Artifical Lighting and Wattage

[Sampdoria]

Hello

 

Ive been trying to figure out and still cant get a control of the Lighting Wattage in Nxt.

 

Previously, after setting the ACAD Sheet in a proper metric environment (mm), it used to be straightforward by placing (for example) 100W Recessed Lights with a Narrow Beam and the results were close to accurate in terms of shape and illuminance.

 

Right now, I cannot get these real world Wattages working in the same environments. If I place a 100W light, it will be too bright. In fact I keep ending up having to tone down the lights to the minimum 0.01W which is highly unrealistic.

 

Any tips on this issue?

 

Also while on this topic, I was wondering how to get those realistic crescent shaped Recessed light fall offs from Lights next to walls? (Refer to image)regards

[Roy Hirshkowitz]

If you're having to turn lights down that low something is very wrong-- stop immediately and send an example. Always use accurate values with nXt. 100 watts = 1700 lumens (I always prefer working in lumens)-- it's scaled to the efficiency of old fashioned 100 watt incandescent bulbs which are rapidly disappearing.

 

The shape you're seeing next to the walls is caused by the shadowing of the light source by the rim of the can.  You'll actually have to recess the light to model this one-- it's tough to do and often not worth it since it involves cutting holes in the ceiling.  The bright shapes above the fireplace is a caustic caused by the lens and can be modeled using nXt....

 

[Sampdoria]

Hello, I have attached the DWG file and also a sample render. I removed the objects and most materials to reduce the size. Its so bright that I even had to turn off the HDRI environment. regards

 

 

[Roy Hirshkowitz]

Basically, you can't adjust the appearance of a single light by changing it's output-- although the value (data) of the pixel will go down accordingly as you mouse over the pixels.  The tone op will compensate for the reduced light.  You must adjust the Brightness of the image if you're trying to mimic low-light levels, such as night renderings.  Here's an article on tone mapping which may help you out a little bit.  Legacy versions of AR worked in a similar fashion.

 

The way light sources work relative to each other, of course, does depend on their individual output.  If one is 1700 lumens and the other is 17 lumens the first will appear much brighter.

 

*********

 

You model a caustic by actually placing a glass lens in front of the source and tagging the lens as a caustic.  I don't necessarily recommend doing this, it's tricky and requires some extra processing (it also works better in the Path Tracer.)

 

 

 

 

[Sampdoria]

Its a new concept I think, since in Ar3 one needed to prop up external lights to 200W to get a nice illuminance in an aerial night scene.

Isn't the Nxt method, therefor a hit and miss where I have to rely on a Tone Op instead of intuition? Is this an issue also in interior renders?

 

For the scene I sent you, I even had to turn off the HDRI otherwise the scene was too bright.

[Roy Hirshkowitz]

It's not a new concept, it was the same in AR3 although the tone operator itself has changed.  

It works a lot like a camera-- if you're taking a picture of something bright, like snow, you may have to overexpose it a bit to get the value you want-- the tone operator (like the camera) fights for a neutral gray.  

In general, the tone operator does a very good job of modeling the way we perceive different light levels.  Both exterior scenes and interior scenes often require little or no modification to the exposure levels.  

One big exception, however, has to do with low light levels, known technically as scotopic vision.  The tone operator we have does not model this well automatically-- you need to turn the brightness down in these cases.

[Sampdoria]

The bright shapes above the fireplace is a caustic caused by the lens and can be modeled using nXt....

 

A silly question, but how do I model a caustic? By placing the light source in a Rim?

 

regards

[Kevin Lockwood]

I think he means model the actual recesses fixture and lense