Thread by @hueforge

There seems to be some confusion about Transmission Distance. (TD)
Namely - is there a different TD for front-lit and back lit? Let me try to explain 1/

It's quite obvious when Filament Painting that the TD of a filament does not equate directly to how thick you can print and still see the layer below.
A filament with a TD of 5 will be fully opaque well before 1mm of depth when front-lit.
What gives? 2/

Since HueForge started as backlit tool, I standardized on the backlit measurement I was already taking. So in a backlit Lithophane, the TD very definitely equates to the literal distance in mm that light will travel through solid filament. (Perceptually, in room lighting)

Okay so if the TD is measured as a backlit value, then it's worthless for Front lit right?
Wrong.
HueForge converts that value into the correct value for Front-lit in the color model. The conversion is CLOSE to a 10x reduction but is not exactly at 10x reduction. 4/

The reduction is actually non-linear (though likely not non-linear enough at very low layer heights). But 10x is a good start.
But you do not need to have two TDs in HueForge. You only need one, and it will do the necessary conversion to make blending correct.

This is why I made the Filascope before the Step Test.
The problem with the FilaScope is that when you isolate ALL other sources of light, you can perceive light coming through the disks well past when you would in normal conditions.
This means it works best with a sensor. 6/

The original Transmission Calibration Grid (which is included with the FilaScope files on Printables) is probably the absolutely easiest one to understand as long as the TD is <= 4mm and you don't mind the roughly 1 hour print time. 7/

Also, note - Backlighting a filament doesn't make it RGB. It's the exact same CMYK blending as Filament Painting.