 1 lumen per square meter. I tend to use lux for things like mimicking daylight because its easy to measure lux with a light meter and adjust accordingly. I do with corona had a lightmeter object though much like mentalray had.
This video demonstrates the relationship between lux and lumens quite well. The torch is always outputting the same amount of light (lumens) but when spread across a wider area, the lux level on the surface decreases the wider the focus is and increases the narrower it gets.
Watts per steridean per square meter is a radiometric measure of the lights percieved brightness and it depends on the size of your light so for every size increase, you’ll get a power increase. It measures radiance which is sort of how much of the emitted power the eye is recieving. Id like to understand this a bit better but basically if you make the light larger, the output of the light will scale to ensure that the lux on the surface stays the same.
Candelas (cd) measure luminous intensity at the source in the same way that lux measures it on a surface. That is, how bright is the light at the source. 1 candela for a wax candle flame is accurate. It uses something called the steridean value which is basiccally the cone of the light. I’m not certain on this one. Again id like to know more.
Here’s a little diagram of how the measurements relate to each other
I wish there was a corona light meter like the old one in mental ray. Then i could actually do some real tests because theres some stuff about Steridean I dont understand (which i think is the technical term for directionality?) and i thought based on the descriptions of the units, the directionality would affect them differently . I asked a fair few years ago about the implementation and maru had some quotes from somewhere. Hopefully one day we might see some sort of implementation when the big list of future bugs/features is running dry. Unfortunately now mental ray is gone you cant really do much.
You can paste the value which you obtain in W/(sr.m^2) from image pixel (or from your lightmeter in real life) into Corona light intensity and then change light units to Lux. But from what I understand this still requires some guessing and is more of a “workaround” than real light metering. Unfortunately, it looks like a proper implementation of light metering tools will be required for this to work.
Another idea is to use native 3ds Max photometric lights as Corona supports them. You can then switch to Mental Ray and use pseudo-color exposure controls or other tools which are built into 3ds Max to get the desired light intensity, and switch back to Corona again.
it seems that the easiest way to recreate real light in Corona (or any other renderer probably) would be to find what is the light’s intensity in lumen (it should be on the box/package if this is some regular light bulb, or there should be some manual with such info if this is some more advanced/photographic light). If you know value in lumen, then you should be able to simply paste that value into Corona light. If you know the power of light in W, and you know the light’s efficiency, then you should also be able to convert this into lumen. Here is an example calculator: Watts to lumens (lm) conversion calculator
There are also some obstacles like light color, or light shape, so it looks like this is always some simplification.
If you are not after scientific precision, I would suggest you taking a number of reference photos so that you would know the camera exposure values (shutter speed, iso, f number, etc), then recreating these exposure values in Corona and tweaking light source intensity so that the image is visually similar to the reference photos. Unfortunately I don’t think there are currently any easy ways of doing this.
