molding-urethane.md

@@ -96,6 +96,15 @@ More aggressive addition of dye can simulate the effect of using an optical filt
 
 ![DSC_2286__1_](uploads/b0d184dc3c2c844e65d2bdc71da15997/DSC_2286__1_.jpg)
 
+While the darkest particles appear opaque to the eye, placing them on a backlit grid shows that all three colors of particle transmit light:
+![DSC_2281__3_](uploads/980c740510a9260057d59896b67ed05b/DSC_2281__3_.jpg)
+
+Indeed, in the blue channel of the image, the particles are nearly indistiguishable:
+![DSC_2281__5_](uploads/50cf42cd651d1955193a54058e37e05a/DSC_2281__5_.jpg)
+
+However in the red channel, we can clearly see that only the darkest particle effectively blocks the red light completely:
+![DSC_2281__4_](uploads/06e1077e26c85f0f2e5d45fbbcf25854/DSC_2281__4_.jpg)
+
 In this case it is helpful to keep in mind that the stress-optic coefficient is a function of wavelength: coloration, in addition to stiffness and material thickness, controls the resolution and dynamic range of your photoelastic measurements.  Longer wavelength (redder) light produces fewer fringes for the same strain than shorter wavelength (bluer) light.