One part epoxy changes from red to clear under UV light

MASTER BOND UV15RCL low viscosity, cationic type UV curing system with a special colour changing feature. The originally red material changes to clear once exposed to UV light, indicating that there is UV light access across the adhesive material. Although this change in colour from red to clear does not indicate a full cure it does confirm that the UV light has reached the polymer. By giving immediate visual feedback, product provides processing and handling advantages over conventional systems. Curing under UV light typically takes 30-60 seconds with a broad-spectrum UV lamp emitting light with a wavelength range between 320-365 nm. The minimum energy required is 20-40 milliwatts per cm2.

click here for more info