Using VrayLightMeter to Estimate the Exposure

This is a guest post by Pablo Conca Bosch from Spain

I think his technique is quite interesting, you can use the vray light meter as a real hand-held incident lightmeter. I must admit, I tried it some time ago, but upon finding out it gives lux values instead of something more usable, I quickly gave up and went back to guessing/using my eyes to determine exposure. After this tutorial, however, I can see that it is not that difficult to interpret the results and I already have a couple of ideas how to use this for fine tuning scenes.


VRay Physical Camera Exposure (initial point to work) using VRay

Due to other reasons…. (money mainly, lol) I don´t have manual camera and how it´s normal I
never use manual cameras, but I try to learn in depth all the theories that I can about
photography, to understand better how to use the VRay Physical Cameras (and a manual when
somebody leave me one).
And for this at first (years ago) I adjusted the VRay Physical Camera settings (F‐number, Shutter
Speed and ISO) with empirical method, one probe…another probe… and like this to find the
correct adjust… Work!.
Then I studied the exposure theory and the behavior of the manual cameras on photography,
and what are the most common settings for photography in different scene situations, and this
gave me an idea that how i could begin to look for a correct exposure on different scene
In this tutorial we have assumed that you have clear the exposure theory, if isn’t in this way
you can consult this great tutorial from Austris:

Vray Physical Camera Tutorial

But at the end I had no a correct number based on the intensity of the light sources on the
scene, units that could be real or unitless…The units used on your light sources no matter
because due to the first measurement we do in this technique you adapt your camera settings
to the light you have at the scene, but I normally use real values for my light sources.
And a Saturday I went to the bed  thinking about exposure theory and other stuff about real
cameras and too my last freelance work on mind (as always I had not satisfied), and on Sunday
morning  when I woke up I thought… yes! I could do this to try to adjust…
The idea is find an initial point of exposure to begin the work in the scene with your VRay
Physical Camera using a Helper included on VRay Renderer (Vray Light Meter) and transform
the data given by this helper (iluminance/luxes/lumen x square meter) to F‐number and
Shutter Speed values, using this formula to obtain an Exposure Value (Ev):
lux=(2^Ev) x 2.5 x100 /ISO
Ev= Log (Lux/2.5)/Log (2)
An this other formula to obtain the final values:
Ev=Log N2/t
Where N is the F‐Number and t is Shutter Speed
but… i think is better use the web… you will see on the tutorial.
In this tutorial you could see two cases, but it will be useful to find this initial point to work in
all the situations you could be immerse.

Here is the tutorial video:

The links shown in the video are:

Equivalent Exposure Value Chart

Exposure Triangle cheat sheet

Lux to Exposure Value (EV) and EV to Lux Converter

Watts / Energy Used vs. Lumens


Microscratches / Swirl marks / Spiderwebbing

Circular ‘spiderweb’ around bright highlights in the reflections, caused by microscratches catching light at different angles. We see this effect quite often on real world surfaces that have been scuffed, scratched and worn out for a while.

Over the years, I’ve tried to achieve this effect in VRay for quite a few times… unsuccessfully

After finding some leads on how to approach this on VRay forums, finally I think I’ve got a method that gives good looking results, naturally I also made a tutorial :)

First of all, forget about using a regular Bump map, it simply doesn’t look right in my experience. You need to use a Normal map to simulate those extremely tiny scratches.
This sort of map is best hand drawn in Photoshop, since you can easily make it tileable. Draw a bunch of scratches in the center of the image, making sure none of them approach the borders. Then, use Offset filter and draw a few more in the white areas. Keep doing until everything is well covered up in scratches.

Here’s a quick video on how I do it and my resulting map.

Now using the NVidia Normal Map filter, generate a Normal map.

Ok, now that we got the map, we can create our shader.
The shader we are creating is going to be pure reflection. So start with a Vray mat with pure black Diffuse, pure White Reflection and no Fresnel.

Add Normal Bump to the Bump slot and load in the map. Make sure it’s Gamma is set to 1.0 and filtering is disabled. I’ll use the strength of the Normals at 1.5

Now convert the material to VrayBlend and duplicate this first layer a few times.
For each subsequent coat layer, lower the Normal strength a bit. This will create a more gradual falloff around the highlights.
So in my example I used 1.5 for the first layer, 1.0 for the second, 0.5 for the third, and finally 0 for the fourth. This last layer gives pure, smooth reflections, without the scratches. You can adjust the blend amount of the last layer to control the strength of the scratches. Just make the pure reflection stronger to make it less worn.

So far the result looks like this:


This is basically our completed Reflection coat.

If you want to add these scratches to a material like polished plastic – it’s easy.
Create a new material, set up your diffuse color/etc and use it as the base layer in a new VrayBlend.

Now add our scratched reflection shader to the Coat slot and use a Falloff map in the blend amount slot, set it to Fresnel (1.4~3)

Ta-da! Your plastic now has a scratched up clear coat.

plastic microscratches

Same technique can be used to add these scratches to any other material. Just look out for the ones that already have strong glossy reflections – it’s best to disable them altogether and just use the coat.

Obviously there are still some things to tweak, like adjusting the normal strength in different layers or increasing the number of layers and so on, but I think this is a pretty good starting point!

Perhaps you use a different method to achieve this result? If you do, please share in the comments!