Essentially all it is is a white slate that you point your DSLR or DSLR copy cat at to accurately tell the sensor what color is true white. You may think that white is white, and although that's true, white will appear differently based on different lighting conditions (including underwater). A properly white balanced camera above water will be completely inaccurate underwater unless re-calibrated using a white card or sometimes called a grey card. This slate is exactly that. This will also change with depth as light only penetrates so far and color loss will happen at certain depths. This slate has each color in the light spectrum on it to show color loss due to depth and the rest is white to allow your re-calibration. If you are using probes and other underwater lighting then color loss is not an issue as you are introducing a light source closer to the object you are shooting. But again, you would still need to account for varying light from your probe plus/minus natural light to get a true reading of white and therefore produce an image with proper colors and not one that just looks all blue or all green etc.
Honestly I don't even need this slate because my other slates would work just fine as they are white and the altitude one is blank on the other side. My adventures in diving multipurpose data carrier also has each of the colors of the spectrum so if I had that underwater I could take advantage of it. The proper photography slate is just more practical and also includes greyscale. If your camera is good enough then it may calibrate for true white, grey and pure black producing the most accurate images. This slate I believe incorporates all of that.
Here's what it looks like:
Hope that gives some insight for ya :cool2:
